C-INTERCAL 0.29
***************

This manual is for C-INTERCAL version 0.29.  It does not replace the old groff manual, nor is it designed to be read in conjunction with it; instead, it serves a different purpose, of providing information useful to users of C-INTERCAL (unlike the other manual, it is not derived from the original INTERCAL-72 manual).

Copyright © 2007 Alex Smith.

     Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.  A copy of the license is included in the section entitled ``GNU Free Documentation License.''

About this manual
*****************

This is the Revamped Instruction Manual for C-INTERCAL (this version is distributed with C-INTERCAL version 0.29).  It is divided into four parts.

The first part is about the C-INTERCAL compiler ick, and how to use it. It covers installing the compiler, using the compiler, what error and warning messages are produced by the compiler, and some information on how to use the debugger.

The second part is about the core INTERCAL language, invented in 1972, and some other commands since then which don't feel like they're extensions.  (This is a pretty arbitrary distinction, but people who write the documentation are entitled to arbitrary distinctions.  The manual's licensed under a license that lets you change it (see Copying), so if you disagree you can move the commands from section to section yourself.)  Mostly only commands that are implemented in C-INTERCAL are covered here (if you're interested in the other commands implemented in other compilers, read CLC-INTERCAL's documentation).  However, a comprehensive guide to portability of these commands between C-INTERCAL and other INTERCAL compilers is given.

The third part covers the INTERCAL extensions and dialects that are implemented by C-INTERCAL, such as TriINTERCAL and Threaded INTERCAL. Again, extensions and dialects not implemented have been mostly left out.

The final part contains appendices (which were known as `tonsils' in the original INTERCAL manual), such as character sets used by INTERCAL, programs other than ick in the C-INTERCAL distribution, information on how to read and update the list of optimizer idioms used by the compiler, and the copyright.

Installation
************

The C-INTERCAL distribution is distributed in source code form; this means that before using it, you first have to compile it.  Don't worry: if you have the right software, it's not at all difficult.  Most Linux-based and UNIX-based computers are likely to have the software needed already; the software needed to compile source-distributed packages is also readily available for free for other operating systems.  The following instructions will help you install the distribution in a method appropriate for your system.

(If you happen to be using Debian or Ubuntu, you can also obtain and install binaries with the command sudo apt-get install intercal, making the rest of this chapter redundant.  If you do this, use Debian's bug tracker to report bugs, rather than reporting them directly to the maintainers.)

Obtaining
=========

C-INTERCAL distributions have been stored in many different places over time; it can sometimes be hard to make sure that you are finding the most recent version.  In order to make sure that you have the most recent version, the easiest way is to look at the alt.lang.intercal newsgroup; all releases of the C-INTERCAL compiler ought to be announced there.  (If you are interested in what other INTERCAL compilers are available, it may also be worth looking there.)  If you don't have access to a newsreader, your newsreader doesn't cover that newsgroup, or the distance between releases has been too large for your news server to keep the message, it's likely that you can find the announcement in an archive on the World Wide Web; at the time of writing (2007), the archives of the newsgroup are stored by Google Groups (http://groups.google.com), and a search for alt.lang.intercal there should tell you where to find a copy.

If you're looking for the latest version, note that the number after the dot represents the major version number; you want to maximise this in favour of the number before the dot, which is the bugfix level within a major version.  (Major versions are released as version 0.whatever; if a new version comes out that fixes bugs but adds no new features, nowadays its number will be of the form 1.whatever, with the same major number. This has not always been the case, though.)

Unpacking
=========

C-INTERCAL is distributed in compressed pax format; for instance, you may find as a .pax.lzma file if you have the unlzma decompression program (this is advised, as it's the smallest); .pax.bz2 is larger and .pax.gz is larger still.  Most computers can decompress files in this format, even if they don't realise it, because pax is forwards-compatible with tar; try renaming the extension from .pax to .tar after decompressing to see if you have a progam that can decompress it.  (If you're wondering why such an apparently non-standard format is being used, this is is actually a case where C-INTERCAL is being perfectly nonstandard by conforming to the standards; tar is no longer specified by POSIX, and pax is its replacement. It's just that pax never really caught on.)

It doesn't matter where you extract the distribution file to: it's best if you don't put it anywhere special.  If you aren't an administrator, you should extract the file to somewhere in your home directory (Linux or UNIX-like systems) or to your My Documents directory (recent versions of Windows; if you're using an older version, then you are an administrator, or at least have the same privileges, and can extract it anywhere).  Some commands that you might use to extract it:

Generic UNIX/Linux

          unlzma ick-0-29.pax.lzma
          tar xvf ick-0-29.pax

     or

          bunzip2 ick-0-29.pax.bz2
          tar xvf ick-0-29.pax

     or

          gunzip ick-0-29.pax.gz
          tar xvf ick-0-29.pax

     On most UNIX-based and Linux-based systems, tar will be available to unpack the installation files once they've been uncompressed with gunzip.  (I've heard that some BSD systems have pax itself to decompress the files, although have not been able to verify this; some Linux distributions also have pax in their package managers.  Both tar and pax should work fine, though.)  gunzip is also likely to be available (and bunzip2 and unlzma are less likely, but use those versions if you have them to save on your bandwidth); if it isn't, you will need to download a copy from the Internet.

Using GNU tar

          tar xzvf ick-0-29.pax.gz

     or

          tar xqvf ick-0-29.pax.bz2

     If you are using the GNU version of tar (which is very likely on Linux), you can combine the two steps into one as shown here, except when using the lzma-compressed version.

Using DJGPP

          djtar -x ick-0-29.pax.gz

     On a DOS system, you will have to install DJGPP anyway to be able to compile the distribution, and once you've done that you will be able to use DJGPP's decompressing and unpacking utility to extract the files needed to install the distribution.  (You will need to type this at the command line; on Windows 95 and later, try choosing Run... from the start menu then typing cmd (or command if that fails) in the dialog box that opens to get a command prompt, which you can exit by typing exit.  After typing any command at a command line, press RET to tell the shell to execute that command.)

On Windows

     If you're running a Windows system, you could always try double-clicking on the ick-0-29.pax.gz file; probably renaming it to have the extension .tgz is likely to give the best results.  It's quite possible that you'll have a program installed that's capable of decompressing and unpacking it. Unfortunately, I can't guess what program that might be, so I can't give you any instructions for using it.

Whatever method you use, you should end up with a directory created called ick-0.29; this is your main installation directory where all the processing done by the installation will be carried out.  You will need to have that directory as the current directory during install (at the command prompt in all the operating systems I know, you can set the current directory by typing cd
ick-0.29).

Simple Installation
===================

There are scripts included in the distribution to automate the process of installing, in various ways.  The simplest method of installing is to use the following routine:

 1. Configure C-INTERCAL, by running configure.  Although building in the distribution directory works; it is recommended that you build elsewhere; create a directory to build in (using mkdir on most operating systems), then run configure from inside that directory (for instance, you could do this from inside the main installation directory:

          mkdir build
          cd build
          ../configure

     to build in a subdirectory of the distribution called ``build'').  You also specify where you want the files to be installed at this stage; the default of /usr/local is good for many people, but you may want to install elsewhere (in particular, if you want to test out C-INTERCAL without installing it, create a new directory somewhere you own and specify that as the place to install it, so the install will actually just copy the files into the right structure for use instead of installing them).  To specify a location, give the option --prefix=LOCATION to configure; for instance, configure --prefix=/usr would install in /usr.

 2. Compile the source code, with the command make.  The Makefile will be set up for your version of make, and to automatically recompile only what needs compiling (it will even recompile the build system if you change that).

 3. Optionally, create libraries from third-party interpreters to add support for more languages to the C-INTERCAL external calls system; see Creating the Funge-98 Library.  (This step can be skipped; you can also do it later, but if you do so you need to run the next step again.)

 4. Install the executables, help files, include files, and libraries, using make install.  (This is the only step that needs root/administrator permissions; so on a system that uses sudo to elevate permissions, for instance, write it as sudo make install if you're installing into a directory that you can't write to as a non-administrative user.)

On all systems, it's worth just trying this to see if it works.  This requires a lot of software on your computer to work, but all of it is standard on Linux and UNIX systems.  The first command is a shell-script which will analyse your system and set settings accordingly; it will explain what it's doing and what settings it detected, and create several files in the installation directory to record its results.  (This is a configure script produced by the GNU autoconf (configure); its autoconf source code is available in the file configure.ac.)  The second command actually compiles the source code to produce binaries; this takes the longest of any of the steps.  You will see all the commands that it's running as it runs them.  The third command will copy the files it's compiled to appropriate shared locations on your system so that anyone on the system can just use ick.

There may be various factors that prevent this simple installation method working.  On a system not based on UNIX or Linux, you may find that you don't have some of the software required to run this (for instance, you may be missing the shell sh, and don't have the shell bash which can emulate it, and so can't run sh config.sh that depends on one of those shells being available) and so this method won't work for you.  In such cases, one solution may be to install all the software required; the GNU project has a version of all the commands required, for instance, and there may be ports available for your operating system.  However, the only software absolutely required is a C compiler (C-INTERCAL was designed to work with gcc and is tested mostly with that compiler, but in theory it should work with other C compilers too, and this is tested on occasion) and the associated software needed to compile C files to object files and executables, combine object files into libraries, etc.

Another possibility that might stop this process working is if your version of the relevant software is incompatible with the GNU versions that were used for testing.  For instance, I have come across proprietary versions of lex that need directives in the source file to say in advance how much memory the lexer-generator needs to allocate.  In such cases, pay attention to the error messages you're getting; normally they will suggest trivial modifications to the source files that will cause the compilation to work again.

Some Linux and UNIX systems (notably Debian and Ubuntu) don't have the required files for compilation installed by default.  To install them, just download and install the required packages: for Ubuntu at the time of writing, they are binutils, cpp, gcc, libc6-dev, make to compile C-INTERCAL, and if you want to modify it, you may also need autoconf, automake, bison, and flex.  For debugging help, you may also want gdb, and to recompile the documentation, you may need groff, tetex-bin, texinfo, and tidy.  Of course, if INTERCAL ever (the author of this sentence was going to write something like ``becomes popular, someone will probably package up a compiler for Debian or Ubuntu anyway'', but at that point on a whim did an Ubuntu package search for 'intercal' and actually came up with two results, which look suspiciously like C-INTERCAL and CLC-INTERCAL, later confirming this.  The author was vaguely aware that there was a CLC-INTERCAL package floating around, but was unaware of the C-INTERCAL package, and as a result was too surprised to finish the sentence.)

If you're trying to do something unusual, you probably want to set some of the settings yourself rather than letting the compilation process guess everything.  In this case, use configure --help to view the options that you can set on configure; there's a wide range of settings that you can set available there, and one of them may be what you want.

Uninstalling
============

It may happen that you decide to uninstall C-INTERCAL after installing it; this may be useful if you want to test the installation system, or change the location you install programs, or for some reason you don't want it on your computer.  It's worth uninstalling just before you install a new version of C-INTERCAL because this will save some disk space; you cannot install two versions of C-INTERCAL at once (at least, not in the same directory; but you can change the --prefix of one of the installations to get two versions at once).

If you installed C-INTERCAL using make install, you can uninstall it by using make uninstall from the installation directory, assuming that it still exists.  If you can't use that method for some reason, you can uninstall it by deleting the files ick and convickt where your computer installs binaries (with an extension like .exe added if that's usual for binaries on your operating system), libick.a, libickmt.a, libickec.a, and libyuk.a where your computer installs libraries, and the subdirectories ick-0.29 in the places where your computer installs data files and include files, and their contents.

You can go further than uninstalling.  Running make clean will delete any files created by compilation; make distclean will delete those files, and also any files created by configuring. It's probably a wise idea to uninstall before doing a distclean, though, as otherwise information needed to uninstall will be deleted, as that information is generated by configure.

Reporting Bugs
==============

If you can't get C-INTERCAL to install at all, or something goes wrong when you're using it, reporting a bug is probably a good idea.  (This is still important even if you figure out how to fix it, and the information isn't in the manual, because the fix can be added to the source code if possible, or at least to the manual, to benefit future users.)  For general help, you may want to post to the alt.lang.intercal news group; to report a bug or submit a patch, email the person who released the most recent C-INTERCAL version (which you can determine by looking at that newsgroup).

If you do find a bug (either the compiler not behaving in the way you'd expect, or if you find a way to cause E778 (see E778) without modifying the source code), it helps a lot if you can submit a bug report explaining what causes it.  If you're not sure, say that; it helps if you give examples of input, command line options, etc. that cause the bug.  There are several debug options (see Debug Options) that you can use to help pin down a bug if you're interested in trying to solve the problem yourself; looking at the output C code can also help pin down a bug if the compiler gets that far.

Information that should be given in a bug report is what you expect to happen, what actually happens, what input and command line options you gave to the compiler, what operating system you're using, any ideas you might have as to what the problem is, and any appropriate debug traces (for instance, -H (see -H) output if you think the bug is in the optimizer).  Core dumps aren't portable between systems, so don't send those; however, if you're getting an internal error and can dump core with -U (see -U), it helps if you can load a debugger (such as gdb) on the core dump, use the debugger to produce a backtrace, and send that backtrace.

If you figure out how to solve the bug yourself, and want to submit the patches to help other users (this also carries the advantage that your patches will then be maintained along with the rest of the distribution, and that you won't have to reapply them every time you upgrade to a newer version of C-INTERCAL), you must first agree to license your code under the same license as the code that surrounds it (normally, that's the GNU General Public License, but if you submit a patch to a file with a different license, like this manual (yes, documentation patches are useful too), you must agree to that license).  You will be credited for the patch in the source code unless you specifically ask not to be or you don't give your name (in both these cases, you must license the code to the public domain so that it can be incorporated without the attribution requirement).  Preferably, patches should be submitted in the format created by the command diff -u; this command is likely to be available on UNIX and Linux systems and versions are also available for DOS and Windows (including a DJGPP port of the GNU version).  If you can't manage that, just submit your new code with enough lines of old code around it to show where it's meant to go, and a description of approximately where in the file it was.  Patches should be submitted by email to the person who most recently released a version of C-INTERCAL.

If you have a suggestion for a new feature, it makes sense to first discuss it on the alt.lang.intercal news group; other INTERCAL compiler maintainers may also want to implement that feature.  If you have developed code to implement that feature in C-INTERCAL, you can submit it the same way that you would submit a patch for a bug.

Distributing
============

Due to the licensing conditions of C-INTERCAL, you are allowed to release your own version or distribution if you want to.  In such cases, it's recommended that you follow the following guidelines:

 1. Make sure the new version is based on the most recent existing version. Looking at the alt.lang.intercal newsgroup will normally let you know what version is most recent.

 2. Increment the version number; if you add any new features, increment the major version number (after the decimal point) and drop the minor version number (before the decimal point) to 0, and otherwise increment the minor version number.  You have to update the version number in the following files: configure.ac, configure, and doc/ick.txi.  You also have to rename the installation directory to reflect the new version number.

 3. Add an entry to the NEWS file explaining what's new in the version that you're releasing, following the same format as the other entries.

 4. Update the README with a description of any new files you may have added.

 5. Remove any autosave or backup files that may be littering the installation directory or its subdirectories.

 6. Run make distcheck, which will make the distribution paxballs, and rename them to have the correct extensions (Automake thinks they're tarballs, so will use .tar rather than .pax, and you have to fix this by hand).  make distcheck will also perform some sanity checks on the build system of the resulting paxball, which will help to ensure that nothing important is missing from it.

 7. Place the new version somewhere on the Internet, and announce the location and the fact that a new version has been released on alt.lang.intercal.

Invoking ick
************

All operations on INTERCAL source code available in C-INTERCAL are currently carried out by the compiler ick.

The syntax is

     ick -options INPUTFILE

(Options can be given preceded by separate hyphens, or all in a row after one hyphen, or a mixture; they're all single characters.)  By default, this compiles one INTERCAL program given as the input file directly to an executable without doing anything fancy; usually you will want to give options, which are described below.

Language-affecting Options
==========================

The following command-line options to ick affect what dialect of the INTERCAL language is compiled by the compiler; you may need to set one or more of these options if your input is not the default C-INTERCAL but instead some other language like INTERCAL-72 or CLC-INTERCAL, or just because you like certainty or like being different with respect to your output. Note that there is no command-line option corresponding to TriINTERCAL (or the base 4-7 versions); instead, the numeric base to use is determined by looking at the filename extension (.i for base 2, the default, or .3i to .7i for the base 3-7 versions.)

-b
     If this option is not given, there is a small chance that a random bug appears in the compiler, which causes the programs it creates to manifest a bug that causes error E774 (see E774).  Giving the option means that this bug will not happen.  (You may wonder why this bug was preserved; it is in fact a bug that was carefully preserved since the days of INTERCAL-72, in this case, but the option to turn it off is available as a workaround.  (There are no plans to fix this or any of the other carefully preserved bugs any time soon, because that would kind of defeat the point of having preserved them.)  Interestingly, the INTERCAL-72 compiler documentation mentions a similar command-line option that is a workaround for the same bug.)

-m
     This option needs to be given to allow any multithreading or backtracking commands or identifiers to be used.  (Unlike with other language features, this is not autodetected because it's legal to have a program with multiple COME FROM (see COME FROM) commands aiming at the same line even when it isn't multithreaded, in which case the commands cause error E555 (see E555) when that line is encountered (with the usual caveats about both commands having to be active at the time).)  Attempts to use non-COME FROM multithreading or backtracking commands without this option produce error E405 (see E405).

-e
     This option makes it possible to link non-INTERCAL programs with INTERCAL programs; instead of giving INTERCAL programs only on the command line, give one INTERCAL program, followed by any number of programs in other languages that have been written to be able to link to INTERCAL programs.  It also allows expansion libraries to be specified on the command line, after the INTERCAL program (expansion libraries are given with no extension). For more information, see External Calls.  Also, both the -a and -e options must be set to use CREATEd operators (regardless of whether external calls are used or not).

-E
     This option causes the system library to never be linked; this option is only useful if your program references a line number in the range 1000 to 1999, contains no line numbers in that range, and yet still doesn't want the system library to be linked in; therefore, it is mostly useful with -e when adding in a custom replacement system library written in a non-INTERCAL language, especially the expansion library syslibc (a system library replacement written in C).

-t
     This option tells the compiler to treat the source code as INTERCAL-72; as a result, any language constructs that are used but weren't available in 1972 will trigger error E111 (see E111).

-a
     This option allows the CREATE statement (see CREATE) to be used. Note that enabling it carries a run-time penalty, as it means that operand overloading code has to be generated for every variable in the program. (This option is not necessarily needed for the external call version of CREATE to work, but the external call version has fewer features without it.)  Note that -e (see -e) also needs to be set to be able to CREATE operators.

-v
     It is possible to write INTERCAL code sufficiently tortuous that it ends up assigning to a constant.  Generally speaking, this isn't what you wanted to do, so the compiler will kindly cause an error (E277; see E277) that stops the insanity at that point, but at the cost of a significant amount of performance you can give this option to tell the compiler to simply change the constant and keep on going anyway.  (Note that unlike CLC-INTERCAL, this only changes uses of the constant preceded by # in your program, not things like line numbers; you want Forte (http://esolangs.org/wiki/Forte) for that.)  This option also allows you to write arbitary expressions on the left of an assignment statement if you wish.

-C
     When this option is given, the generated programs will write the number 4 as IIII rather than IV, in case you're writing a clock program.

-P
     This tells the compiler to treat the input as PIC-INTERCAL (see PIC-INTERCAL) rather than ordinary C-INTERCAL input, and generate PIC output code accordingly.  There are a lot of options that are incompatible with this, as well as many language features, due to the limited memory available on a PIC.  If you get error E256 (see E256), you have this option given when it shouldn't be; likewise, if you get error E652 (see E652), you should be using this option but aren't.  (A few simple programs are C-INTERCAL/PIC-INTERCAL polyglots, but such programs are incapable of doing input or output, meaning that they aren't particularly useful.)

-X
     The C-INTERCAL and CLC-INTERCAL compilers use different notation for various things, sometimes to the extent where the same notation is legal in both cases but has a different meaning.  As this is the C-INTERCAL compiler, it rather guessably uses its own notation by default; however, the CLC-INTERCAL notation can be used as the default instead using this option.  (In most situations where there isn't an ambiguity about what something means, you can use the `wrong' syntax freely.) The option causes ambiguous characters like ? to be interpreted with Princeton rather than Atari meanings.

-x
     This option causes some constructs with different meanings in C-INTERCAL and CLC-INTERCAL to use the CLC-INTERCAL meaning rather than the C-INTERCAL meaning. At present, it affects the abstention of a GIVE UP (see GIVE UP) command by line number, which is possible as long as this switch isn't given; reading through the INTERCAL-72 manual, there are a lot of things that imply that this probably wasn't intended to be possible, but as far as I can tell that manual doesn't actually say anywhere that this particular case is disallowed, even though it rules out all other similar cases.  It also causes I/O on array variables to be done in CLC-INTERCAL's extended Baudot syntax, rather than using the Turing Tape method.

Debug Options
=============

Sometimes things will go wrong with your program, or with the way ick was installed.  There may even be unknown bugs in ick itself (if you find one of these, please report it).  The following options are used to debug the whole system on various levels.

-d

     If you think that something has gone wrong with the parser, or you want to see how your program is being parsed, you can give this option on the command line.  All the debug output produced by the parser and lexical analyser will be output.

-g

     This option allows debugging of the final executable at the C code level.  Any C code generated will be left in place, and the -g option will be given to the C compiler that's used to compile the code, so all the information needed for a C debugger to be used on the executable will be present there.

-h
-H
-hH

     These options allow debugging of the optimiser, or produce output helpful for understanding how your program has been summarised. -h produces a summary of what optimiser rules were used, the initial expression and what it was optimised to; -H produces a more expanded view that shows each intermediate step of optimisation, and -hH shows the same output as -H, but written completely using C syntax (the other options output in a strange mix of INTERCAL and C).

-l

     This option turns on generation of warnings (see Warnings).  To make sure that they aren't actually useful, or are only marginally useful, the warning generator is far too sensitive, and there is no way to decide which warnings are given and which ones aren't; you either get all of them or none.

-p

     This option causes the program to run immediately after being compiled, and profiles the resulting program to identify performance bottlenecks, etc.  The usefulness of this depends on the resolution of the timers on the computer and operating system; DOS, in particular, is really bad with timer resolution.  The output will be saved in a file called yuk.out when the program finishes running.  It's legal to turn on both the profiler and the interactive debugger at the same time, but if you do this the profiler will also identify bottlenecks in the person typing in commands to step through the program! The profiler will, in fact, identify all the timings that particular commands in the program take; so WRITE IN instructions will often show up as taking a long time due to their need to wait for input.

-w

     This option causes the produced program to support the printflow option fully; when this option is not given, printflow will in most cases have partial or no support (except in multithreaded programs, where this option is redundant), because not all the code needed for it will be included in the program to save space.

-u

     When you are getting problems with finding files -- for instance, the compiler can't find the skeleton file (see E999) or the system library (see E127) -- this option will let you know, on standard error, where the compiler is looking for files.  This may hopefully help you pin down where the file-finding problems are coming from, and also offers the option of simply placing copies of the files where the compiler is looking as a last resort.

-y

     This is the main debugging option: it loads yuk, an interactive INTERCAL debugger with ability to step through the program, set breakpoints, view and modify variables, etc.  See yuk.

-Y

     This options causes the command line to be displayed for all calls to other programs that ick makes (mostly to gcc); it is therefore useful for debugging problems with the command lines used when using the external calls system (see External Calls).

-U

     The internal error E778 (see E778) should never happen.  However, there are all sorts of potential problems that may come up, and if part of the code detects something impossible, or more usually when the operating system detects things have got too insane and segfaults, normally this error will just be generated and that's that.  (I most often get this when I've been writing a new section of code and have made a mistake; hopefully, all or at least most of these errors are fixed before release, though.)  If you want more information as to what's going on, you can give the -U option, which will cause the compiler to raise an abort signal when an internal error happens.  This can generally be caught by a debugger that's being run on ick itself at the time; on many systems, it will also cause a core dump.

Output Options
==============

These options allow you to control how far to compile (all the way to an executable, or only to C, etc.), and where the output will be created. Note that the output options may change depending on the other options selected; for instance, many of the debug options will prevent the code being compiled all the way to an executable.

-c

     By default, the original INTERCAL code will be compiled all the way to an executable, and the intermediate C and object files produced will be deleted.  Giving this option causes the compiler to stop when it has finished producing the C file, leaving the C file there as the final output of the compiler.  (Its filename is the same as the source file, but with .c as its extension/suffix rather than the source file's extension.)  Without this option, an executable will be produced with the extension changed to whatever's appropriate for the system you are on (or omitted entirely if that's appropriate for the system).

     This option also places verbose comments in the output C file.

-o

     This option causes the compiler to progress no further than producing the C output file, but instead of writing it to a file writes it directly to standard output.  This might occasionally be useful when using ick as part of a pipe; it can also be useful to see how far the compiler gets with compiling code before an error happens, when you're trying to track down an error.

Optimizer Options
=================

There are various command line options that can be used to tell ick whether and in what ways to optimize code.

-f

     This option requests the compiler to attempt to analyse the flow of the program and optimize accordingly; for instance, it will detect which commands can't possibly be ABSTAINED from and refrain from generating code to check the abstention status of those commands.

-F

     This option tells the compiler to optimize the output for speed.  This is done to crazy extremes; the compiler may take several hours/days analysing the program in some cases and still not come up with an improvement.  It turns on all the other optimizer options.  Note that not all systems accept this option, because it sometimes outputs a shell script disguised as an executable rather than an actual executable.

-O

     This option tells the compiler to apply optimizer idioms to the expressions in the code given, when appropriate.  The list of idioms is stored in the file src/idiotism.oil; note that it is compiled into the compiler, though, so you will have to rebuild and reinstall the compiler if you change it.  For more information about changing the list of idioms, see Optimizer Idiom Language.

Other Options
=============

Some options just can't be classified.

-@

     If this option is given, the compiler doesn't run at all, but instead prints a set of instructions for using it, explaining which options are available on the system you're on and which options conflict with which other options.

Options to Generated Programs
=============================

Once the compiler runs and produces an output executable, that executable itself will accept a range of options that control the way it runs.  None of these options have to be used; a default value will be assumed if they aren't.

+help
-help

     Whether + or - is given at the start of this option, it will cause the program to print out what options are available and what state they are in.  It will then cause the program to exit via an internal error.

+wimpmode
-wimpmode

     If the + version of this is given (rather than the default -), then the program will print a message explaining that you are a wimp (the mode itself is known as wimpmode), and for the rest of execution will input in Arabic numerals (123 rather than ONE TWO THREE) and likewise will output in Arabic numerals rather than Roman numerals (such as CXXIII).  True INTERCAL programmers should rarely have to use this mode.

+traditional
-traditional

     This option does not actually appear to do anything.

+instapipe
-instapipe

     This option causes standard output to be flushed whenever any characters are output when the + version is used, rather than on each newline (the default - version).  It is most useful for more responsive pipes when outputting binary data, and also useful for debugging very slow programs.

+printflow
-printflow

     The usual debugging methods don't work with multithreaded or backtracking programs.  This option exists to give at least a slim chance of working out what is going on with them.  It causes the program to print the line number of the command it thinks it may be executing next (i.e. the line number that would be printed if that line had an error) immediately after executing each command, and also an internal identifier for the thread that that command was in.  It also prints a trace of what parts of the multithreader are being activated; so for instance, it will tell you when a thread is being forked into multiple threads or when a choicepoint has been deleted. Note that the -w option (see -w) must be given to gain full support for flow printing in non-multithreaded non-backtracking programs, because otherwise the required code to print this information will not be generated.

+mystery
-mystery

     This option is occasionally capable of doing something, but is deliberately undocumented.  Normally changing it will have no effect, but changing it is not recommended.

Environment Variables
=====================

Various environment variables can be set to affect the operation of ick.

Variable   Meaning
ICKINCLUDEDIR
ICKLIBDIR
ICKDATADIR

These three environment variables suggest locations in which
ick should look to find various files that it needs: the
skeleton file, system library, C header files and libraries that it
needs, constant-output optimiser, and the GNU General Public License
(which the debugger needs to be able to display on demand for legal
reasons).
CC

The name of a C compiler to use (defaults to gcc, which is the only
compiler with which C-INTERCAL has been tested recently).  This option has
no effect on DJGPP, where gcc is always used.
ICKTEMP
TMPDIR
TEMP
TMP

On DJGPP, ick creates temporary files to pass options to gcc
as a method of getting around the limit on the length of a command line
that can sometimes affect DOS programs.  These four environment
variables are tried (in this order) to determine a location for the
temporary file; if none of them are set, the current directory is used.

Errors and Warnings
*******************

Things may go wrong, either during the compilation or the execution of your program.  Note that some things that would be compile-time errors in many other languages -- such as syntax errors -- are in fact run-time errors in INTERCAL.

Errors and warnings appear as an error code starting with ICL, followed by a three digit number, followed by I for an error or W for a warning.  However, they will be notated here as E000, etc., to save space and because consistency was never a strong point of INTERCAL.  This is followed by a text description of the error, and a hint as to the location of the error.  This is not the line on which the error occurred, but rather the line on which the next command to be executed is.  To add to the fun, the calculation of the next command to be executed is done at compile-time rather than runtime, so it may be completely wrong due to things like abstention on COME FROMs or computed COME FROMs.  The moral of this story is that, if you really want to know where the error is, use a debugger.  Note also that if the error happens at compile-time, there is no guarantee that the line number given makes any sense at all.  Some errors don't give next line numbers, mostly those for which it doesn't make logical sense, such as E633 (see E633).  After this is a suggestion to correct (or reconsider) the source code and to resubnit it.  (This typo has been carefully preserved for over a decade.)

Errors
======

This is a list of the error messages that might be produced during the compilation or execution of an INTERCAL program.

E000

     This is an unusual error; it's what's printed when a syntax error is encounted at runtime, in a situation in which it would be executed.  (An ABSTAINed syntax error, for instance, would not be executed; this is one of the mechanisms available for writing comments.)  The text of the error message is simply the statement that couldn't be decoded.

E017

          DO YOU EXPECT ME TO FIGURE THIS OUT?

     This error occurs when there is an attempt to use a constant with a value outside the onespot range; it's a compile-time error.

E079

          PROGRAMMER IS INSUFFICIENTLY POLITE

     The balance between various statement identifiers is important.  If less than approximately one fifth of the statement identifiers used are the polite versions containing PLEASE, that causes this error at compile time.

E099

          PROGRAMMER IS OVERLY POLITE

     Of course, the same problem can happen in the other direction; this error is caused at compile time if over about one third of the statement identifiers are the polite form.

E111

          COMMUNIST PLOT DETECTED, COMPILER IS SUICIDING

     This error happens when you give the -t option (see -t) but you use a language construct that wasn't available in INTERCAL-72.  If this happens, then either there's a mistake in the program that prevents it being INTERCAL-72 or you shouldn't be compiling it as INTERCAL-72 in the first place.

E123

          PROGRAM HAS DISAPPEARED INTO THE BLACK LAGOON

     There is a hard limit of 80 NEXTs at a time; this is to discourage excessive use of NEXTING for things like recursion. (Recursive programs are entirely legal; you simply have to figure out how to do it with computed COME FROM instead.  (For the record, it is possible.  (Using lots of nested brackets when talking about recursion is great (yay!).)))  Another problem with writing the source code that can cause this error is a failure to properly FORGET the entry on the NEXT stack created when trying to simulate a goto.

E127

          SAYING 'ABRACADABRA' WITHOUT A MAGIC WAND WON'T DO YOU ANY GOOD

     Your program asked to include the system library (by specifying a line number from 1000 to 1999 inclusive without including a line with that number), but due to installation problems the compiler couldn't find the system library to include.  You could try using the -u (see -u) option to see where the compiler's looking; that may give you an idea of where you need to copy the system library so that the compilation will work.  This error happens at compile time and doesn't give a next command line number.

E129

          PROGRAM HAS GOTTEN LOST

     This error happens at compile time when the compiler can't figure out where a NEXT command is actually aiming (normally due to a typo in either the line label given or the line label on the line aimed for).  The logic behind this error means that the next line to be executed is unknown (after all, that's the whole point of the error) and is therefore not given.  The -e command-line option (see -e) makes this error into a run-time error, because it allows NEXT commands to dynamically change targets at runtime, as well as line labels to dynamically change values, and thus the error is impossible to detect at compile time.

E139

          I WASN'T PLANNING TO GO THERE ANYWAY

     This error happens at compile time when an ABSTAIN or REINSTATE references a non-existent target line.  This generally happens for much the same reasons as E129 (see E129).

E182

          YOU MUST LIKE THIS LABEL A LOT!

     At present, it's impossible to have more than one line with the same line number.  That would make NEXT act too much like COME FROM in reverse to be interesting.  This error happens at compile time.  (For inconsistency, it is possible to have multiple lines with the same number as long as at most one of them is in an INTERCAL program (the others have to be in programs in other languages included via the external calls system).)

E197

          SO!  65535 LABELS AREN'T ENOUGH FOR YOU?

     Legal values for line labels are 1 to 65535 (with values from 1000 to 1999 reserved if you want to use the system library, and 1600 to 1699 reserved if you're using other expansion libraries).  This error comes up if you use nonpositive or twospot values for a line label.

E200

          NOTHING VENTURED, NOTHING GAINED

     You used a variable that isn't actually in your program.  Failing that (which would be quite impressive and is probably impossible, at least in the present version of C-INTERCAL), you specified an illegal number for a variable (legal numbers are positive and onespot).  This error happens at compile time, at least for illegal variable numbers.

E222

          BUMMER, DUDE!

     In INTERCAL, you're allowed to STASH as much as you like; this makes the language Turing-complete and allows for unlimited recursion when combined with computed COME FROM in the right way. Unfortunately, real computers aren't so idealised; if you manage to write a program so memory-intensive that the computer runs out of memory to store stashes, it causes this error at runtime.  To fix this error, you either have to simplify the program or upgrade your computer's memory, and even then that will only help to some extent.

E240

          ERROR HANDLER PRINTED SNIDE REMARK

     Arrays have to be large enough to hold at least one element; you tried to dimension an array which isn't large enough to hold any data.  This error happens at run time.

E241

          VARIABLES MAY NOT BE STORED IN WEST HYPERSPACE

     This error happens at run time when the subscripts given to an array are inconsistent with the way the array was dimensioned, either because there were the wrong number of subscripts or because a subscript was too large to fit in the array.  It can also happen when a multidimensional array is given to a command, such as WRITE IN, that expects it to be monodimensional.

E252

          I'VE FORGOTTEN WHAT I WAS ABOUT TO SAY

     This run-time error message is caused by the compiler running out of memory whilst trying to do I/O; at present, it can only happen during CLC-INTERCAL-style I/O.

E256

          THAT'S TOO HARD FOR MY TINY BRAIN

     Some commands simply aren't available in PIC-INTERCAL.  I mean, PICs generally have less than a kilobyte of memory; you're not going to be able to use some of the more confusing language features with that sort of resource limitation.  The solution is to replace the affected command, or to not give the -P option (see -P) if you didn't mean to compile as PIC-INTERCAL in the first place.

E275

          DON'T BYTE OFF MORE THAN YOU CAN CHEW

     This error happens when there is an attempt to store a twospot value in a onespot variable.  The actual size of the value is what matters when counting its spots; so you can store the output of a mingle in a onespot variable if it happens to be less than or equal to 65535, for instance. (This is not necessarily the case in versions of INTERCAL other than C-INTERCAL, though, so you have to be careful with portability when doing this.)

E277

          YOU CAN ONLY DISTORT THE LAWS OF MATHEMATICS SO FAR

     Reverse assignments are not always mathematically possible.  Also, sometimes they require changing the value of a constant; this is only legal if you specifically specified that it was legal by using the -v option.  In the case of an impossible reverse assignment (including a situation in which operand overloading causes a reverse assignment to happen), this error happens at runtime.

     This error can also come up when a scalar variable is overloaded to an array (which doesn't make sense, but could happen if someone exploited bugs in the CREATE statement (see CREATE)), and an attempt is made to read or assign to that variable.  (Subscripting a scalar variable is a syntax error, so there is no use for doing such an overload anyway.)

E281

          THAT MUCH QUOTATION AMOUNTS TO PLAGIARISM

     There is a limit of 256 on the number of nested spark/ears groups allowed.  If you somehow manage to exceed that limit, that will cause this error.  Try breaking the expression up into smaller expressions. (The limit is trivial to increase by changing SENESTMAX in ick.h; if you ever actually come across a program that hits the limit but wasn't designed to, just email the maintainer to request a higher limit.)

E333

          YOU CAN'T HAVE EVERYTHING, WHERE WOULD YOU PUT IT?

     Your program references so many variables that the compiler couldn't cope.  This error is unlikely to ever happen; if it does, try reducing the number of variables you use by combining some into arrays.  This is a compile-time error.

E345

          THAT'S TOO COMPLEX FOR ME TO GRASP

     This is another compile-time error that's unlikely to ever happen; this one signifies the compiler itself running out of memory trying to compile your program.  The only solutions to this are to simplify your program, or to make more memory available to the compiler.

E404

          I'M ALL OUT OF CHOICES!

     Your program asked that a choicepoint be backtracked to or removed, but there aren't any choicepoints at the moment.  This runtime error usually indicates a logic mistake in your program.  In backtracking programs translated from other backtracking languages, this indicates that the program has failed.

E405

          PROGRAM REJECTED FOR MENTAL HEALTH REASONS

     Your program used a construct that only makes sense when multithreading or backtracking (WHILE, MAYBE, GO BACK, or GO AHEAD), but you didn't specify the -m option (see -m).  If you meant to write a multithreaded or backtracking program, just give that option; if you didn't, be careful what words you use in comments! This error happens at compile-time.

E436

          THROW STICK BEFORE RETRIEVING!

     In order to RETRIEVE a variable, it has to be STASHed first; if it isn't, then this error happens at runtime.

E444

          IT CAME FROM BEYOND SPACE

     A COME FROM aiming at a line label --- as opposed to a computed COME FROM, which is allowed to be pointing at a nonexistent line --- must point to a valid line label.  The same applies to NEXT FROM.  This error happens at compile time if a nonexistent line label is found in one of these contexts.

E533

          YOU WANT MAYBE WE SHOULD IMPLEMENT 64-BIT VARIABLES?

     This error is like E275 (see E275), but applies when an attempt is made at runtime to store a threespot value (or even a fourspot or morespot value) in a twospot variable, or a threespot or greater value is produced as an intermediate during a calculation (for instance by a mingle operation).  No values above twospot are allowed at any point during an INTERCAL program; if you want to process higher numbers, you have to figure out a different way of storing them.

E553

          BETTER LATE THAN NEVER

     Oops!  The compiler just noticed that it had a buffer overflow. (Normally programs catch buffer overflows before they happen; C-INTERCAL catches them just afterwards instead.)  This only happens on systems which don't have a modern C standard library. Try using shorter or fewer filenames on the command line, to reduce the risk of such an overflow.

E555

          FLOW DIAGRAM IS EXCESSIVELY CONNECTED

     Aiming two COME FROMs at the same line only makes sense in a multithreaded program.  In a non-multithread program, doing that will cause this error at compile time (if neither COME FROM is computed) or at run time (if the command that has just finished running is simultaneously the target of two or more COME FROMs).  This either indicates an error in your program or that you've forgotten to use the -m option (see -m) if you are actually trying to split the program into two threads.

E562

          I DO NOT COMPUTE

     The program asked for input, but for some reason it wasn't available. (This is a runtime error, obviously.)   The error may happen because the input is being piped in from a command or file which has reached end-of-file, or because the user typed CTRL-D (UNIX/Linux) or CTRL-Z (DOS/Windows) while the program was trying to WRITE IN some data.

E579

          WHAT BASE AND/OR LANGUAGE INCLUDES STRING?

     When reading spelt-out-digit input, the input didn't seem to be a valid digit in English, Sanskrit, Basque, Tagalog, Classical Nahuatl, Georgian, Kwakiutl, Volapük, or Latin.  This seems to have languages covered pretty well; what on earth were you using, or did you just make a spelling mistake?

E621

          ERROR TYPE 621 ENCOUNTERED

     The compiler encountered error E621 (see E621).  This happens at runtime when the program requests that no entries are removed from the NEXT stack (which is possible), but that the last entry removed should be jumped to (which given the circumstances isn't, because no entries were removed).

E632

          THE NEXT STACK RUPTURES.  ALL DIE.  OH, THE EMBARRASSMENT!

     When an attempt is made to RESUME past the end of the NEXT stack, the program ends; however, this cause the program to end in a manner other than via GIVE UP or DON'T TRY AGAIN, so an error message must be printed, and this is that error message.

E633

          PROGRAM FELL OFF THE EDGE

     You can't just let execution run off the end of the program.  At least, that is, if it doesn't end with TRY AGAIN.  An attempt to do that causes this error at runtime.  Note that if your program references the system library, then it counts as being appended to your program and so the program will run into the first line of the system library rather than cause this error.  As it happens, the first line of the system library is a syntax error, so doing this will cause E000 (see E000) with the error text PLEASE KNOCK BEFORE ENTERING.  There isn't a next statement to be executed with E633, so the next statement won't be given in the error message.

E652

          HOW DARE YOU INSULT ME!

     The PIN command doesn't make much sense for anything bigger than a PIC; using it in a non-PIC program causes this error at compile-time.  Try using the normal input and output mechanisms instead.  This error may also be a clue that you are trying to compile a PIC-INTERCAL program without giving the -P option (see -P).

E666

          COMPILER HAS INDIGESTION

     There isn't a limit on the length of an input program other than your computer's memory; if your computer does run out of memory during compilation, it causes this error.  This error can also be caused if too many input files are specified on the command line; if you suspect this is the problem, split the compilation into separate compilations if you can, or otherwise you may be able to concatenate together your input files into larger but fewer files.  Yet another potential cause of this error is if a line in an input program is too long; sensible line-wrapping techniques are encouraged.

E774

          RANDOM COMPILER BUG

     No compiler is perfect; sometimes errors just happen at random.  In this case, the random error is E774.  If you don't like the idea that your program may be shot down by a random compiler bug, or you are doing something important, you can use the -b option (see -b) to prevent this bug happening.  (You may wonder why this bug is in there at all if it's so easily prevented.  The answer is that such a bug was present in the original INTERCAL-72 compiler, which also had an option to turn the bug off.  It's also a reward for people who actually read the manual.)

E777

          A SOURCE IS A SOURCE, OF COURSE, OF COURSE

     You specified a file to compile on the command line, but the compiler couldn't find or couldn't open it.  This is almost certainly because you made a typo specifying the file.

E778

          UNEXPLANED COMPILER BUG

     This should never come up, either at compile time or at run time.  It could come up at either when an internal check by the compiler or the runtime libraries realises that something has gone badly wrong; mistakes happen, and in such cases the mistake will have been detected.  (If this happens at compile time you can use the -U option (see -U) to cause the compiler to send an abort signal -- which normally causes a core dump -- when the error happens, to help debug what's causing it.)  More often, this error comes up when the operating system has noticed something impossible, like an attempt to free allocated memory twice or to write to a null pointer, and tells the compiler an error has occured, in which case the same response of putting up this error happens.  The point is that in all cases this error indicates a bug in the compiler (even if it happens at run time); in such cases, it would be very helpful if you figure out what caused it and send a bug report (see Reporting Bugs).

E810

          ARE ONE-CHARACTER COMMANDS TOO SHORT FOR YOU?

     This is a debug-time error caused when you give too much input to the debugger when all it wanted was to know what you wanted to do next.

E811

          PROGRAM IS TOO BADLY BROKEN TO RUN

     There's a limit to how many breakpoints you can have in a program; you've broken the limit and therefore broken the debugger.  This is a debug-time error.

E888

          I HAVE NO FILE AND I MUST SCREAM

     The output file couldn't be written, maybe because the disk is full or because there's already a read-only file with the same name.  This is a compile-time error.

E899

          HELLO? CAN ANYONE GIVE ME A HAND HERE?

     This error occurs at compile-time if a file type was requested for which the required libraries are unavailable.  (Support for Funge does not ship with the compiler; instead, you need to generate the library yourself from the cfunge sources.  For more information, see Creating the Funge-98 Library.)

E991

          YOU HAVE TOO MUCH ROPE TO HANG YOURSELF

     There is no limit on the number of threads or choicepoints that you can have in a multithreaded or backtracking program (in a program that isn't multithreaded or backtracking, these are obviously limited to 1 and 0 respectively).  However, your computer may not be able to cope; if it runs out of memory in the multithreader, it will cause this error at runtime.

E993

          I GAVE UP LONG AGO

     TRY AGAIN has to be the last command in a program, if it's there at all; you can't even follow it by comments, not even if you know in advance that they won't be REINSTATEd.  This error happens at compile time if a command is found after a TRY AGAIN.

E995

          DO YOU REALLY EXPECT ME TO HAVE IMPLEMENTED THAT?

     Some parts of the code haven't been written yet.  There ought to be no way to cause those to actually run; however, if you do somehow find a way to cause them to run, they will cause this error at compile time.

E997

          ILLEGAL POSSESSION OF A CONTROLLED UNARY OPERATOR

     Some operators (such as whirlpool (@) and sharkfin (^)) only make sense in TriINTERCAL programs, and some have a minimum base in which they make sense.  This error happens at compile-time if you try to use an operator that conflicts with the base you're in (such as using TriINTERCAL operators in an INTERCAL program in the default base 2).

E998

          EXCUSE ME, YOU MUST HAVE ME CONFUSED WITH SOME OTHER COMPILER

     This error occurs just before compile-time if a file is encountered on the command line that C-INTERCAL doesn't recognise.  (If this error occurs due to a .a, .b98, .c, or .c99, then you forgot to enable the external calls system using -e (see -e).)

E999

          NO SKELETON IN MY CLOSET, WOE IS ME!

     The skeleton file ick-wrap.c or pickwrap.c is needed to be able to compile INTERCAL to C.  If the compiler can't find it, it will give this error message.  This indicates a problem with the way the compiler has been installed; try using the -u option (see -u) to find out where it's looking (you may be able to place a copy of the skeleton file in one of those places).

Warnings
========

This is a list of the warnings stored in the warning database. Warnings only come up when the -l option (see -l) is given; even then, some of the warnings are not currently implemented and therefore will never come up.

W016

          DON'T TYPE THAT SO HASTILY

     The positional precedence rules for unary operators are somewhat complicated, and it's easy to make a mistake.  This warning is meant to detect such mistakes, but is not currently implemented.

W018

          THAT WAS MEANT TO BE A JOKE

     If an INTERCAL expression has been translated from another language such as C, the optimiser is generally capable of translating it back into something similar to the original, at least in base 2.  When after optimisation there are still INTERCAL operators left in an expression, then this warning is produced.  (Therefore, it's likely to come up quite a lot if optimisation isn't used!)  The system library produces some of these warnings (you can tell if a warning has come up in the system library because you'll get a line number after the end of your program).

W112

          THAT RELIES ON THE NEW WORLD ORDER

     This warning comes up whenever the compiler recognises that you've added some code that didn't exist in INTERCAL-72.  This allows you to check whether your code is valid INTERCAL-72 (although -t (see -t) is more useful for that); it also warns you that code might not be portable (because INTERCAL-72 is implemented by most INTERCAL compilers, but more recent language features may not be).

W128

          SYSLIB IS OPTIMIZED FOR OBUSCATION

     There is an idiom used in the system library that does a right-shift by selecting alternate bits from a twospot number and then mingling them the other way round.  A rightshift can much more easily be done with a single rightshift, so this is a silly way to do it, and this warning warns that this idiom was used.  However, the present optimizer is incapable of recognising whether this problem exists or not, so the warning is not currently implemented.

W276

          YOU CAN'T EXPECT ME TO CHECK BACK THAT FAR

     It's an error to assign a twospot value (a value over 65535) to a onespot variable, or to use it as an argument to a mingle.  If the optimizer can't guarantee at compile time that there won't be an overflow, it issues this warning.  (Note that this doesn't necessarily mean there's a problem --- for instance, the system library generates some of these warnings --- only that the optimiser couldn't work out for sure that there wasn't a problem.)

W239

          WARNING HANDLER PRINTED SNIDE REMARK

     Your code looks like it's trying to assign 0 to an array, giving it no dimension; this is an error.  This warning is produced at compile time if it looks like a line in your code will cause this error, but it isn't necessarily an error because that line of code might never be executed.

W278

          FROM A CONTRADICTION, ANYTHING FOLLOWS

     It's sometimes impossible to reverse an assignment (a reverse assignment can happen if the -v option (see -v) is used and an expression is placed on the left of an assignment, or in operand overloading); if the compiler detects that a reversal failure is inevitable, it will cause this warning.  Note that this doesn't always cause an error, because the relevant code might never be executed.

W450

          THE DOCUMENTOR IS NOT ALWAYS RIGHT

     There is no way to get this warning to come up; it isn't even written anywhere in C-INTERCAL's source code, is not implemented by anything, and there are no circumstances in which it is even meant to come up. It is therefore not at all obvious why it is documented.

W534

          KEEP LOOKING AT THE TOP BIT

     C-INTERCAL uses a slightly different typing mechanism to some other INTERCAL compilers; types are calculated at compile time rather than run time. This only makes a difference in some cases involving unary operators. It's impossible to detect at compile time for certain whether such a case has come up or not, but if the compiler or optimizer thinks that such a case might have come up, it will issue this warning.

W622

          WARNING TYPE 622 ENCOUNTERED

     Your code looks like it's trying to resume by 0; this is an error.  This warning is produced at compile time if it looks like a line in your code will cause this error, but it isn't necessarily an error because that line of code might never be executed.

The yuk debugger
****************

The C-INTERCAL distribution contains a runtime debugger called `yuk'. Unlike most other debuggers, it is stored as object code rather than as an executable, and it is compiled into the code rather than operating on it.  To debug code, add -y (see -y) to the command line of ick when invoking it; that tells it to compile the debugger into the code and then execute the resulting combination.  (The resulting hybrid debugger/input executable is deleted afterwards; this is to prevent it being run by mistake, and to prevent spreading the debugger's licence onto the code it was compiled with.)

yuk can also be used as a profiler using the -p option (see -p); this produces a file yuk.out containing information on how much time was spent running each command in your program, and does not prompt for debugger commands.

Note that some command line arguments are incompatible with the debugger, such as -m and -f.  In particular, this means that multithreaded programs and programs that use backtracking cannot be debugged using this method; the +printflow option (see +printflow) to a compiled program may or may not be useful for debugging multithreaded programs.

When the debugger starts, it will print a copyright message and a message on how to access online help; then you can enter commands to run/debug the program.  The debugger will show a command prompt, yuk007 , to let you know you can input a command.

Here are the commands available.  Commands are single characters followed by newlines, or followed by a line number (in decimal) and a newline or a variable name (a ., ,, : or ; followed by a number in decimal; note that some commands only allow onespot and twospot variables as arguments).

Command   Description
aLINE

All non-abstained commands on line LINE become abstained from
once.
bLINE

A breakpoint is set on line LINE.  The breakpoint causes execution
with c to stop when it is reached.
c

The program is run until it ends (which also ends the debugger) or a
breakpoint is reached.
dLINE

Any breakpoint that may be on line LINE is removed.
eLINE

An explanation of the main expression in each command on line LINE
is printed to the screen.  The explanation is in the same format as the
format produced by -h (see -h) and shows what the optimiser
optimised the expression to (or the original expression if the optimiser
wasn't used).
fLINE

Removes the effect of the m command on line LINE.
gLINE

Causes the current command to be the first command on LINE (if not
on that line already) or the next command on LINE, as if that line
was NEXTed to and then that NEXT stack item was forgotten.
h

Lists 10 lines either side of the current line; if there aren't 10 lines
to one or the other side of the current line, instead more lines will be
shown on the other side to compensate, if available.
iVAR

Causes variable VAR to become IGNOREd, making it read-only.
jVAR

Causes variable VAR to become REMEMBERed, making it no
longer read-only.
k

Continues executing commands until the NEXT stack is the same
size or smaller than it was before.  In other words, if the current
command is not a NEXT and doesn't have a NEXT FROM
aiming at it, one command is executed; but if a NEXT does
happen, execution will continue until that NEXT returns or is
forgotten.  A breakpoint or the end of the program also end this.
lLINE

Lists 10 lines of source code either side of line LINE, the same
way as with h, but using a line stated in the command rather than
the current line.
mLINE

Produces a message onscreen every time a command on line LINE is
executed, but without interrupting the program.
n

Show the NEXT stack on the screen.
o

Continue executing commands until the NEXT stack is smaller
than it was before.  If you are using NEXTs like procedures,
then this effectively means that the procedure will run until it
returns.  A breakpoint or the end of the program also end this.
p

Displays the value of all onespot and twospot variables.
q

Aborts the current program and exits the debugger.
rLINE

Reinstates once all abstained commands on line LINE.
s

Executes one command.
t

Continues execution until the end of the program or a breakpoint: each
command that executes is displayed while this command is running.
uLINE

Continues execution of the program until just before a command on line
LINE is run (or a breakpoint or the end of the program).
vVAR

Adds a `view' on variable VAR (which must be onespot or twospot),
causing its value to be displayed on the screen whenever a command is
printed on screen (for instance, because the command has just been
stepped past, or due to the m or t commands).
w

Displays the current line and current command onscreen.
xVAR

Removes any view and any action that may be associated with it on
variable VAR (which must be onespot or twospot).
yVAR

Adds a view on variable VAR; also causes a break, as if a
breakpoint was reached, whenever the value of that variable changes.
zVAR

Adds a view on variable VAR; also causes a break, as if a
breakpoint was reached, whenever that variable's value becomes 0.
VAR

A onespot or twospot variable written by itself prints out the value of
that variable.
<VAR

WRITEs IN a new value for variable VAR.  Note that input
must be in the normal ONE TWO THREE format; input in any other
format will cause error E579 (see E579) and as that is a fatal
error, the debugger and program it's debugging will end.
*

Displays the license conditions under which ick is
distributed.
?

Displays a summary of what each command does.  (@ does the same
thing.)

While the code is executing (for instance, during a c, k, o, t or u command), it's possible to interrupt it with CTRL-C (on UNIX/Linux) or CTRL-BREAK (on Windows/DOS); this will cause the current command to finish running and the debugger prompt to come back up.

Syntax
******

INTERCAL programs consist of a list of statements.  Execution of a program starts with its first statement; generally speaking each statement runs after the previous statement, although many situations can change this.

Whitespace is generally insignificant in INTERCAL programs; it cannot be added in the middle of a keyword (unless the keyword contains whitespace itself) or inside a decimal number, but it can be added more or less anywhere else, and it can be removed from anywhere in the program as well.

An INTERCAL statement consists of an optional line label, a statement identifier, an optional execution chance, the statement itself (see Statements), and optionally ONCE or AGAIN.

Princeton and Atari Syntax
==========================

The history of INTERCAL is plagued with multiple syntaxes and character sets.  The result has settled down with two versions of the syntax; the original Princeton syntax, and the Atari syntax (which is more suited to the operating systems of today).

Princeton syntax
----------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
some versions   version 0.18+   all versions   no

The original INTERCAL-72 compiler was the Princeton compiler, which introduced what has become known as the Princeton syntax for INTERCAL; this is the syntax used in the original manual, for instance, and can be considered to be the `original' or `official' INTERCAL syntax.  It is notable for containing various characters not found in some character sets; for instance, it writes the operator for mingle as a cent sign (known as `change').  The other operator that often causes problems is the bookworm operator V, backspace, -, which is used for exclusive-or; the backspace can cause problems on some systems (which was probably the original intention).  This syntax is also the default syntax in the CLC-INTERCAL compiler, which is the de
facto standard for expanding the Princeton syntax to modern INTERCAL features that are not found in INTERCAL-72; however, it does not appear to have been used as the default syntax in any other compilers. Nowadays, there are other ways to write the required characters than using backspace; for instance, the cent sign appears in Latin-1 and UTF-8, and there are various characters that approximate bookworms (for instance, CLC-INTERCAL uses the Latin-1 yen symbol for this, which just to make things confusing, refers to a mingle in modern Atari syntax).

Atari syntax
------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
some versions   yes   version 0.05+   yes

The other main syntax is the Atari syntax, so called because it was originally used by the ``Atari implementation'' of INTERCAL-72.  It is not clear why the implementation was given this name, because the authors of the implementation are not known; it is not clear whether it has any connection with Atari other than running on an Atari computer, for instance.  The syntax was designed to work better on ASCII-based systems, by avoiding the change character (although it can still be written as c, backspace, /, which the Atari compiler documentation claims that the Princeton compiler supported) in favour of a `big money' character ($), and using the `what' (?) as an alternative character for exclusive-or.  This is the syntax that C-INTERCAL and J-INTERCAL have always used, and is the one most commonly used for communicating INTERCAL programs on Usenet and other similar fora (where ASCII is one of the most reliable-to-send character sets).  It is also the syntax used for examples in this manual, for much the same reason.  The Atari syntax for constructs more modern than INTERCAL-72 is normally taken to be that used by the C-INTERCAL compiler, because it is the only Atari-syntax-based compiler that contains non-INTERCAL-72 constructs that actually need their own notation.

Line Labels
===========

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

The first part of an INTERCAL statement is a line label that specifies what its line number is.  This is optional; it's allowed to have a statement without a line number, although that prevents other commands referring to it by number.  Line numbers must be constants, and unique within the program.  However, they do not have to be in order; unlike some other languages with line numbers, a line with a higher number can come earlier in the program than a line with a lower number, and the numbers don't affect the order in which commands are executed.

A line label is a integer expressed in decimal within a wax/wane pair (( and )).  For instance, this is a valid line label:

     (1000)

Note that line numbers from 1000 to 1999 are used by the system library, so using them within your own programs may produce unexpected errors if the system library is included.  Apart from this, line numbers from 1 to 65535 are allowed.

It has become reasonably customary for people writing INTERCAL libraries to pick a range of 1000 line numbers (for instance, 3000 to 3999) and stick to that range for all line numbers used in the program (apart from when calling the system library), so if you want to write an INTERCAL library, it may be a good idea to look at the existing libraries (in the pit/lib directory in the C-INTERCAL distribution) and choose a range of numbers that nobody else has used.  If you aren't writing a library, it may be a good idea to avoid such number ranges (that is, use only line numbers below 1000 or very high numbers that are unlikely to be used by libraries in the future), so that you can easily add libraries to your program without renumbering in the future.

Statement Identifiers
=====================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

After the line label (if a statement has one) comes the statement identifier, which marks where the statement starts.  Either the label or the statement identifier, whichever one comes first, marks where the preceding statement finishes.

The main statement identifier is DO.  It also has two synonyms, PLEASE and PLEASE DO; these synonyms are the 'polite' forms of statement identifiers.  Although the three identifiers have the same meaning, using either polite or non-polite identifiers too much can cause an error; the correct proportion is approximately 3 non-polite identifiers for every polite identifier used.  None of these identifiers actually does anything else apart from marking where the statement starts; they leave the statements in the default `reinstated' state.

Adding NOT or N'T to the end of any of these identifiers, to create a statement identifier such as DO NOT or PLEASE DON'T, also creates a valid statement identifier.  These differ in meanings from the previous set of identifiers, though; they cause the statement they precede to not be executed by default; that is, the command will be skipped during execution (this is known as the `abstained' state).  This applies even if the command in question is in fact a syntax error, thus causing this to be a useful method of writing comments.  One common idiom is to write code like this:

     PLEASE NOTE: This is a comment.

The statement identifier (PLEASE NOT) is the only part of this statement that is valid INTERCAL; however, because the statement identifier is in the negated form that contains NOT, the syntax error won't be executed, and therefore this is a valid statement.  (In INTERCAL, syntax errors happen at runtime, so a program containing a statement like DOUBT THIS WILL WORK will still compile, and will not end due to the syntax error unless that statement is actually executed.  See E000.)

The ABSTAIN and REINSTATE statements can override the NOT or otherwise on a statement identifier; see ABSTAIN.

In backtracking programs, MAYBE is also a valid statement identifier; see MAYBE.  It comes before the other keywords in the statement identifier, and an implicit DO is added if there wasn't one already in the statement identifier (so MAYBE, MAYBE DO, MAYBE DON'T, MAYBE PLEASE, and so on are all valid statement identifiers).

Execution Chance
================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   version 0.02+   all versions

It's possible to specify that a command should be run only a certain proportion of the time, at random.  This is a rarely used feature of INTERCAL, although it is the only way to introduce randomness into a program.  (The C-INTERCAL compiler approximates this with pseudorandomness.) An execution chance specification comes immediately after the statement identifier, but before the rest of the statement, and consists of a double-oh-seven (%) followed by an integer from 1 to 99 inclusive, written in decimal; this gives the percentage chance of the statement running.  The execution chance only acts to prevent a statement running when it otherwise would have run; it cannot cause a statement that would otherwise not have run to run.  For instance, the statement DO %40 WRITE OUT #1 has a 40% chance of writing out I, but the statement DON'T %40 WRITE OUT #1 has no chance of writing out I or anything else, because the N'T prevents it running and the double-oh-seven cannot override that.

ONCE and AGAIN
==============

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.25+   no   no

The last part of a statement is an optional ONCE or AGAIN. ONCE specifies that the statement is self-abstaining or self-reinstating (this will be explained below); AGAIN specifies that the statement should behave like it has already self-reinstated or self-abstained.  Whether the behaviour is self-abstention or self-reinstatement depends on whether the statement was initially abstained or not; a ONCE on an initially reinstated statement or AGAIN on an initially abstained statement indicates a self-abstention, and a ONCE on an initially abstained statement or AGAIN on an initially reinstated statement indicates a self-reinstatement.

The first time a self-abstaining statement is encountered, it is executed as normal, but the statement is then abstained from and therefore will not run in future.  Likewise, the first time self-reinstating statement is encountered, it is not executed (as is normal for an abstained statement), but then becomes reinstated and will run in future.  In each of these cases, the ONCE effectively changes to an AGAIN; the ONCE only happens once, as might be expected.

REINSTATING a currently abstained self-abstaining statement or ABSTAINING (that is, with the ABSTAIN or REINSTATE commands) a currently reinstated self-reinstating statement causes the AGAIN on the statement to change back into a ONCE, so the statement will again self-abstain or self-reinstate.  Likewise, REINSTATING a currently abstained self-reinstating statement or ABSTAINING a currently reinstated self-abstaining statement causes its ONCE to turn into an AGAIN.

Historical note: ONCE was devised by Malcom Ryan as a method of allowing synchronisation between threads in a multithreaded program (ONCE is atomic with the statement it modifies, that is, there is no chance that threads will change between the statement and the ONCE).  AGAIN was added to Malcom Ryan's Threaded Intercal standard on the suggestion of Kyle Dean, as a method of adding extra flexibility (and to allow the ONCEs to happen multiple times, which is needed to implement some multithreaded algorithms).

Expressions
***********

Many INTERCAL statements take expressions as arguments.  Expressions are made up out of operands and operators between them.  Note that there is no operator precedence in INTERCAL; different compilers resolve ambiguities different ways, and some versions of some compilers (including the original INTERCAL-72 compiler) will cause error messages on compiling or executing an ambiguous expression, so it's safest to fully group each expression.

Constants and Variables
=======================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

The basic operands in INTERCAL are constants and variables.  These together make up what in other languages are known as `lvalues', that is, operands to which values can be assigned.  (Constants can also be lvalues in INTERCAL, but by default C-INTERCAL turns this off because it carries an efficiency penalty and can be confusing; this can be turned on with the -v option (see -v).)

Constants can have any integer value from 0 to 65535 inclusive; higher values (up to 4294967295) can be generated in programs, but cannot be specified literally as constants.  (The usual way to work around this limitation is to interleave two constants together; see Mingle.) A constant is written as a mesh (#) followed by a number in decimal.  At the start of the program, all constants have the same value as the number that identifies them; for instance, #100 has 100 as its value, and it's strongly advised not to change the value of a constant during the execution of a program.

There are four types of variable: 16-bit and 32-bit unsigned integers, and arrays of 16-bit and 32-bit unsigned integers.  These are represented with a spot, twospot, tail, and hybrid (., :, ,, and ;) respectively.  For this reason, integers within the range 0 to 65535 inclusive are known as `onespot numbers', and integers within the range 0 to 4294967295 inclusive are known as `twospot numbers'; variables with those ranges are known as onespot and twospot variables.  (Note that arrays did not work in C-INTERCAL before version 0.7.)

Variables are represented with a character representing their data type, followed by an integer from 1 to 65535 inclusive, written in decimal. Non-array variables don't need to be declared before they are used; they automatically exist in any program that uses them.  For instance, .1 and .001 are the same variable, onespot number 1. Array variables need to be dimensioned before they are used, by assigning dimensions to them; see Calculate.

Grouping Rules
==============

Because there are no operator precedences in INTERCAL, there are various solutions to specifying what precedences actually are.

The portable solution

     All known versions of INTERCAL accept the INTERCAL-72 grouping rules.  These state that it's possible to specify that an operator takes precedence by grouping it inside sparks (') or rabbit-ears ("), the same way as wax/wane pairs (parentheses) are used in other programming languages.  INTERCAL-72 and earlier C-INTERCAL versions demanded that expressions were grouped fully like this, and this practice is still recommended because it leads to portable programs and is easier to understand.  Whether sparks or rabbit-ears (often called just `ears' for short) are used normally doesn't matter, and programmers can use one or the other for clarity or for aesthetic appeal.  (One common technique is to use just sparks at the outermost level of grouping, just ears at the next level, just sparks at the next level, and so on; but expressions like ''#1~#2'~"#3~#4"'~#5 are completely unambiguous, at least to the compiler.)

     There are, however, some complicated situations involving array subscripting where it is necessary to use sparks and ears at alternate levels, if you want to write a portable program.  This limitation is in C-INTERCAL to simplify the parsing process; INTERCAL-72 has the same limitation, probably for the same reason.  Compare these two statements:

          DO .1 <- ,3SUB",2SUB.1".2
          DO .1 <- ,3SUB",2SUB.1".2~.3"".4

     The problem is that in the first statement, the ears close a group, and in the second statement, the ears open a group, and it's impossible to tell the difference without unlimited lookahead in the expression. Therefore, in similar situations (to be precise, in situations where a group is opened inside an array subscript), it's necessary to use the other grouping character to the one that opened the current group if you want a portable program.

     One final comment about sparks and rabbit-ears; if the next character in the program is a spot, as often happens because onespot variables are common choices for operands, a spark and the following spot can be combined into a wow (!).  Unfortunately, none of the character sets that C-INTERCAL accepts as input (UTF-8, Latin-1, and ASCII-7) contain the rabbit character, although the Hollerith input format that CLC-INTERCAL can use does.

Positional precedences: CLC-INTERCAL rules

     The precedence rules used by CLC-INTERCAL for grouping when full grouping isn't used are simple to explain: the largest part of the input that looks like an expression is taken to be that expression.  The main practical upshot of this is that binary operators right-associate; that is, .1~.2~.3 is equivalent to .1~'.2~.3'.  C-INTERCAL versions 0.26 and later also right-associate binary operators so as to produce the same results as CLC-INTERCAL rules in this situation, but as nobody has yet tried to work out what the other implications of CLC-INTERCAL rules are they are not emulated in C-INTERCAL, except possibly by chance.

Prefix and infix unary operators

     In INTERCAL-72 and versions of C-INTERCAL before 0.26, unary operators were always in the `infix' position.  (If you're confused about how you can have an infix unary operator: they go one character inside a group that they apply to, or one character after the start of a constant or variable representation; so for instance, to portably apply the unary operator & to the variable :1, write :&1, and to portably apply it to the expression '.1~.2', write '&.1~.2'.)  CLC-INTERCAL, and versions of C-INTERCAL from 0.26 onwards, allow the `prefix' position of a unary operator, which is just before whatever it applies to (as in &:1).  This leads to ambiguities as to whether an operator is prefix or infix.  The portable solution is, of course, to use only infix operators and fully group everything, but when writing for recent versions of C-INTERCAL, it's possible to rely on its grouping rule, which is: unary operators are interpreted as infix where possible, but at most one infix operator is allowed to apply to each variable, constant, or group, and infix operators can't apply to anything else.  So for instance, the C-INTERCAL '&&&.1~.2' is equivalent to the portable '&"&.&1"~.2' (or the more readable version of this, "&'"&.&1"~.2'", which is also portable).  If these rules are counter-intuitive to you, remember that this is INTERCAL we're talking about; note also that this rule is unique to C-INTERCAL, at least at the time of writing, and in particular CLC-INTERCAL is likely to interpret this expression differently.

Operators
=========

Operators are used to operate on operands, to produce more complicated expressions that actually calculate something rather than just fetch information from memory.  There are two types of operators, unary and binary operators, which operate on one and two arguments respectively. Binary operators are always written between their two operands; to portably write a unary operator, it should be in the `infix' position, one character after the start of its operand; see Prefix and infix unary operators for the full details of how to write unary operators portably, and how else you can use them if you aren't aiming for portability.  This section only describes INTERCAL-72 operators; many INTERCAL extensions add their own operators.

Mingle
------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes, all versions, all versions, all versions

Mingle, or interleave, is one of the two binary operators in INTERCAL-72. However, different INTERCAL compilers represent it in different ways, so it is impossible to write a mingle in a program completely portably, because it differs between Princeton and Atari syntax, and worse, the sequence of character codes needed to represent it in each syntax has varied from compiler to compiler.

The original INTERCAL-72 compiler (the Princeton compiler) used the 'change' (cent) character for a mingle, represented as c, backspace, /.  (By the way, this is still the most portable way to write a mingle; both C-INTERCAL and CLC-INTERCAL accept it, at least if a lowercase c is used, the Atari compiler accepted it, and its documentation claimed that the Princeton compiler also accepted it; CLC-INTERCAL also accepts a capital C before the backspace and /, and allows | rather than /.)  The Atari compiler, another INTERCAL-72 compiler, used a 'big money' character ($) as the mingle character; this character is also the only one accepted for mingle by the J-INTERCAL compiler.  C-INTERCAL originally also used the $ character for mingle, and this character is the one most commonly seen in existing C-INTERCAL programs, and most often used when giving examples of INTERCAL on Usenet, because it exists in the ASCII-7 character set, and because it doesn't contain control characters.  From version 0.18 of C-INTERCAL onwards, various other units of currency (change, quid, and zlotnik if Latin-1 is used as the input, and euro if Latin-9 is used as the input) are accepted; from version 0.20 onwards, in addition to the Latin-1 characters, all the currency symbols in Unicode are accepted if UTF-8 is used as the input format.  CLC-INTERCAL has always used the change character (either the Latin-1 version or the version that contains a backspace) for mingle. In this manual, mingle will be represented as $, but it's important to bear in mind that this character is not the most portable choice.

The mingle operator should be applied to two operands or expressions. To be portable, the operands must both be onespot expressions, that is expressions which have a 16-bit result; C-INTERCAL relaxes this rule slightly and only requires that the result be in the onespot range. (This is because the data type of a select operator's value is meant to be determined at runtime; C-INTERCAL determines all data types at compile time, so has to guess a 32-bit result for a select with a 32-bit type as its right operand even when the result might actually turn out to be of a 16-bit type, and so this behaviour prevents an error when a select operation returns a value with a 16-bit data type and is used as an argument to a mingle.)  The result is a 32-bit value (that is, it is of a 32-bit data type, even if its value fits into the onespot range), which consists of bits alternated from the two arguments; to be precise, its most significant bit is the most significant bit of its first argument, its second most significant bit is the most significant bit of its second argument, its third most significant bit is the second most significant bit of its first argument, and so on until its least significant bit, which is the least significant bit of its second argument.

One of the most common uses of interleaving is to create a constant with a value greater than 65535; for instance, 65536 is #256$#0.  It is also commonly used in expressions that need to produce 32-bit results; except in some simple cases, this is usually coded by calculating separately the odd-numbered and even-numbered bits of the result, and mingling them together at the end.  It is also used in expressions that need to left-shift values or perform similar value-increasing operations, as none of the other operators can easily do this; and mingle results are commonly used as the argument to unary binary logic operators, because this causes them to behave more like the binary logic operators found in some other languages.

Select
------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

The select operator is one of the two binary operators in INTERCAL-72; unlike mingle, every known implementation of INTERCAL ever has used the sqiggle character (~) as the representation of the select operator, meaning that writing it portably is easy.

The select operator takes two arguments, which can be of either datatype (that is, 16- or 32-bit).  It returns a value made by selecting certain bits of its first operand indicated by the second operand, and right-justifying them.  What it does is that it ignores all the bits of the first operand where the second operand has a 0 as the corresponding bit, that is, deletes them from a copy of the operand's value; the bits that are left are squashed together towards the least-significant end of the number, and the result is filled with 0s to make it up to 16 or 32 bits.  (In INTERCAL-72 the minimum multiple of 16 bits possible that the result fits into is chosen, although if :1 has the value 131061 (in hex, 1FFFF) the expression #21~:1 produces a 32-bit result because 17 bits were selected, even though many of the leading bits were zeros; in C-INTERCAL the data type of the result is the same as of the right operand of the select, so that it can be determined at compile time, and so using a unary binary logic operator on the result of select when the right operand has a 32-bit type is nonportable and not recommended.)  As an example, #21~:1 produces 21 as its result if :1 has the value 131061, 10 as its result if :1 has the value 30 (1E in hex; the least significant bit of 21 is removed because it corresponds to a 0 in :1), and 7 as its result if :1 has the value 21 (because three bits in 21 are set, and those three bits from 21 are therefore selected by 21).

Select is used for right-shifts, to select every second bit from a number (either to produce what will eventually become an argument to mingle, or to interpret the result of a unary binary logic operator, or occasionally both), to test if a number is zero or not (by selecting it from itself and selecting 1 from the result), in some cases as a limited version of bitwise-and (that only works if the right operand is 1 less than a power of 2), and for many other purposes.

Unary Binary Logic
------------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

There are three unary operators in INTERCAL-72, each of which carries out a binary logic operation on adjacent bits of the number. The operators are and, or, and exclusive or; and and or are represented by an ampersand (&) and book (V) respectively, and exclusive or has the same notational problems as mingle, as it differs between Princeton and Atari syntax.  It was represented by a bookworm, written V, backspace, -, in the Princeton INTERCAL-72 implementation, and this is still the most portable way to write it (C-INTERCAL and CLC-INTERCAL accept it).  The Atari implementation of INTERCAL-72 wrote it with a what (?), and this is the representation originally used by C-INTERCAL (and still accepted), the only representation accepted by J-INTERCAL, the one most commonly used on Usenet, and the one used in this manual (although again, it's worth pointing out that this isn't portable).  CLC-INTERCAL approximates a bookworm with the yen character, which being a currency character is one of the possible representations for mingle in C-INTERCAL; C-INTERCAL uses the rather confusing method of interpreting a yen character as exclusive-or if input in Latin-1 but as mingle if input in UTF-8. (This usually does the right thing, because CLC-INTERCAL doesn't support UTF-8.)  In the same way, CLC-INTERCAL has a C-INTERCAL compatibility option to allow the use of ? for exclusive-or.

The operators take each pair of consecutive bits in their arguments (that is, the least significant with the second least significant, the second least significant with the third least significant, the third least significant with the fourth least significant, and so on, with the pair consisting of the most significant and least significant being used to calculate the most significant bit of the result), and perform an appropriate logic operation on them; and sets a bit of the result if and only if both bits in the pair were set, or sets each bit corresponding to each pair where either bit was set, and exclusive or sets if and only if the bits in the pair had different values (that is, one was set, but not both).  So for instance, #&26 is 16 (26 is 1A in hexadecimal or 11010 in binary); #V26 is 31 (11111 in binary), and #?26 is 23 (10111 in binary).

The most commonly seen use for these operators is to carry out bitwise ands, ors, and exclusive ors between two different 16-bit expressions, by mingling them together, applying a unary binary logic operator, and selecting every second bit of the result; such code often results due to people thinking in terms of some other language when writing INTERCAL, but is still often useful.  (Historically, the first idiom added to the optimizer, apart from constant folding, was the mingle/unary/select sequence.)  There are more imaginative uses; one impressive example is the exclusive or in the test for greater-than from the original INTERCAL-72 system library:

     DO :5 <- "'?":1~'#65535$#0'"$":2~'#65535$#0'"'
          ~'#0$#65535'"$"'?":1~'#0$#65535'"$":2~'#0$
          #65535'"'~'#0$#65535'"
     DO .5 <- '?"'&"':2~:5'~'"'?"'?":5~:5"~"#65535~
          #65535"'~'#65535$#0'"$#32768'~'#0$#65535'"
          $"'?":5~:5"~"#65535$#65535"'~'#0$#65535'"'
          "$"':5~:5'~#1"'~#1"$#2'~#3

The first statement works out the value of :1 bitwise exclusive or :2; the second statement then works out whether the most significant set bit in :5 (that is, the most significant bit that differs between :1 and :2) corresponds to a set bit in :2 or not.  In case that's a bit too confusing to read, here's the corresponding optimizer idiom (in OIL):

     ((_1~:2)~((?32(:2~:2))^#2147483648))->(_1>(:2^_1))

(Here, the ^ refers to a bitwise exclusive or, an operation found in OIL but not in INTERCAL, which is why the INTERCAL version is so much longer.) The INTERCAL version also has some extra code to check for equality and to produce 1 or 2 as the output rather than 0 or 1.

Array Subscript
---------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   version 0.7+   all versions   all versions

In order to access the elements of an array, either to read or write the array, it is necessary to use the array subscript operator SUB.  Note that an array element is not a variable, so it is not accepted as an acceptable argument to statements like IGNORE; however, it can be assigned to.

The syntax for an array element is the array, followed by the keyword SUB, followed by an expression for the element number in the array.  In the case of a multidimensional array, more than one expression is given after the keyword SUB to give the location of the element in each of the array's dimensions.  The first element in an array or array dimension is numbered 1.

For instance, this is a legal (but not particularly useful) INTERCAL program with no syntax errors that shows some of the syntaxes possible with array subscripting:

     PLEASE ,1 <- #2
     DO .1 <- #2
     DO ,1 SUB .1 <- #1
     DO ,1 SUB #1 <- ,1 SUB #2
     PLEASE ;1 <- #2 BY #2
     DO ;1 SUB #1 #2 <- ,1 SUB ,1 SUB .1
     DO READ OUT ;1SUB#1.1
     DO GIVE UP

Grouping can get complicated when nested array subscripting is used, particularly with multiple subscripts.  It is the programmer's job to write an unambiguous statement, and also obey the extra grouping rules that apply to array subscripts; see Grouping Rules.

Statements
**********

There is a wide range of statements available to INTERCAL programs; some identifiably belong to a particular variant or dialect (such as Backtracking INTERCAL), but others can be considered to be part of the 'core language'.  The statements listed here are those that the C-INTERCAL compiler will accept with no compiler switches to turn on particular dialect options.  Note that many statements have slightly different effects in different implementations of INTERCAL; known incompatibilities are listed here, but it's important to check your program on multiple compilers when attempting to write a portable program.

Syntax Error
============

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   version 0.15+   all versions   all versions

One of the more commonly-used commands in INTERCAL is the syntax error. A properly-written syntax error looks nothing like any known INTERCAL command; a syntax error that looks vaguely like a command but isn't may confuse C-INTERCAL before version 0.28, and possibly other compilers, into bailing out at compile time in some situations (this is known as a `serious syntax error'), and so is not portable.  For other syntax errors, though, the semantics are easily explained: there is a run-time error whenever the syntax error is actually executed, and the line containing the syntax error is used as the error message.

One purpose of this is to allow your programs to produce their own custom errors at run time; however, it's very important to make sure that they start and end in the right place, by manipulating where statement identifiers appear.  Here's a correct example from the system library:

     DOUBLE OR SINGLE PRECISION ARITHMETIC OVERFLOW

This is a valid INTERCAL command, that produces an error when run (note the DO at the start).  An even more common use is to produce an initially abstained syntax error by using an appropriate statement identifier, for instance

     PLEASE NOTE THAT THIS IS A COMMENT

This would produce an error if reinstated somehow, but assuming that this isn't done, this is a line of code that does nothing, which is therefore equivalent to a comment in other programming languages. (The initial abstention is achieved with the statement identifier PLEASE NOT; the extra E causes the command to be a syntax error, and this particular construction is idiomatic.)

Referring to the set of all syntax errors in a program (or the set of all commands of any other given type) is achieved with a special keyword known as a `gerund'; gerund support for syntax errors is resonably recent, and only exists in CLC-INTERCAL (version 1.-94.-3 and later, with COMMENT, COMMENTS, or COMMENTING), and C-INTERCAL (COMMENT in version 0.26 and later, and also COMMENTS and COMMENTING in version 0.27 and later). Therefore, it is not portable to refer to the set of all syntax errors by gerund; using a line label is a more portable way to refer to an individual syntax-error command.

Calculate
=========

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

At present, the only INTERCAL command that contains no keywords (apart from the statement identifier and possibly ONCE or AGAIN) is what is known as the `calculate' command.  It is used to assign values to variables, array elements, and arrays; assigning a value to an array changes the number of elements that that array can hold, and causes the values of all elements previously in that array to be lost.  The syntax of a calculate command is as follows:

     DO .1 <- ':2~:3'~#55

That is, the command is written as a variable or array element, then the <- operator (known as an `angle-worm' and pronounced `gets'), then an expression to assign to it.  In the special case when an array is being dimensioned by assigning a value to it, the expression can contain the keyword BY to cause the array to become multidimensional; so for a 3 by 4 by 5 array, it would be possible to write

     DO ,1 <- #3 BY #4 BY #5

The calculate command always evaluates the expression, even if for some reason the assignment can't be done (for instance, if the variable being assigned to is read-only); this is important if the expression has side-effects (for instance, giving an overflow error). If the variable does happen to be read-only, there is not an error; the expression being assigned to it is just evaluated, with the resulting value being discarded.

The gerund to refer to calculations is CALCULATING; however, if you are planning to use this, note that a bug in older versions of C-INTERCAL means that assignments to arrays are not affected by this gerund before version 0.27.

CLC-INTERCAL from 1.-94.-4 onwards, and C-INTERCAL from 0.26 onwards, allow arbitrary expressions on the left hand side of an assignment (C-INTERCAL only if the -v option is used); for more information on how such `reverse assignments' work, see Operand Overloading.

NEXT, FORGET and RESUME
=======================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   see text   all versions

The only flow-control commands in INTERCAL-72 were NEXT, RESUME, and FORGET; together these manipulate a stack of locations in the program known as the `NEXT stack'.  Although all INTERCAL compilers have implemented these, from CLC-INTERCAL version 0.05 onwards CLC-INTERCAL has considered them obsolete, and therefore a special command-line switch needs to be used to enable them.  (They are still the most portable flow-control commands currently available, though, precisely because INTERCAL-72 implements nothing else.)  Note that there is a strict limit of 80 locations on the NEXT stack, enforced by all known INTERCAL compilers; this helps to enforce good programming style, by discouraging NEXT-stack leaks (which are otherwise quite easy to write).

Here are examples to show the syntax of these three statements:

     DO (1000) NEXT
     DO FORGET '.1~.1'~#1
     DO RESUME .5

The NEXT command takes a line label as its argument (unlike most other INTERCAL commands, it comes after its argument rather than before); both FORGET and RESUME take expressions. (CLC-INTERCAL from version 0.05 onwards also allows an expression in NEXT, rather than a label, to give a computed NEXT, but this behaviour was not implemented in other compilers, and is deprecated in CLC-INTERCAL along with noncomputed NEXT; if computed NEXT is ever implemented in C-INTERCAL, it will likely likewise be deprecated upon introduction).  (Update: it was implemented in C-INTERCAL version 0.28, but only as part of the external calls system, so it cannot be used in ordinary programs; a sample expansion library gives in-program access to a limited form of computed NEXT, but should probably not be used.)  Running a NEXT causes the program control to transfer to the command whose line label is referenced, and also saves the location in the program immediately after the NEXT command on the top of the NEXT stack.

In order to remove items from the NEXT stack, to prevent it filling up (which is what happens with a naive attempt to use the NEXT command as an equivalent to what some other languages call GOTO), it is possible to use the FORGET or RESUME commands.  They each remove a number of items from the NEXT stack equal to their argument; RESUME also transfers control flow to the last location removed from the NEXT stack this way.  Trying to remove no items, or more items than there are in the stack, does not cause an error when FORGET is used (no items or all the items are removed respectively); however, both of these cases are errors in a RESUME statement.

Traditionally, boolean values in INTERCAL programs have been stored using #1 and #2 as the two logic levels.  This is because the easiest way to implement an if-like construct in INTERCAL-72 is by NEXTING, then NEXTING again, then RESUMING either by 1 or 2 according to an expression, and then if the expression evaluated to 1 FORGETTING the remaining NEXT stack entry.  By the way, the previous sentence also explained what the appropriate gerunds are for NEXT, RESUME, and FORGET.

STASH and RETRIEVE
==================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

The NEXT stack is not the only stack available in an INTERCAL program; each variable used in the program also has its own stack, which holds values of the same type as the variable.  The STASH command pushes a variable's value onto that variable's stack; RETRIEVE can be used in the same way to pop the top element of a variable's stack to replace that variable's value.  The syntax is the same as most other INTERCAL commands, with the word STASH or RETRIEVE followed by the variable or variables to stash or retrieve:

     DO STASH .1 + ;2
     DO RETRIEVE ,3

Note that it is possible to stash or retrieve multiple variables at once, by listing their names separated by intersections (+); it's even possible to stash or retrieve a variable twice in the same statement.

It is not entirely clear how RETRIEVE interacts with IGNORE in historical INTERCAL-72 compilers; the three modern INTERCAL compilers all use different rules for the interaction (and the C-INTERCAL maintainers recommend that if anyone decides to write their own compiler, they choose yet another different rule so that looking at the interaction (the so-called `ignorret test') can be used as a method of determining which compiler is running):

  * C-INTERCAL treats a retrieval just like an assignment.  That is, a read-only variable will not be changed by a retrieval, and will remain read-only, but the side-effect of popping that variable's stash will still happen.

  * J-INTERCAL ignores the read-only status of a variable when retrieving it from a stash; the value of the variable changes despite being read-only.  The variable remains read-only; the RETRIEVE simply allows a change to its value despite the read-only status.

  * CLC-INTERCAL is the most complicated; it stores read-only status or otherwise in the stash, along with the variable's value.  This means that if a variable was stashed when read-only, retrieving it will retrieve its value and set it to read-only regardless of its previous read-only status; likewise, if a variable was stashed when read-write, retrieving it will retrieve its value and set it to read-write even if it was previously read-only.

The appropriate gerunds for STASH and RETRIEVE are STASHING and RETRIEVING respectively.

IGNORE and REMEMBER
===================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

Variables in INTERCAL can be either read-write or read-only.  At the start of a program, all variables are read-write, but this status can be changed dynamically during execution of a program using the IGNORE and REMEMBER statements (whose gerunds are IGNORING and REMEMBERING respectively).  The syntax is the same as for STASH and RETRIEVE: the command's name followed by an intersection-separated list of variables.  For instance:

     DO IGNORE .4
     DO REMEMBER ,4 + ;5

Using the IGNORE statement sets a variable to be read-only (or does nothing if it's read-only already); REMEMBER sets it to be read-write.  Any attempt to assign to a read-only variable silently fails.  One place that this is used is in the system library; instead of not assigning to a variable in certain control flow paths, it instead sets it to be read-only so that subsequent assignments don't change its value (and sets it to be read-write at the end, which succeeds even if it was never set read-only in the first place); the advantage of this is that it doesn't need to remember what flow path it's on except in the variable's ignorance status.

The interaction between IGNORE and RETRIEVE was never defined very clearly, and is in fact different in C-INTERCAL, CLC-INTERCAL and J-INTERCAL; for more details, see RETRIEVE.

ABSTAIN and REINSTATE
=====================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

The statement identifier of a statement determines whether it's in an abstained or reinstated state at the start of a program; these states determine whether the statement runs at all when it's encountered.  It is, however, possible to change this state dynamically during a program's execution, and the statements to do this are rather appropriately named ABSTAIN and REINSTATE.  There are two forms of each, one which takes a single line label (which must be constant in most compilers, but can instead be an expression in recent CLC-INTERCAL versions), and one which takes an intersection-delimited list of gerunds.  They look like this:

     DO ABSTAIN FROM ABSTAINING + REINSTATING
     DO ABSTAIN FROM (10)
     DO REINSTATE CALCULATING
     DO REINSTATE (22)

(This also illustrates the gerunds used for these commands; note that ABSTAINING from REINSTATING is generally a bad idea!) The line referenced, or every command represented by any gerund referenced, are reinstated or abstained as appropriate (effectively changing the DO to DON'T (or PLEASE to PLEASE DON'T, etc.), or vice versa).  Using these forms of ABSTAIN and/or REINSTATE won't abstain from a command that's already abstained, or reinstate a command that's already reinstated.

There is a strange set of restrictions on ABSTAIN and REINSTATE that has existed since INTERCAL-72; historically such restrictions have not always been implemented, or have not been implemented properly.  They together define an unusual interaction of ABSTAIN and GIVE UP (note, for instance, that there isn't a gerund for GIVE UP).  The wording used in the INTERCAL-72 manual is:

     [...] the statement DO ABSTAIN FROM GIVING UP is not accepted, even though DON'T GIVE UP is. [...] DO REINSTATE GIVING UP is invalid, and attempting to REINSTATE a GIVE UP statement by line label will have no effect.  Note that this insures that DON'T GIVE UP will always be a "do-nothing" statement.

This restriction was not implemented at all in the only CLC-INTERCAL version before 0.02 (i.e. version 0.01), or in C-INTERCAL versions before 1.26.  The restriction was implemented in C-INTERCAL version 1.26 and CLC-INTERCAL versions 0.02 and later as ``GIVE UP cannot be REINSTATED or ABSTAINED FROM''; however, this is not strictly the same as the definition used by INTERCAL-72 (C-INTERCAL still uses this definition in CLC-INTERCAL compatibility mode).  The J-INTERCAL implementation of this restriction is to make REINSTATING or ABSTAINING from a line label that refers to a GIVE UP statement a compile-time error, but this does not fit the INTERCAL-72 definition either.  The definition adopted with version 0.27 and later of C-INTERCAL, which is hopefully correct, is to allow abstaining from a GIVE UP statement by line number but to rule out the other three cases (reinstating by line number silently fails, reinstating or abstaining by gerund is impossible because there is no gerund).

As well as CLC-INTERCAL's extension to abstain/reinstate by computed line number, there is also (since version 0.25) a C-INTERCAL-specific extension to ABSTAIN, also known as `computed abstain', but with a different syntax and different semantics.  It's written like an ordinary ABSTAIN, but with an expression between the words ABSTAIN and FROM, for instance:

     DO ABSTAIN #1 FROM (1000)
     DO ABSTAIN .2 FROM WRITING IN

Unlike non-computed ABSTAIN, this form allows a command to be abstained from even if it's already been abstained from; so if the first example command is run and line (1000) is already abstained, it becomes `double-abstained'.  The number of times the statement is abstained from is equal to the number of times it was already abstained from, plus the expression (whereas with non-computed abstain, it ends up abstained once if it wasn't abstained at all, and otherwise stays at the same abstention status).  Reinstating a statement always de-abstains it exactly once; so double-abstaining from a statement, for instance, means it needs to be reinstated twice before it will actually execute.

There are many uses for ABSTAIN (both the computed and non-computed versions) and REINSTATE, especially when interacting with ONCE and AGAIN (see ONCE and AGAIN); the computed version, in particular, is a major part of a particular concise way to write conditionals and certain kinds of loops.  They also play an important role in multithreaded programs.

READ OUT and WRITE IN
=====================

The READ OUT and WRITE IN commands are the output and input commands in INTERCAL; they allow communication between the program and its user.  There was a numeric I/O mechanism implemented in INTERCAL-72, and it (or trivial variants) have been likewise implemented in all more modern variants.  However, it had some obvious deficiences (such as not being able to read its own output) which meant that other methods of I/O were implemented in C-INTERCAL and CLC-INTERCAL.

The syntax of READ OUT and WRITE IN is the same in all cases: the name of the command followed by an intersection-separated list of items; the form of each item, the compiler you are using, and its command line arguments together determine what sort of I/O is used, which can be different for different elements in the list.

INTERCAL-72 I/O
---------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   see text   all versions

INTERCAL-72 had its own versions of I/O commands; these commands are available in all modern INTERCAL compilers as well (but CLC-INTERCAL implements output slightly differently).  To distinguish INTERCAL-72 input and output from the other more modern types of I/O, the READ OUT and WRITE IN commands must take one of the following values: a onespot or twospot variable, a single element of a tail or hybrid array, or (in the case of READ OUT) a constant, meaning that these are some examples of the possible forms:

     READ OUT .1
     READ OUT ;2 SUB .3:4
     READ OUT #3
     WRITE IN :4
     WRITE IN ,5 SUB #6

The statements do what you would expect; READ OUT outputs its argument to the user, and WRITE IN inputs a number from the user and assigns it to the variable or array element referenced.  (If the variable, or the array that contains the array element, happens to be read-only, the input or output still happens but in the case of WRITE IN silently skips the assignment, instead throwing away the input.)  The formats used for input and output are, however, different from each other and from the formats used by most mainstream languages.

Input is achieved by writing a number in decimal, one digit at a time, with each digit written out as a word; so to input the number 12345, a user would have to type ONE TWO THREE FOUR FIVE as input (if they were using English, the most portable choice of language).  In INTERCAL-72 only English is accepted as a language, but other compilers accept other languages in addition.  C-INTERCAL from version 0.10 onwards accepts English, Sanskrit, Basque, Tagalog, Classical Nahuatl, Georgian, and Kwakiutl; also Volapük from version 0.11 onwards, and Latin from version 0.20 onwards.  J-INTERCAL accepts the same languages, except with Esperanto instead of Latin; from version 0.05 of CLC-INTERCAL onwards, the same list of languages as C-INTERCAL is supported (apart from Latin, which was added in version 1.-94.-8), plus Scottish Gaelic.

The format that output can be read in is a modified form of Roman numerals, known as `butchered' Roman numerals.  INTERCAL-72, C-INTERCAL and J-INTERCAL do this the same way; CLC-INTERCAL is somewhat different. The characters I, V, X, L, C, D, and M mean 1, 5, 10, 50, 100, 500 and 1000 respectively, placing a lower-valued letter after a higher-valued letter adds them, and placing a lower-valued letter before a higher-valued letter subtracts it from the value; so XI is 11 and IX is 9, for instance.  In INTERCAL-72, C-INTERCAL, and J-INTERCAL, a bar over a numeral multiplies its value by 1000, and writing a letter in lowercase multiplies its value by 1000000; however, CLC-INTERCAL uses lowercase to represent multiplication by 1000 and for multiplication by 1000000 writes a backslash before the relevant numeral.

C-INTERCAL I/O
--------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.07+   version 0.05+   no

C-INTERCAL's method of character-based (rather than numeric) input and output is known as the Turing Tape method; it is a binary (character-set-agnostic) input/output mechanism.  To specify that C-INTERCAL-style I/O is being used, an array must be used as the argument to READ OUT or WRITE IN; as the syntax is the same as for CLC-INTERCAL's I/O, command-line arguments and the capabilities of the version of the compiler being used serve to distinguish the two mechanisms.

The character-based input writes as many characters into a tail or hybrid array as will fit, one character in each element.  The number that's written into the array is not the character code, though, but the difference between the character code and the previous character code, modulo 256.  (To be precise, the code is the new character minus the previous character, or 256 minus (the previous character minus the new character) if the previous character had a higher character code; the 'previous character' is the previous character from the input, not the previous character written into the array.)  End-of-file causes 256 to be written into the array.  The concept is that of a circular tape containing all the characters, where the program measures how many spaces it needs to move along the tape to reach the next character.  The 'previous character' starts at 0, but is preserved throughout the entire program, even from one WRITE IN to the next.

Character-based output uses a similar model, but conceptually the output device moves on the inside of the tape, rather than on the outside.  Therefore, the character is that is actually output is the bit-reversal of the difference between the last character output before it was bit-reversed and the number found in the array (subtracting in that order, and adding 256 if the result is negative).  (Rather than trying to parse the previous sentence, you may find it easier to look either at the source code to the compiler if you have it (the relevant part is binout in src/cesspool.c) or at some example C-INTERCAL programs that do text-based I/O.)

CLC-INTERCAL I/O
----------------

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   see text   all versions   no

There are also two CLC-INTERCAL-specific I/O mechanisms.  These are Baudot-based text I/O (which is also implemented from C-INTERCAL version 0.27 onwards), and CLC-INTERCAL generalised binary I/O (not implemented in C-INTERCAL).

Baudot text-based I/O is specified by using a tail array as an argument to WRITE IN or READ OUT.  (A tail array can also be used to specify C-INTERCAL-style Turing Tape I/O.  In order to determine which is used: both C-INTERCAL and CLC-INTERCAL use their own sort of I/O unless a command-line argument instructs them to use the other.)  In the case of WRITE IN, one line of input is requested from the user (C-INTERCAL requires this to be input in Latin-1, and will then automatically convert it; CLC-INTERCAL gives the option of various character sets for this input as command-line options); the final newline is removed from this line, then it is converted to extended Baudot and stored in the tail array specified (causing an error if the array is too small).  Because Baudot is only a 5-bit character set, each element is padded to 16 bits; CLC-INTERCAL pads with zeros, C-INTERCAL pads with random bits.  Trying to input at end-of-file will act as if the input were a blank line.  READ OUT is the reverse; it interprets the array as extended Baudot and converts it to an appropriate character set (Latin-1 for C-INTERCAL, or whatever was specified on the command line for CLC-INTERCAL), which is output to the user, followed by a newline.  Note that the Baudot is often longer than the corresponding character in other character sets due to the need to insert shift codes; for information on the extended Baudot character set, Character Sets.

Generalised binary I/O is specified using a hybrid array as an argument to WRITE IN or READ OUT.  Input works by reading in a number of bytes equal to the length of the array (without trying to interpret them or translating them to a different character set), prepending a byte with 172 to the start, padding each byte to 16 bits with random data, then replacing each pair of consecutive bytes (that is, the first and second, the second and third, the third and fourth, and so on) with (the first element selected from the second element) mingled with (the complement of the first element selected from the complement of the second element).  Output is the exact opposite of this process.  End-of-file reads a 0, which is padded with 0s rather than random data; if a non-end-of-file 0 comes in from the data, its padding will contain at least one 1.  Any all-bits-0-even-the-padding being read out will be skipped.

GIVE UP
=======

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   all versions   all versions

The GIVE UP command causes the program to end (or, in a multithreaded program, causes the current thread to end).  It is written simply as GIVE UP.  There is not much else to say about it, except to mention that it is the only way to end the program without an error unless the last line of the program is TRY AGAIN, and that it has an unusual interaction with ABSTAIN; for details of this, see ABSTAIN.  (Going past the last command in the program is an error.)

There is no gerund for GIVE UP; in particular, GIVING UP is a syntax error.

TRY AGAIN
=========

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.25+   no   no

The TRY AGAIN command is a very simple command with many limitations; its effect is to place the entire program in a loop.  If it exists, it must be the very last command in the program (it cannot even be followed by syntax errors), and it causes execution of the program to go back to the first command.  If the TRY AGAIN command is abstained or for some other reason doesn't execute when reached, it exits the program without the error that would usually be caused by going past the last line of code.

The gerund for TRY AGAIN is TRYING AGAIN.

COME FROM and NEXT FROM
=======================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   see text   see text   see text

The COME FROM statement (incidentally also invented in 1972, but not in connection with INTERCAL) is the main control-flow command in CLC-INTERCAL (which deprecates NEXT), and one of two main control flow structures in other modern INTERCAL compilers.  It takes either a label or an expression as its argument; these forms are noncomputed COME FROM and computed COME FROM.

Noncomputed COME FROM was implemented in version 0.5 of C-INTERCAL, but did not conform to modern-day semantics until version 0.7; it is available in every version of CLC-INTERCAL and J-INTERCAL.  Computed COME FROM support is available in every version of CLC-INTERCAL and in C-INTERCAL from version 0.25 onwards, but not in J-INTERCAL; the variant NEXT FROM of COME FROM is available from CLC-INTERCAL version 1.-94.-8 and C-INTERCAL version 0.26 (both computed and noncomputed).  C-INTERCAL and CLC-INTERCAL also have a from-gerund form of COME FROM and NEXT FROM, which was also implemented from CLC-INTERCAL version 1.-94.-8 and C-INTERCAL version 0.26.

The basic rule of COME FROM is that if a COME FROM statement references another statement, whenever that statement is reached, control flow will be transferred to the COME FROM after that statement finishes executing.  (NEXT FROM is identical except that in addition to the COME FROM behaviour, the location immediately after the statement that was nexted from is saved on the NEXT stack, in much the same way as if the statement being nexted from was itself a NEXT.)

Here are examples of noncomputed, computed, and from-gerund COME FROM:

     DO COME FROM (10)
     DO COME FROM #2$'.1~#1'
     DO COME FROM COMING FROM

(The last example is an infinite loop.  If it said DO NEXT FROM NEXTING FROM, it would not be an infinite loop because the NEXT stack would overflow and cause an error.  This also establishes the gerunds used for COME FROM and NEXT FROM.)

There are some things to be careful with involving COME FROM and NEXT FROM.  First, if the statement come from or nexted from happens to be a NEXT, the NEXT doesn't count as 'finishing executing' until the NEXT stack entry created by the NEXT is RESUMEd to.  In particular, this means that if FORGET is used to remove the entry, or a RESUME with a large argument resumes a lower entry, the COME FROM doesn't steal execution at all.

Second, you may be wondering what happens if two COME FROMs or NEXT FROMs aim at the same line.  In a non-multithreaded program (whether a program is multithreaded or not is determined by a compiler option for those compilers that support it), this is an error; but it is only an error if the statement that they both point to finishes running, and both COME FROMs or NEXT FROMs try to execute as a result (they might not if, for instance, one is abstained or has a double-oh-seven causing it not to run some of the time).  If both COME FROMs or NEXT FROMs are noncomputed, however, a compiler can (but does not have to) give a compile time error if two COME FROMs or NEXT FROMs share a label, and so that situation should be avoided in portable code. (If it is wanted, one solution that works for C-INTERCAL and CLC-INTERCAL is to use computed COME FROMs or NEXT FROMs with a constant expression.)

System Library
**************

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
yes   all versions   no   all versions

INTERCAL has a system library, called syslib.  It is included automatically at the end of your program by the compiler whenever your program refers to a line from (1000) to (1999) without defining any line in that range in the program.  (Although it is not added automatically by the compiler in CLC-INTERCAL, it is trivial to concatenate a copy onto the end of the program; copies of the system library are available from many sources on the Internet, including a version in the example code that comes with C-INTERCAL.)

The intention of the system library is to provide a range of useful capabilities, like multiplication, that can otherwise be hard to write in INTERCAL.  System library routines are used by NEXTING to their line number (see NEXT), where they will make changes to certain variables depending on certain other variables (depending on which routine is called), and RESUME back to the original program.  As the system library is itself written in INTERCAL, there are some restrictions that need to be obeyed for calls to it to be guaranteed to work; none of the variables it uses (.1 to .6 and :1 to :5) should be read-only or overloaded (although the value of any variables that aren't mentioned in the routine's description will be preserved by the routine), and none of the lines in it should have their abstention status changed by lines outside it (this can happen with blatant infractions like DO ABSTAIN FROM (1500) or more subtle problems like gerund-abstention) or have COME FROMs or NEXT FROMs aiming at them.

The system library is currently available in all bases from 2 to 7 (see TriINTERCAL), but not every command is available in every base, and C-INTERCAL is the only one of the three compilers listed above that use the system library to ship with a version in bases other than 2.  (This table was originally based on the INTERCAL-72 manual, but has had extra information added for bases other than 2.) Here, ``overflow checked'' means that #1 is assigned to .4 if there is not an overflow, and #2 is assigned to .4 if there is; ``overflow captured'' means that if there is overflow, the digit that overflowed is stored in the variable referenced.  In all cases, division by 0 returns 0.

Line   Description   Bases
(1000)
.3 <- .1 plus .2, error exit on overflow
2, 3, 4, 5, 6, 7
(1009)
.3 <- .1 plus .2, overflow checked
2, 3, 4, 5, 6, 7
(1010)
.3 <- .1 minus .2, no action on overflow
2, 3, 4, 5, 6, 7
(1020)
.1 <- .1 plus #1, no action on overflow
2, 3, 4, 5, 6, 7
(1030)
.3 <- .1 times .2, error exit on overflow
2, 3, 4, 5, 6, 7
(1039)
.3 <- .1 times .2, overflow checked
2, 3, 4, 5, 6, 7
(1040)
.3 <- .1 divided by .2
2, 3, 4, 5, 6, 7
(1050)
.2 <- :1 divided by .1, error exit on overflow
2, 3, 4, 5, 6, 7
(1200)
.2 <- .1 times #2, overflow captured in .3
4, 6
(1210)
.2 <- .1 divided by #2, one digit after the quartic or sextic point
stored in .3
4, 6
(1500)
:3 <- :1 plus :2, error exit on overflow
2, 3, 4, 5, 6, 7
(1509)
:3 <- :1 plus :2, overflow checked
2, 3, 4, 5, 6, 7
(1510)
:3 <- :1 minus :2, no action on overflow
2, 3, 4, 5, 6, 7
(1520)
:1 <- .1 concatenated with .2
2, 3, 4, 5, 6, 7
(1530)
:1 <- .1 times .2
2, 3, 4, 5, 6, 7
(1540)
:3 <- :1 times :2, error exit on overflow
2, 3, 4, 5, 6, 7
(1549)
:3 <- :1 times :2, overflow checked
2, 3, 4, 5, 6, 7
(1550)
:3 <- :1 divided by :2
2, 3, 4, 5, 6, 7
(1700)
:2 <- :1 times #2, overflow captured in .1
4, 6
(1710)
:2 <- :1 divided by #2, one digit after the quartic or sextic point
stored in .1
4, 6
(1720)
:2 <- :1 times the least significant digit of .1, overflow captured in
.2
5, 7
(1900)
.1 <- uniform random number from #0 to #65535
2, 3, 4, 5, 6, 7
(1910)
.2 <- normal random number from #0 to .1, with standard deviation .1
divided by #12
2, 3, 4, 5, 6, 7

If you happen to be using base 2, and are either using the external call system (see External Calls) or are willing to use it, it is possible to use a version of the system library written in C for speed, rather than the default version (which is written in INTERCAL). To do this, use the command line options -eE (before the INTERCAL file), and syslibc (at the end of the command line).

TriINTERCAL
***********

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.7+   version 1.-94.-8+   no

One extension to INTERCAL that is implemented by both C-INTERCAL and CLC-INTERCAL is known as TriINTERCAL, and extends INTERCAL to bases other than binary.  Unlike ordinary INTERCAL programs, which have the extension .i, TriINTERCAL programs in bases from 3 to 7 (the only allowed bases) have extensions from .3i to .7i respectively.

The change of numeric base only affects expressions, and in particular the behaviour of operators, and the range of variables.  (The onespot and twospot ranges become the highest number of trits or other digits in the base required that fit inside the binary ranges, so for instance, the maximum value of a onespot variable in ternary is 59048, or 3 to the power 10 minus 1.) Interleave/mingle is the simplest to explain; it alternates digits just as it alternated bits in binary. The other operators all change, as follows:

  * Exclusive-or becomes two operators, known either as subtract-without-borrow and add-without-carry due to their mathematical interpretations, or what and sharkfin after their Atari syntax representations (? and ^).  (In Princeton syntax, these are the bookworm or yen sign, and a spike (|).)  The two operators do the same thing in binary, but differ in higher bases. (Nevertheless, it is an error to use a sharkfin in binary, because it is a so-called `controlled unary operator', as are the rest of the new operators defined in this section, which has a lower limit on which base it is allowed in.)  Instead of doing the exclusive-or operation, the bits being combined are either subtracted or added; if the result is out of range for the base being used, the base is added or subtracted from the result until it is in range.  (For the subtraction, the bit that was less significant is subtracted from the bit that was more significant in any given pair of bits, except for the subtraction between the most and least significant bits, where the most significant bit is subtracted from the least.)

  * The AND and OR operators are generalised into an entire sequence of operators; the number of operators available is the same as the base that is being used.  In base 3, the third operator is known as BUT; in other bases, there are no commonly accepted names for the extra operators, so names that reflect the notation are used here.

     The way to think of it is this: in base 2, an AND gives the result 0 if either argument is a 0, and otherwise a 1, and likewise, an OR gives the result 1 if either argument is a 1, and otherwise a 0; they could be said to favour 0 over 1 and 1 over 0 respectively.  In base 3, AND favours 0 over 2 over 1, OR favours 2 over 1 over 0, and BUT favours 1 over 0 over 2.  (The symbol for BUT is @ (a `whirlpool', which is another name for the BUT operation) in Atari syntax, and ? in Princeton syntax.)  The pattern continues: in base 4, AND favours 0 over 3 over 2 over 1, BUT favours 1 over 0 over 3 over 2, 2BUT (written 2@ or 2?) favours 2 over 1 over 0 over 3, and OR favours 3 over 2 over 1 over 0.  (This can be extended to higher bases following the same pattern, introducing operators 3@ or 3?, etc., to favour 3, etc., when neither AND (which always favours 0) nor OR (which favours the highest digit in the base) are available.)  All the whirlpool operators are controlled unary operators, which are only legal when both the base contains the favoured digit, and they aren't redundant to AND nor OR.

  * Select has its semantics modified to deal with more than one nonzero digit in the base.  It starts off by doing an AND (in whatever base is being used) between the two numbers being selected; then the bits of the result are sorted by the bits of the right-hand argument to the select, with bits corresponding to 0s in the right-hand argument ending up more significant than bits corresponding to high digits in the right-hand argument.  In base 2, this has the same effect as a traditional-style select.

Note that the base doesn't affect anything other than variable ranges and expressions; in particular, it doesn't affect the bit-reversal used by Turing Tape I/O.  (The tape still has characters written on it in binary, even though the program uses a different base.)

Multithreading and Backtracking
*******************************

The multithreading and backtracking extensions to INTERCAL were originally invented by Malcom Ryan, who implemented COME
FROM-based multithreading as a modified version of C-INTERCAL, known as Threaded INTERCAL, but did not implement backtracking.  (The same functionality is implemented in C-INTERCAL today, but with different code. Most likely, this means that the original code was better.)  He also invented the original version of Backtracking INTERCAL, but did not implement it; the only known implementation is the C-INTERCAL one.  A different version of multithreading, using WHILE, was implemented as part of CLC-INTERCAL (like all extensions first available in CLC-INTERCAL, it is most likely due to Claudio Calvelli) and then added to C-INTERCAL, although its implications were not noticed for some time afterwards.

So nowadays, three freely-mixable threading-like extensions to INTERCAL exist, all of which are implemented in C-INTERCAL.  (A fourth, Quantum INTERCAL, is implemented in CLC-INTERCAL but not C-INTERCAL, and so will not be discussed further here.)  If you're wondering about the description of backtracking as a threading-like extension, it's implemented with much of the same code as multithreading in C-INTERCAL, because the INTERCAL version can be seen as roughly equivalent to multithreading where the threads run one after another rather than simultaneously.  (This conceptualisation is probably more confusing than useful, though, and is also not strictly correct.  The same could probably be said about INTERCAL as a whole, for that matter.)

Multithreading using COME FROM
==============================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.25+   version 0.05+   no

The original multithreading implementation worked by giving a new meaning to what was previously an error condition.  If in a multithreaded program (a program is marked as multithreaded using options to a compiler) two or more COME FROMs or NEXT FROMs (or a mixture of the these) attempt to steal control simultaneously, the original thread splits into multiple threads, one for each of the commands trying to take control, and a different command gains control of the program in each case.

From then on, all the threads run simultaneously.  The only thing shared between threads (apart from the environment in which they run) is the abstained/reinstated status of each command; everything else is separate.  This means, for instance, that it's possible to change the value of a variable in one thread, and it will not affect the corresponding variable in other threads created this way.  Likewise, there is a separate NEXT stack in each thread; if both a COME FROM and a NEXT FROM aim at the same line, for instance, the NEXT FROM thread will end up with a NEXT stack entry that isn't in the COME FROM thread, created by the NEXT FROM itself.  This is known as unwoven thread creation; none of the threads created this way are `woven' with any of the other threads created this way.  (Whether threads are woven depends on how they were created.)  If the thread being split was itself woven with other threads, exactly one of the resulting threads after the split is woven with the threads that the original thread was woven to, but the rest will not be woven to anything.  (If that seems a somewhat unusual rule: well, this is INTERCAL.)

In C-INTERCAL, there are other guarantees that can be made about unwoven threads (that is, threads not woven to any other thread).  In particular, they can all be guaranteed to run at approximately the same speed; to be more precise, the number of commands that have been given the chance to execute in any given thread will not differ by more than 2 from the number of commands that have been given the chance to execute in any other thread that was created at the same time.  (However, COME FROMs and NEXT FROMs can make this relationship less precise; it is unspecified (in the technical sense that means the compiler can choose any option it likes and change its mind on a whim without telling anyone) whether a COME FROM or NEXT FROM aiming at the current command counts towards the command total or not, thus causing the relationship to become weaker the more of them have the chance to execute.  In versions of C-INTERCAL from 0.27 onwards, there is a third guarantee; that if a COME FROM comes from itself, it will actually give other threads at least some chance to run, at some speed, by counting itself as a command every now and then; previously this requirement didn't exist, meaning that a COME FROM could block all threads if it aimed for itself due to the speed restrictions and the fact that COME FROMs need not count towards the total command count.)  Also, all commands, including any ONCE or AGAIN attached to the command, are atomic; this means that it's impossible for another thread to conflict with what the command is doing.  (In a departure from the usual INTERCAL status quo, these guarantees are somewhat better than in most other languages that implement threading, amusingly continuing to leave INTERCAL with the status of being unlike any other mainstream language.)

The only way to communicate between unwoven threads is by changing the abstention status of commands; this always affects all threads in the program, whether woven or not.  (The combination of ABSTAIN and ONCE is one way to communicate atomically, due to the atomic nature of ONCE.)

If there are at least two threads, the GIVE UP command ends the current thread, rather than the current program.

Multithreading using WHILE
==========================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.27+   version 0.05+   no

The WHILE command (which is not strictly speaking a command, but more a sort of metacommand that joins commands) is a second method of achieving multithreading.  (In CLC-INTERCAL, there are at least two other meanings for WHILE, but only the one implemented in C-INTERCAL is discussed here.)  The syntax is somewhat unusual, and consists of two commands separated by the WHILE keyword, but sharing the statement identifier, execution chance, and any ONCE/AGAIN keyword that may be present.  For instance:

     (1) DO COME FROM ".2~.2"~#1 WHILE :1 <-
           "'?.1$.2'~'"':1/.1$.2'~#0"$#65535'"$
           "'"'&.1$.2'~'#0$#65535'"$#0'~#32767$#1"

(OK, maybe that's an unnecessarily complicated example, and maybe it shouldn't have included the / operator which is part of another INTERCAL extension (see Operand Overloading).  Still, I thought that maybe you'd want to see how addition can be implemented in INTERCAL.)

A WHILE command starts two threads (the original thread that ran that command and a new one), one of which runs the command to the left of the WHILE and one of which runs the command to the right.  Any line number applies to the left-hand command, not the WHILE as a whole, which is a metalanguage construct.  NEXTING FROM, ABSTAINING FROM or similar behaviour with respect to the WHILE itself is impossible, although it's certainly possible to abstain from either of its operands (and abstaining from the left operand has much the same effect as abstaining from the WHILE itself; the right-hand thread deliberately takes a bit of time to get started just so that this behaviour happens).  The right-command thread starts just before the left command is run (so NEXTING, etc., directly to the left command will not start that loop in the first place); if that command finishes (which may be almost immediately for something like a calculate command, or take a long time for something like NEXT), that thread loops and reruns that command as long as the left command has not finished; COMING FROM that command, or a NEXT/NEXT FROM from/aiming at that command, doesn't count as finishing that command until it is RESUMEd back to (if possible; if it's come from, that command can never end and the right-hand loop will continue forever, or until it GIVEs UP or the loop ends due to the command ending later in another thread).  A WHILE command itself exists across all threads of a multithreaded program in a way; for each left-hand command that ends (in any thread), the next time a right-hand command of the same WHILE ends it will cause the thread it's looping in to end, regardless of whether that thread corresponds to the thread in which the left-hand command ended.  (As well as a right-hand command ending, there's also the possibility that it never got started; there is a delay before the right-hand command runs during which a left-hand command ending can prevent the right-hand thread starting in the first place; this counts as the same sort of event as terminating a right-hand loop, and can substitute for it anywhere a right-hand command ending is mentioned.)  There is one exception, in that if two or more left-hand commands end in a space of time in which no right-hand commands for that WHILE ends, they together only cause one right-hand command to end.  (What, did you expect the logical and orthogonal behaviour?)

The two threads produced by a WHILE (the original thread and a new copy of it) have more in common than ordinary INTERCAL threads created by COME FROM; ordinary threads share only ABSTAIN/REINSTATE information, whereas the WHILE-produced threads count as `woven' threads which also share variables and stashes.  (They still have separate instruction pointers, separate instruction pointer stacks, such as the NEXT stack, and separate choicepoint lists.  Overloading information is shared, though.)  Being woven is a relationship between two or more threads, rather than an attribute of a thread, although a thread can be referred to as being unwoven if it is not woven to any other thread.

Ordinary multithreading cannot create woven threads. When threads are created by multiple COME FROMs from an original thread, which was woven with at least one other thread, one of the resulting threads counts as the `original' thread and remains woven; the rest are `new' threads which initially start out with the same data as the original, but are not woven with anything.  Backtracking in a thread (see Backtracking) causes it to unweave with any threads it may be woven with at the time (so the data in the thread that backtracks is set back to the data it, and the threads it was woven with at the time, had at the time of the MAYBE, but the other threads continue with the same data as before).  The only way to cause three or more threads to become woven is with a new WHILE inside one of the threads that is already woven, which causes all the new threads to be woven together (the weaving relationship is transitive).

Backtracking
============

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.25+   no   no

A somewhat unusual threading construct that's available is backtracking.  In case you haven't come across it before (the concept exists in other languages but is implemented differently and usually in a less general way), the basic idea is that instead of executing or not executing a command, you can MAYBE execute a command.  This causes the command to be executed, but also creates a dormant thread in which the command wasn't executed; at any time later, the program can either decide that it liked the consequences of the command and GO AHEAD and get rid of the dormant thread, or decide that it didn't like the consquences of the command and GO BACK to the dormant thread, discarding the current one.  The dormant thread is more commonly called a `choicepoint', that is, a point at which a choice was made but a different choice can still be made, and is generally not thought of as a thread at all by most programmers.  (In case you're wondering: dormant threads are always unwoven.)

To create a choicepoint, the statement identifier MAYBE is used, rather than the more usual DO or PLEASE. (Combination statement identifiers are still allowed, but must be in the order MAYBE PLEASE DO NOT with optionally some parts omitted, or different versions of NOT used, or both.) Here's an example:

     MAYBE DON'T GIVE UP

When a command whose statement identifer contains MAYBE is reached, it is executed or not executed as normal, but in addition a choicepoint is created containing the program as it is at that time. Only ABSTAIN and REINSTATE, which always affect all threads in a program (even choicepoints), can alter the values stored in the choicepoint; so in this way, a choicepoint is also somewhat similar to the concept of a continuation in other languages.  The choicepoint is placed on a choicepoint stack, which is maintained separately for each thread in much the same way that stashes and the NEXT stack are.

The choicepoint does not actually do anything immediately, but if the program doesn't like the look of where it's ended up, or it decides to change its mind, or just wants to try all the possibilities, it can call the GO BACK command (which has no arguments, and is just the statement identifier, optional execution chance, GO BACK, and optional ONCE or AGAIN).  This causes the current thread to unweave from all other threads and then replace itself with the thread created by the choicepoint on top of the choicepoint stack. The difference is that this time, the abstention or reinstatement status of the command that was modified with MAYBE is temporarily reversed for determining whether it runs or not (this reversal only lasts immediately after the GO BACK, and does not affect uses of the command in other threads or later in the same thread), so unless it has been ABSTAINed or REINSTATEd in the meantime it will run if and only if it wasn't run the first time.  The choicepoint stack's top entry is replaced by a `stale' choicepoint that definitely isn't a thread; attempting to GO BACK to a stale choicepoint instead causes the stale choicepoint to be deleted and the program to continue executing.  (This is what gives INTERCAL's backtracking greater flexibility in some ways than some other languages; to get backtracking without the stale choicepoints having an effect, simply run COME FROM the GO BACK as the previous statement.)

Note that, though, when a thread splits into separate threads (whether woven or unwoven), the choicepoint stack doesn't split completely, but remains joined at the old top of stack.  The two choicepoint stacks can add and remove items independently, but an attempt to GO BACK to before the current thread split off from any other threads that are still running instead causes the current thread to end, although it will GO BACK as normal if all other threads that split off from it or that it split off from since the top choicepoint of the stack was created have ended since.  This means that it's possible to backtrack past a thread splitting and get the effect of the thread unsplitting, as long as both resulting threads backtrack; this is another way in which INTERCAL's backtracking is more flexible than that of some other languages.

If, on the other hand, a program decides that it likes where it is and doesn't need to GO BACK, or it wants to GO BACK to a choicepoint lower down the stack while skipping some of the ones nearer the top of the stack, it can run the GO AHEAD command, which removes the top choicepoint on the stack, whether it's a genuine choicepoint or just a stale one.

Both GO AHEAD and GO BACK cause errors if an attempt is made to use them when the choicepoint stack is empty.

Operand Overloading
*******************

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.26+   version 0.05+   no

(Operand overloading in C-INTERCAL is nowhere near as advanced as it is in CLC-INTERCAL.  This chapter only explains the partial implementation used by C-INTERCAL; for a full implementation, see CLC-INTERCAL and its documentation.)

Operand overloading is a method of using a onespot or twospot variable as a substitute for an expression.  When a variable is overloaded to an expression (which could be another variable, or something more complex), any uses of that variable cause the expression to be substituted instead.

At the beginning of the program, all variables stand for themselves; so .1 really does mean .1, for instance.  The meaning of a variable can be overloaded using the slat operator (/), which is the same in both Princeton and Atari syntax: it is a binary operator whose left argument must be a onespot or twospot variable and whose right argument can be any expression.  The slat operator returns the true value of its left argument, but as a side effect, changes the meaning of its left argument to be its right argument.  Here is an example:

     DO .1 <- .2/'.3~.4'

The example causes .2's true value to be assigned to .1 (unless of course .1 is read-only), but also causes .2 from then on to actually mean '.3~.4', except when it's the left operand of a slat operator.  So for instance, DO .1 <- .2 would actually assign '.3~.4' to .1.  Somewhat confusingly, this also works in the other direction; DO .2 <- .1 would assign .1 to '.3~.4', which would have the effect of changing the values of .3 and .4 so that '.3~.4' had the correct value, or throw an error if it couldn't manage this.  (The general rule in this case is that any variable or constant in the expression that overloads the variable is at risk of being changed; this is known as a `reverse assignment'.  Code like DO .1 <- .1/#1 is entirely capable of changing the value of #1, although to protect new INTERCAL users C-INTERCAL will refuse to carry out operations that change the value of constants unless a command-line switch (see -v) is used to give it permission.  In C-INTERCAL, changing the value of a constant only changes meshes with that value, but in CLC-INTERCAL it can also change non-mesh uses of that constant, so doing so is not portable anyway.)

When multiple overloading rules are in effect, they are all applied; overloading .1 to '.2~.3' and .2 to '.3$.4' will cause .1 to refer to ''.3$.4'~.3'. However, this expansion stops if this would cause a loop; to be precise, overloading is not expanded if the expansion is nested within the same expansion at a higher level (so .1/.2 and .2/.1 together cause .1 to expand to .2, which expands to .1, which cannot expand any further).  In C-INTERCAL, the expression on the right hand side of a slat is not evaluated and not expanded by operand overloading.

STASHING a variable causes its overloading information to be stashed too; RETRIEVING it causes its overload rule to also be retrieved from the stash (or any overload rule on the variable to be removed if there wasn't one when the variable was stashed).

Overloading a onespot variable to a twospot variable or vice versa is possible, but the results are unlikely to be predictable, especially if a onespot variable is used to handle a twospot value.  Possible outcomes include truncating the value down to the right bitwidth, throwing an error if a value outside the onespot range is used, and even temporarily handling the entire twospot value as long as it doesn't end up eventually being assigned a value greater than twospot.

Note that reverse assignments can cause unpredictable behaviour if an attempt is made to reverse-assign the same variable twice in the same expression.  In particular, sequences of commands like DO .1 <- .2/'.3$.3' DO .2 <- #6 are liable to succeed assigning garbage to .3 rather than failing as they ought to do, and likewise any situation where a variable is reverse-assigned twice in the same expression may assign garbage to it.  This behaviour is seen as unsatisfactory, though, and plans exist to improve it for future versions.

PIC-INTERCAL
************

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.26+   no   no

PIC-INTERCAL is a simplified version of INTERCAL designed especially for embedded systems, designed to minimise code and data usage by INTERCAL programs so that they can fit on devices whose memory is measured in bytes rather than megabytes.  (It is named after the first microcontroller for which code was successfully generated, and which influenced the choices of commands, the PIC16F628 manufactured by Microchip, and is most likely to be portable to other microcontrollers in the same range.)  C-INTERCAL only compilers as far as C code when producing PIC-INTERCAL; it is up to the user to find the appropriate cross-compiler to translate this C into the relevant dialect of machine code.  (Two header files in the distribution, src/pick1.h and src/pick2.h, don't have any affect on the compiler but are referenced by the generated code, and the intent is for the user to change them to suit the behaviour of the PIC compiler used, because these are not as standardised as C compilers for everyday systems.)

There are several restrictions on PIC-INTERCAL programs:

  * It's impossible to store more than one item in the STASH for any given variable.

  * Many errors are not checked at runtime; it is the programmer's job to ensure that runtime errors are avoided.  (The few errors that are detected will send the program into an infinite loop, as there is no sensible way to exit from a PIC program.)

  * Double-oh-sevens are not randomised, but instead depend merely on whether the probability given is greater than 50% or not.

  * The NEXT stack is limited to 16 entries rather than the usual 80.

  * ABSTAIN and REINSTATE still work, but cannot be computed ABSTAINs, and will not necessarily work when used to affect the system library or calls to it.

  * READ OUT and WRITE IN don't work.  (See below for a replacement.)

  * COME FROM and NEXT FROM must aim at a label, not an expression or gerund.

  * Multithreading and TriINTERCAL cannot be used on PICs.  (Operand overloading can be used, but only if your PIC cross-compiler supports function pointers; some don't.)

In order to provide I/O capabilities, a new command PIN is available. It controls up to 16 I/O pins on the PIC or other embedded system; an I/O pin is capable of receiving or sending voltages to an electrical or electronic circuit.  This explanation assumes that the device being controlled is a PIC16F628A, and therefore has its pins in two blocks of 8 named PORTA and PORTB; for other microcontrollers, adapting the code in src/pick1.h is likely to be necessary to tell the compiler how to control the I/O pins, and the way in which this done will affect which I/O pins it is that the program will end up being able to communicate with.

The PIN command takes one twospot variable as its argument, like this:

     DO PIN :2

The twospot variable is conceptually divided into 4 blocks of 8 bits. The highest two blocks control the directions of the pins in PORTB (most significant block) and PORTA (second most significant block); a 1 on any bit means that the corresponding I/O pin should be set to send data, and a 0 means that it should be set to receive data.  The lower two blocks control the values on the pins that are sending (and are ignored for receiving pins); the second least significant block controls PORTB and the least significant block controls PORTA, with a 1 causing the program to set the output voltage to that of the microcontroller's negative voltage supply rail, and a 0 causing the program to set the output voltage to that of the microcontroller's positive voltage supply rail.  (These voltages may vary on other systems; consult your system's datasheet and the changes you made to the header files.) After setting the pins, the PIN command then reads them as part of the same operation, this time setting the values of the lower blocks that are receiving, rather than setting the pins from the lower blocks that are sending.  However, 1 and 0 bits on all bits of the twospot variable have the opposite meaning when doing this, so that 1 means receiving/positive voltage rail and 0 means sending/negative voltage rail.  There is no way to input without output, or vice versa, but it's trivial to just send the same output again (which has no effect, because the voltage on sending pins is maintained at the same level until it is changed), or to ignore the input received.

CREATE
******

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.28+   see text   no

The CREATE command allows the creation of new syntax at runtime.  CLC-INTERCAL has had such a command since 1.-94.-8, but its syntax is completely different and incompatible with the C-INTERCAL version, and so is not documented here (see the CLC-INTERCAL documentation for more details).  The C-INTERCAL version is only defined if the -a option is used on the command line (and a runtime error E000 otherwise), because it forces the operand overloading code to be introduced and so slows down every variable access in the program.

The syntax of the CREATE command is to write CREATE, then a line label, then anything.  OK, well not quite anything; you're restricted to syntax that is supported by the `just-in-case' compiler that runs on comments at compile time just in case they gain a meaning later (see below).  The anything provides an example statement to CREATE; statements which look the same (but may differ in details) are created. Typical syntax for a CREATE statement would therefore look something like this:

     DO CREATE (5) SWITCH .1 WITH .2

There is also computed CREATE, working identically to ordinary CREATE except that the line number is taken from an expression and the created command must start with a letter (to avoid an ambiguity if the expression giving the line label happens to be an array reference), with a syntax like this:

     DO CREATE .5 SWITCH .1 WITH .2

Here, a new SWITCH WITH statement (there is no such statement in INTERCAL normally) is being created.  This command makes it possible to do this:

     DO SWITCH .3 WITH .4

Normally that line would be an error (E000) due to being unrecognised, but having been CREATEd, it's now a real statement.  (The gerund to affect created statements is COMMENT, just like before they were created; the gerund to affect CREATE itself is CREATION (CREATING is also allowed, but not as elegant).)  When the created statement is encountered, it NEXTs to line (5), the line number specified in the CREATE statement. In order for the code there to be able to affect the variables mentioned in the statement, the variables :1601 (for the first variable or expression mentioned), :1602 (for the second variable or expression mentioned), and so on, are STASHed and then overloaded to the respective expressions or variables mentioned in the created command; so :1601 has been overloaded to mean .3 and :1602 has been overloaded to mean .4 at this point.  Then, the code at (5) runs; if it returns via a RESUME #1, :1601 and :1602 will be RETRIEVEd automatically and the program will continue from after the created statement.  (If you do not resume to that point, say if you're creating a flow control statement, you'll have to deal with the stashes for the 1600-range variables yourself.)

So what syntax is available in created statements?  All the capital letters except V (which is an operator in INTERCAL) are available and can be used freely and as many times as desired; they match themselves literally.  However, they are not allowed to spell an INTERCAL keyword at any point (so watch out for DO and FROM, for instance).  Whitespace is allowed, but is ignored (both in the CREATE template statement, and in the code being created; so DO SW ITCH :8 WITH :50 will also have been created).  Then, there are three groups of matchable data: scalar variables (onespot or twospot variables, as used in the examples above) match other scalar variables, array elements (like ,4 SUB '.5~.6') match other array elements, and other expressions match other other expressions.  Two matchable data may not appear consecutively in a created command, but must be separated by at least one capital letter (to prevent array-subscript-related ambiguities; remember that the just-in-case compiler has to compile these statements at compile time without knowing what they are).  The actual expressions used in the CREATE statement don't matter; they're just examples for the runtime to match against.

It is also possible (from C-INTERCAL version 0.29 onwards) to create new operators.  Such operators are always binary operators (that is, they take two arguments and parse like mingle or select), and always return 32-bit results.  There are three types of legal names for such operators, all of which are treated equivalently: lowercase letters, punctuation marks otherwise unused in INTERCAL, and overstrikes consisting of a character, a backspace, and another character (apart from overstrikes already used for built-in INTERCAL operators).  The syntax for creating an operator looks like one of these:

     DO CREATE (5) x
     DO CREATE .5 =

The arguments to the operator will be overloaded onto :1601 and :1602 (which are, like with CREATEd statements, stashed before the overloading happens), and the return value is read from :1603 (which is stashed, then overloaded to itself).  All these three variables are retrieved again after the operator finishes evaluating.

Note that it is a very unwise idea to use a CREATEd operator in the expression for a computed COME FROM or NEXT FROM, because this always leads to an infinite regress; whenever any line label is reached (including the line label that the CREATE statement pointed at), the expression needs to be evaluated in order to determine whether to COME FROM that point, which in turn involves evaluating lines which have labels.

Some other points: a newer CREATE statement supercedes an older CREATE statement if they give equivalent templates, multiple CREATE statements may aim at the same line (this is the recommended technique for creating a statement that can handle expressions even if they're array elements or variables; you do this by specifying multiple templates in multiple CREATE statements), and strange things happen if a twospot variable in the 1600-range is used as an argument to a created statement itself (because of the stash/retrieve, such a variable can usually be read, but may not always be able to be written without the data being lost).

External Calls
**************

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.28+   no   no

C-INTERCAL has a feature allowing INTERCAL and non-INTERCAL code to be mixed.  This is achieved by causing the non-INTERCAL programs to participate in the INTERCAL line-numbering model.  The same feature allows expansion libraries to be linked into the code.

To create a combined program containing INTERCAL and non-INTERCAL code, use ick as the compiler as normal, but specify both the INTERCAL and non-INTERCAL source files on the command line, and use the -e command-line option.  ick will invoke other compilers as necessary, after modifying the source files accordingly. At present, external calls are only supported to and from C.

In each case, it will be the INTERCAL program that is invoked first. (This means that it is impossible to link together more than one INTERCAL program, but you probably don't want to, because concatenating the programs is likely to have a similar effect.)  You can get the INTERCAL program to NEXT to the non-INTERCAL program immediately, or the non-INTERCAL program to COME FROM or NEXT FROM the INTERCAL program immediately, to obtain the effect of running the non-INTERCAL program first.

Note that external calls are incompatible with PIC-INTERCAL and with multithreading; note also that you must use gcc as your compiler, and GNU cpp and ld, for them to work in the current version of C-INTERCAL.

External Calls to C
===================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.28+   no   no

Linking C and INTERCAL programs is achieved by placing various constructs into the C programs that are equivalent to various INTERCAL constructs.  It is possible to simulate a line with a label and a dummy command (which serves as a COME FROM suckpoint and NEXT target), a command with a line label, NEXT, RESUME, and FORGET, and COME FROM and NEXT FROM.  Onespot and twospot variables are accessible from inside the C program, where they can be read and written; however, the INTERCAL program cannot access any variables inside the C program that weren't part of the INTERCAL program originally.

To prevent various logical impossibilities, there are restrictions on where these can be used and what preparation is needed before they are used.  Also, the semantics are not always exactly what you might expect for technical reasons.

It should be observed that the INTERCAL link intrudes on the user namespace.  To prevent possible namespace clashes, no identifiers starting with ick_ or ICK_ should be used anywhere in the linked C program for any reason, except where specified in this manual.

External C Call Infrastructure
------------------------------

For a C program to be connected to an INTERCAL program, it needs to be marked with the correct header file, and needs to have functions marked for communication with the INTERCAL program.

#include <ick_ec.h>
     The header file ick_ec.h must be included using the preprocessor in any file which uses any of the INTERCAL external call functions, variables, or macros.  (Note that this file may not necessarily exist, or exist in the usual place; ick will deal with making sure the correct header file is included.)  This will include stdint.h (a standard C header file, which must exist on your system), so that you can access INTERCAL variables (the INTERCAL types onespot, twospot, tail, hybrid correspond to the C types uint16_t, uint32_t, uint16_t*, uint32_t* respectively); also, it will provide the prototypes for all the functions and definitions for all the macros needed to use the external calls system with C.

ICK_EC_FUNC_START
ICK_EC_FUNC_END
     Many of the INTERCAL interface macros (ick_linelabel, ick_comefrom, and ick_nextfrom) make it possible to jump from an INTERCAL program to a C program.  Because C doesn't allow jumping into the middle of a function, there has to be some way to create a block of code which can be jumped into. This is what these two macros achieve.

     This declaration and definition:

          ICK_EC_FUNC_START(identifier)
          {
            /* code goes here */
          }
          ICK_EC_FUNC_END

     is equivalent to this:

          void identifier(void)
          {
            /* code goes here */
          }

     except that it is possible to jump from an INTERCAL program into the declared and defined program.  (If you need to write a prototype for the function early, void identifier(void); is perfectly acceptable, but an early prototype is not required unless you call the function from earlier within the C code.)  Of course, you can substitute any identifier that's legal as a function name for identifier (as long as it doesn't start with ick_ or ICK_).  The resulting function is a function (for instance, you can take its address or call it in the usual ways); the only differences are that it can be jumped into from INTERCAL code and that it is constrained to take no arguments and return no data.  (It can still access global and INTERCAL variables.)  If the function is jumped into from INTERCAL code, but then control flow reaches the end of the function, or the function return;s but was not called from C, the resulting behaviour is undefined; C-INTERCAL will attempt to continue by some means at that point, but may fail.  If a function is unsure whether it gained control from C or from INTERCAL code, it may use ick_return_or_resume (described below).

     Because you are not allowed to declare two C functions with the same name (even in different modules), all functions declared with ICK_EC_FUNC_START must have unique names across the entire compilation.

ick_startup
-----------

It is sometimes necessary for a C program to do its own initialisation before the INTERCAL program starts running.  To do so, it can use the ick_startup macro inside a function declared with ICK_EC_FUNC_START; the syntax is ick_startup(block), where the argument is an expression, statement, or compound statement to run.  The argument itself must not contain any ick_-prefixed macros or functions except possibly ick_create, may have side effects, and must fit the C preprocessor's idea of what a macro argument should look like (it's more used to parsing expressions than blocks; the general rule is to avoid commas except when they're directly or indirectly inside parentheses or strings).

ick_linelabel
-------------

A line label is something that can be NEXTed to and COME
FROM.  Unlike an INTERCAL line label, it does not label a statement, and therefore attempts to ABSTAIN or REINSTATE it may be errors, or may be ignored (it's unspecified which, which means that either may happen for any or no reason, but exactly one will happen in any given case, although the choice might not be consistent).

The macro ick_linelabel(expression); may appear anywhere a compound statement would normally be able to appear.  (That is, it looks like a function call being used as a standalone expression, but in fact the places where it can appear are more limited.)  In contrast to ordinary line labels, an expression can be used rather than just a constant; however, the behaviour is undefined if the expression has side-effects.  Upon encountering the line label, any COME FROMs or NEXT FROMs aiming at the line label (including ick_comefroms and ick_nextfroms) will steal control from the program; RESUMING after a NEXT FROM will work, but suffers from the same caveats as setjmp/longjmp do (any auto variables that change their value between the NEXT FROM and RESUME will have their value clobbered (i.e. their value is no longer reliable and should not be accessed)).  Note that the INTERCAL variables are immune to this problem.  You can also avoid the problem by marking variables as volatile in the C program.

It is possible to NEXT or ick_next to a ick_linelabel, which has the same effect as saving the NEXT stack, calling the function containing the ick_linelabel and then immediately doing a C goto to an imaginary label preceding it.  Due to this possibility, an ick_linelabel is only allowed within a function defined with ICK_EC_FUNC_START.

ick_labeledblock
----------------

In INTERCAL programs, labels don't stand on their own, but instead label a statement.  The difference between a standalone line label and a line label that labels a statement is that COME FROMs will come from the label itself (which is before the next statement) when aiming at a standalone line label, but the end of the statement when aiming at a labeled statement.  To achieve the same effect in C, the macro ick_labeledblock is available; it can be used as ick_labeledblock(expression,expression) or ick_labeledblock(expression,statement); the first argument is the label, and the second argument is an expression or statement to label (if an expression is labeled, it will be converted to a statement that evaluates it for its side effects and discards the result).  It is even permitted to label a block statement in this way. Note, however, that you have to contend with the C preprocessor's ideas of where macro arguments begin and end when doing this.  Other than the position of the COME FROM target created by the label, this behaves the same way as ick_linelabel (so for instance, computed line labels are allowed, but the expression that computes them must not have side effects, and it is only allowed within a function defined with ICK_EC_FUNC_START).

ick_comefrom and ick_nextfrom
-----------------------------

The ick_comefrom and ick_nextfrom macros are, like the other INTERCAL flow control macros (as opposed to functions), only allowed within a function defined with ICK_EC_FUNC_START.  They act almost exactly like the INTERCAL statements of the same name (although note that C statements cannot be ABSTAINed FROM even if they act the same way as INTERCAL statements); they are written as ick_comefrom(expression); and ick_nextfrom(expression); respectively (note that they must be called as statements, and cannot be used as part of an expression). Whenever a standalone line label is encountered whose expression evaluates to the same number as the expression inside the ick_comefrom or ick_nextfrom, and that number is at most 65535, then control will be transferred to the ick_comefrom or ick_nextfrom, leaving a NEXT stack entry behind in the case of ick_nextfrom; likewise, if the end of a labeled statement, expression or block is reached and the label has the right number.  Some caveats: the expression need not be constant, but must not have side effects, must not be negative, and must fit into the range of an unsigned long in the C program (and the statement will do nothing if the expression evaluates to a value larger than 65535).  In keeping with the best C traditions, these caveats are not checked, but instead result in undefined behaviour if breached.

There are also versions ick_comefromif and ick_nextfromif, which take a second parameter, which is a condition that specifies whether control is actually stolen from the target.  The condition may have side effects, and is only run when the line numbers match; it should return 0 or NULL to leave control flow alone, or nonzero to steal control, and should be either an integral type or a pointer type. Although side effects are allowed, the condition must not look at or alter auto or register variables in the enclosing function, not even if they are also marked volatile.  (Global and static variables are fine, though.)

ick_next
--------

ick_next is a macro that acts like the INTERCAL statement NEXT.  Contrary to the other INTERCAL-like macros, it can be used in any function regardless of whether it was defined with ICK_EC_FUNC_START; however, it must still be used as a statement by itself, and a call to it looks like ick_next(expression);.  The expression is the label to NEXT to, and works under the same rules as the expressions for ick_comefrom; it need not be constant (unlike in C-INTERCAL!), but must not have side effects, must not be negative, must fit into the range of an unsigned long, and is ignored if it is over 65535.  If there happen to be multiple labels with the correct value at the time, the compiler will NEXT to one of them.  Bear in mind that there is a limit of 80 entries to the NEXT stack, and that this limit is enforced.

If the resulting NEXT stack entry is RESUMEd to, the program will continue after the ick_next as if via setjmp, with all the usual restrictions that that entails; if the resulting NEXT stack entry is forgotten, then the ick_next call will never return.  (Notice the 'as if via setjmp' condition allows you to preserve the vales of auto and alloca-allocated storage as long as its value has not changed since the ick_next was called, which is a significantly more lenient condition than that normally imposed on such variables (see External Calls and auto).

ick_resume
----------

ick_resume is a macro, but there are few restrictions on its use; it is permitted to use it inside an expression (but it returns void, making this not particularly useful), and acts like a function which takes an unsigned short argument, returns void, and has a prototype (but you cannot take its address; if you need to be able to do that, write a wrapper function for it).  It can be used within any function regardless of how it was declared, and never returns; instead, it pops the specified number of NEXT stack entries and resumes execution at the last one popped, just as the INTERCAL statement does.  This causes the same errors as the INTERCAL statement if the number of entries popped is zero or larger than the NEXT stack.

There is also a macro ick_return_or_resume();; it can only be used inside a function defined with ICK_EC_FUNC_START, and is equivalent to return; if the function was called from C, or ick_resume(1); if the function was called from INTERCAL.  It's therefore a safe way to return from such a C function if you don't know how control reached it in the first place.

ick_forget
----------

The ick_forget macro removes NEXT stack entries, and the corresponding C stack entries. It must be called as a statement by itself, and its invocation looks like this: ick_forget(expr);, where the expression is the number of NEXT stack entries to forget (all of them will be forgotten if the number is higher than the number of entries).  The expression will be casted to an unsigned short.

ick_forget can only be used inside a function declared with ICK_EC_FUNC_START.  As it is removing stack entries both in INTERCAL and in C, it will clobber the value of all auto variables created since the highest remaining NEXT stack entry came into being (or since the start of the program, if the NEXT stack is emptied by the command) and also deallocate all alloca storage allocated since then.  It also causes the return address of the current function to become undefined, so that function must not return; control may leave it via RESUME, or via COME FROM, or via NEXT or NEXT FROM followed by the relevant NEXT stack entry being forgotten (the function is still 'running' but suspended while the NEXT stack entry still exists).  (Note that these restrictions are stronger than those on RESUME; this is because RESUME preserves most of the stack, but FORGET destroys parts of the stack and therefore cannot avoid destroying the data stored there.  It could be much worse; a previous (never released) version of the code didn't remove those parts of the stack in many circumstances, leading to a stack leak that caused programs to segfault after a while.)

ick_get/setone/twospot
----------------------

This class of four functions make it possible to get and set INTERCAL scalar variables from C code.  Their prototypes are as follows:

     uint16_t ick_getonespot(unsigned short varnumber);
     void ick_setonespot(unsigned short varnumber, uint16_t newvalue);
     uint32_t ick_gettwospot(unsigned short varnumber);
     void ick_settwospot(unsigned short varnumber, uint32_t newvalue);

The program will error out with a fatal error (see E200) if the variable you request is mentioned nowhere in the INTERCAL program; if you attempt to set an IGNOREd variable, the attempt will silently fail (just as if you assigned to it in an INTERCAL program). The get functions are safe to use in a computed line label, so you can use them to produce computed line labels that depend on INTERCAL variables.  (uint16_t and uint32_t are standard C data types; if your system doesn't provide them, get better system header files.)

If you care about speed, note that .1 is the fastest variable of all to access, and otherwise variables first mentioned near the top of the INTERCAL program will be faster to access than variables mentioned lower down.

ick_create
----------

The ick_create function (prototype: void ick_create(char*, unsigned long)) allows the external calls system to be used to create new INTERCAL syntax; to do this, you give a `signature' representing the syntax you want to define and a line number to the function (which are its two arguments, respectively). The signature defines the syntax that you are defining; whenever that syntax is encountered within the INTERCAL program, it will NEXT to the line number you specify, which can do various clever things and then RESUME back to the INTERCAL program (or if you're defining a flow-control operation, you might want to leave the NEXT stack entry there and do other things).  However, note that the overloading of :1601, etc., will still take place as in the INTERCAL version of CREATE if the -a option is used (see -a), so care is needed when writing flow control statements that they work both with and without the option and don't cause STASH leaks (which means no FORGETting the relevant NEXT stack entry, and no looking at 1600-range variables). This allows the external calls system to define whole new INTERCAL commands, with the same power as any other programming language.

There are various restrictions on what syntax you can CREATE with this method, which are best explained by an explanation of the relevant C-INTERCAL compiler internals.  When an INTERCAL program is compiled by C-INTERCAL, any unrecognised statements it comes across are compiled by a `just-in-case' compiler that attempts to compile them anyway with no knowledge of their syntax, just in case the syntax becomes defined later.  (E000 (see E000) will be thrown when such statements are encountered at runtime, unless the syntax has been CREATEd since to give a statement a meaning.)  For the just-in-case compiler to run, the resulting statement must be completely unrecognised; this means that it may contain no keywords (not even a sequence of letters that forms a keyword, such as FROM or DO), it must consist only of variable names, expressions, and capital letters other than V (because V is a unary operator, so otherwise there would be ambiguity), and in which any two variable names or expressions are separated by at least one capital letter.  The compiler will produce a `signature' for the unknown command that can be defined.

A signature consists of a sequence of characters (and is represented as a null-terminated string; the runtime makes a shallow copy of the string and keeps it until the end of the program, so arrangements must be made to ensure that the storage in which the string is allocated stays around that long, but this opens up interesting possibilities in which the signature that was actually CREATEd can be modified retroactively); whitespace is not allowed in a signature.  Capital letters can be used (apart from V), and match the same capital letters literally in the INTERCAL syntax being created; also available are the special characters .,;~, which match respectively a scalar variable (a onespot or twospot variable such as :1), an array variable (such as ;2), an array element (such as ,3 SUB #4 #5), and an expression that isn't a variable name and isn't an array element (such as .4$.5).  If you want to be able to match other things (say, to be able to match all expressions), you will need to submit multiple signatures using multiple calls to ick_create; maybe you could write a library to do that automatically.

CREATEd operators also have signatures, but of quite a different form.  The signature for a single-character operator is a lowercase u, followed by its character code in hexadecimal (no leading zeros, and in lowercase); the signature for an overstrike is a lowercase o, followed by the lower relevant character code in hexadecimal, followed by a lowercase x, followed by the higher relevant character code in hexadecimal.

The routine that is NEXTed to will presumably want to be able to see what in the INTERCAL program was matched by the signature, so a range of function-like macros is provided to access that.  They must be run from within the invocation of the function which was NEXTed into by the created syntax (see External Calls and auto for when a function invocation ends, which could be sooner than you think when the C-INTERCAL external calls system is used), and are undefined behaviour when that invocation did not gain control from a CREATEd statement.  Here are their effective prototypes:

     int ick_c_width(int);
     int ick_c_isarray(int);
     unsigned short ick_c_varnumber(int);
     uint32_t ick_c_value(int);
     /* These require -a to work */
     uint32_t ick_c_getvalue(int);
     void ick_c_setvalue(int, uint32_t);

The first argument to all these macros is the position of the match in the signature (0 for the first non-capital-letter match in the signature, 1 for the second, and so on until no more items are left in the signature to match); specifying a position that isn't in the signature is undefined behaviour.

ick_c_width returns the data type, as a width in bits, of the expression (or the width in bits of an element of the passed in array), and ick_c_isarray returns 1 if the argument was an array variable or 0 if it was an expression (array elements and scalar variables are expressions).  ick_c_varnumber returns the variable's number (for instance 123 for .123), or 0 if the corresponding argument was not a variable; in the cases where the argument was a variable, these three functions together provide enough information to figure out which variable (which is useful if you're writing an extension which takes a variable name as an argument).

ick_c_value returns the value of the corresponding expression at the time the CREATEd command was called; ick_c_getvalue is almost equivalent, but only works if the -a option (see -a) was used during compilation, and returns the value of the corresponding expression now.  (The uint32_t return type is large enough to hold either a onespot or twospot value, and will be zero-extended if the corresponding expression had onespot type.)  ick_c_setvalue also depends on -a, and will assign to the corresponding expression (be careful not to provide a value that is too large for it!).  In the case that the corresponding expression is not a variable, this will attempt to perform a reverse assignment to the expression, and can produce ordinary INTERCAL errors if it fails.  It is not possible to redimension an array this way, as this is assignment, not a calculate operation.

External Calls and auto
-----------------------

Because the external calls system merges the INTERCAL NEXT stack with the C return value and data storage stack (note for pedants: the C standards nowhere mandate the existence of such a stack, or even mention one, but the restrictions stated in them imply that implementations have to act as if such a stack existed, because of the way the scoping rules and recursion work), the external calls system therefore has severe effects on data that happens to be stored there.  (In INTERCAL terms, imagine what would happen if data could be stored on the NEXT stack; if C used the more sensible system of having a STASH for each variable, these problems would never occur in the first place, instead causing an entirely different set of problems.)  Similar considerations apply to the common nonstandard C extension alloca, which dynamically alters the size of the stack; also, in what goes below, register variables should be considered to be auto, because the compiler may choose to allocate them on the stack.  Theoretical considerations would lead one to conclude that variable-length arrays should obey most of the same restrictions; in practice, though, it's unwise to attempt to mix those with INTERCAL code at all, except by separating them into separate functions which aren't flagged with ICK_EC_FUNC_START and use no ick_-prefixed identifiers, even indirectly.  (They may cause a compile to fail completely because they don't mix well with goto.)

In the description below, INTERCAL commands should be taken to include the equivalent C macros.

NEXT/NEXT FROM paired with RESUME have the least effect, and the most obvious effect, on auto variables in the function that was NEXTed from, which is the same effect that the standard C function longjmp has.  That is, alloca storage stays intact, and auto variables have their values `clobbered' (that is, their value is no longer reliable and should not be used) if they changed since the corresponding NEXT and are not marked as volatile.  (This is a very easy restriction to get around, because changing the values of such variables is quite difficult without using statically-allocated pointers to point to them (a dubious practice in any case), and volatile is trivial to add to the declaration.)

COME FROM has more restrictions; it deallocates all alloca storage in the function that was COME FROM, and functions that called it or that called functions that called it, etc., using C calls (as opposed to NEXT), and those invocations of the functions will cease to exist (thus destroying any auto variables in them), even in the case of COMING FROM a function into the same function. auto variables in the function that is come into will start uninitialised, even if initialisers are given in their declaration, and it will be a `new' invocation of that function.  (It is quite possible that the uninitialised values in the auto variables will happen by chance to have the values they had in some previous invocation of the function, though, because they are likely to be stored in much the same region of memory; but it is highly unwise to rely on this.)  Note that volatile will not help here.  Observant or source-code-reading readers may note that there is a mention of an ick_goto in the source code to C-INTERCAL; this is undocumented and this manual does not officially claim that such a macro exists (after all, if it did, what in INTERCAL could it possibly correspond to?), but if such a macro does exist it obeys the same restrictions as COME FROM.

FORGET is the worst of all in terms of preserving data on the stack; it deallocates alloca data and clobbers or deletes auto variables in all function invocations that have come into existence since the NEXT that created the topmost remaining NEXT stack entry was called, or since the start of the program if the NEXT stack is emptied, and the current function will continue in a new invocation.  volatile is useless in preventing this, because the relevant parts of the stack where the data were stored are deleted by the command (that's what FORGET does, remove stack). If any of these data are required, they have to be backed up into static storage (variables declared with static or global variables), or into heap storage (as in with malloc), or other types of storage (such as temporary files) which are not on the stack.  (Incidentally, suddenly deleting parts of the stack is excellent at confusing C debuggers; but even RESUME and COME FROM tend to be sufficient to confuse such debuggers.  More worrying is probably the fact that the C standard provides a portable method for deleting the stack like that, and in fact the external calls runtime library is written in standard freestanding-legal C89 (with the exception of +printflow debug output which requires a hosted implementation), meaning that in theory it would be possible to split it out to create an implementation of a C-plus-COME-FROM-and-NEXT language, and doing so would not be particularly difficult.)

Note that INTERCAL variables are not stored on the C stack, nor are any of the metadata surrounding them, and so are not affected unduly by control flow operations.

External Calls to Funge-98
==========================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.29+   no   no

C-INTERCAL supports linking INTERCAL programs with Funge-98 programs (to be precise, only Befunge-98 programs are currently supported).  However, it does not ship with a Funge-98 interpreter, and such an interpreter needs to be linked to the resulting program in order to run the Befunge program.  Therefore, you need to convert a third-party Funge-98 interpreter to a library usable by C-INTERCAL before you can use this part of the external calls system (see Creating the Funge-98 Library); however, this only has to be done once.

Once the library has been created, you can link an INTERCAL program with a Befunge-98 program by invoking ick like this:

     ick -e intercalprogram.i befungeprogram.b98

You can link no more than one Befunge-98 program at once (just like you can link no more than one INTERCAL program at once).  Also, the INTERCAL program must come first on the command line.

It is legal to link INTERCAL, C, and Befunge-98 simultaneously; however, the identifiers used in the third-party Funge-98 interpreter have not been mangled to avoid collisions, and therefore problems may be caused if the C program uses the same identifiers as the Funge-98 interpreter.

Creating the Funge-98 Library
-----------------------------

Before external calls to Funge-98 can be used, the relevant library must be compiled.  (After the library has been compiled, then you will need to reinstall C-INTERCAL; however, you will not need to recompile C-INTERCAL.)

At present, only the cfunge Funge-98 interpreter can be converted into a library suitable for use by C-INTERCAL; also, doing this is only supported on POSIX systems (although if someone gets it to work on DOS/Windows, the author of this manual would be interested to hear about it.)  Also, a source-code distribution (rather than a binary distribution) is needed.  One way to obtain the latest cfunge sources is via the bzr version-control system, using the following command (correct as of the time of writing, but as always, links can become dead):

     bzr branch http://rage.kuonet.org/~anmaster/bzr/cfunge

(As a licensing note, note that cfunge is licensed under the GNU General Public licence version 3, whereas C-INTERCAL is licensed under version 2 and all later versions of that licence; although these terms are obviously compatible with each other, you must ensure yourself that your program has appropriate licensing terms to allow a GPLv3 library to be linked to it.)

Once you have downloaded the cfunge sources, you need to compile them into a library suitable for use with C-INTERCAL (note that this is a somewhat different process to compiling them into a standalone Funge-98 interpreter).  There is a script provided in the C-INTERCAL distribution to do this, etc/cftoec.sh.  It must be run in the etc subdirectory of the C-INTERCAL distribution (i.e. the directory the script itself is in), and must be given the path to the root directory of the cfunge source distribution (that is, the directory that contains the src, lib and tools subdirectories of that distribution) as its only argument.  Note that it may give some compiler warnings on compilation; my experience is that warnings about C99 inlining can be safely ignored (they reflect a deficiency in gcc itself that luckily seems to be irrelevant in this case), but other warnings may indicate problems in the exact versions of the sources that you downloaded (and errors definitely indicate such problems).

Once the library has been created, it will appear as the new file lib/libick_ecto_b98.a in the C-INTERCAL distribution (the cfunge distribution will be left unchanged); reinstalling C-INTERCAL will install this file to its proper location.  (It is also in a valid location to be able to be run if you aren't installing C-INTERCAL but instead just running it from the distribution's directory.)

The IFFI Fingerprint
--------------------

This section will not make much sense to a non-Funge programmer; therefore, if you are not used to Funge, you probably want to skip it.

To a Funge program, the external calls interface is accessed via a Funge-98 'fingerprint' defined by the interpreter.  The name of the fingerprint is 0x49464649, or as text, IFFI.

When a program formed by linking INTERCAL and Befunge-98 is run, the first thing that happens is some internal INTERCAL initialisation which is not visible to either program, and then initialisation routines specified in the Befunge-98 program run (if an initialisation routine is also specified in a linked C program using ick_startup, it is unspecified whether the C or Befunge-98 initialisation happens first.) In the Befunge program, the initialisation routine consists of everything that happens until the Y command in the IFFI fingerprint is run; the author of the Funge-98 must load the IFFI fingerprint themselves during this initialisation to access that command.  (This is so that the Befunge program ends up complying with the Funge-98 standard; commands not defined in that standard cannot be used until a fingerprint is loaded.)  During initialisation, no commands from the IFFI fingerprint may be used except Y and A.  (If a different command is used, C, M, and X remove the arguments they would use from the stack (if any) but otherwise do nothing, and the other commands in the IFFI fingerprint reflect.)

After the Y command is called, the INTERCAL program starts running; in order for the Befunge program to regain control, it has to be NEXTed to from the INTERCAL program, or COME or NEXT FROM the INTERCAL program, or contain the line label to which syntax in the INTERCAL program was CREATEd.  (In other words, the normal INTERCAL ways of transferring information between parts of a program.)  In order to do this, therefore, line labels and INTERCAL control flow statements must be placed into the Befunge program.

Code like COME FROM (100) is a single statement in INTERCAL, but several statements in Funge-98; therefore, some method of telling the interpreter where to start executing to look for COME FROMs, NEXT FROMs, and line labels is needed.  The method used by C-INTERCAL is that of the 'marker'; a marker is represented by character 0xB7 (a mid-dot in Latin-1) in the input Funge-98 program, but is transformed to a capital M by ick.  (The reason for using a special character for a marker and transforming it rather than just using M is to prevent occurences of M in comments and string literals, etc., having an effect on the control flow of the program.)  Whenever a NEXT or line label is encountered (in the INTERCAL program, the Funge program or elsewhere), the Funge program is executed starting from each marker in each cardinal direction to look for line labels or COME/NEXT FROMs respectively.  Therefore, COME FROM (100) is written in Funge-98 as Maa*C (where the M is a marker in the source code), and likewise the line label (100) would be written as Maa*L.  (This code can be written in any cardinal direction, that is left to right, top to bottom, right to left, or bottom to top, but not diagonally or flying.)  There are likely to be unused directions from markers, which will be evaluated too; you can (and must) close these off by reflecting code execution back into that marker, another marker, or a non-marker M.  Note also that a marker remains in Funge-space even if the M on the same square is deleted (the marker itself is not visible to the g command, though).

Here are the commands in the IFFI fingerprint:

A
     This command pops a line number and then an 0"gnirts"-format string from the stack; they are used as the line number and signature to CREATE syntax in the INTERCAL program; for details of the format of the signature, see ick_create.  Although using this command during speculative execution works, doing so is not recommended; if the target line number for CREATEd syntax is changed during speculative execution to find the line that that syntax corresponds to, its effect is delayed until after the original line is found and execution continues from that point.  (Side effects during speculative execution are never recommended, because they might or might not be optimised away.)

C
     During speculative execution to find COME FROMs and NEXT FROMs, pops a line label off the top of the stack and does a COME FROM that location.  During speculative excution to find line labels, pops the top of the stack and ends that particular speculative execution as a failure.  When not doing speculative execution, pops and discards the top element of the stack.

D
     This command must only be used when the Funge program is executing a CREATEd command, and allows access to the arguments that command has.  It pops an integer off the top of the stack, and treats it as an argument position (0-based, so 0 refers to the first argument, 1 refers to the second, and so on).  Note that providing an invalid argument number, or running this command when not implementing a CREATEd command, leads to undefined behaviour (possibly a reflection, possibly a segfault, possibly worse).

     The command pushes information about the argument chosen onto the stack; the following information is pushed from bottom to top:

       * The data type of the argument, in bits (16 if the argument was a onespot variable or expression of onespot type, and 32 if the argument was a twospot variable or expression of twospot type).  Note that this is under the C-INTERCAL typing rules, rather than the INTERCAL-72 typing rules (that is, select's type is always that of its right argument no matter how many bits are actually selected).

       * 1 if the argument is an array variable, 0 if it is a scalar value.

       * 0 if the argument is not a variable, or the variable's number if it is (e.g. .123 would push 123 here, but .1~.2 would push 0).

       * The argument's value at the time that the CREATEd instruction was called.

       * The argument's value now, or alternatively a repeat of the previous stack element if -a (see -a) was not used.  (Without -a, the information needed to re-evaluate the expression is not available.)

F
     During speculative execution, this command reflects; otherwise, this command pops an integer from the top of stack, and FORGETs that many NEXT stack entries (or all of them if the argument given is negative).

G
     This command pops an integer from the top of stack.  If it is positive, the value of the onespot variable whose name is the popped integer is pushed onto the stack; if it is negative, the value of the twospot variable whose name is minus the popped integer is pushed onto the stack; and if it is zero, the command reflects.  If the referenced variable is not in the INTERCAL program at all, this causes an INTERCAL error due to referencing a nonexistent variable.

L
     During speculative execution to find COME FROMs and NEXT FROMs, this command pops and discards the top stack element, then ends that speculative execution.  During speculative execution to find a line label, this command pops an integer from the top of stack and succeeds with that integer as the line label (that is, it is possible to NEXT to an L in the Funge program if a marker, followed by code to push the correct line number onto the stack, precedes that L).  When not doing speculative execution, the integer on the top of the stack is used as a line label (assuming it is in the range 1--65535, otherwise it is popped and discarded), and a search is made for COME FROMs and NEXT FROMs aiming for that line label (including in the INTERCAL program and the Befunge program itself, as well as programs in any other language which may be linked in).  Note that just as in INTERCAL, it is possible to NEXT to a line label which has a COME FROM aiming for it, in which case the COME FROM will come from that line label as soon as the NEXT transfers control to it.

M
     Does nothing if not in speculative execution, or ends the current speculative execution with failure.  (This is so that code like

           v
          >M5C
           ^

     does exactly the same thing as COME FROM (5), even when, for instance, it is entered from the left in the Funge program, rather than gaining control from the line label (5).)

N
     During speculative execution, reflects.  Otherwise, pops the top stack element, interprets it as a line label, and NEXTs to that line label (this may start speculative execution to look for line labels, but might not if it isn't needed, for instance if the line label in question is in the INTERCAL program or in a C program linked to the Befunge program).

R
     During speculative execution, reflects.  Otherwise, pops the top stack element, removes that many items from the NEXT stack, and RESUMEs at the last item removed.  (If the top stack element was zero, negative, or too large, this will cause a fatal error in the INTERCAL program.)

S
     Pops a variable number (interpreted as onespot if positive, or minus the number of a twospot variable if negative) and an integer from the stack, and sets the referenced variable to the integer.  This reflects if an attempt is made to set the nonexistent variable 0, causes a fatal error in the INTERCAL program if an attempt is made to set a variable that doesn't exist there, and does not set read-only variables (but pops the stack anyway).  If the integer is too high for the variable it is being stored in, only the least significant 16 or 32 bits from it will be used; and likewise, if it is negative, it will be treated as the two's complement of the number given.

V
     Pops a CREATEd argument index and an integer from the top of stack.  (This is undefined behaviour if not in the implementation of a CREATEd statement, or if the referenced argument does not exist; as with the D instruction, 0 refers to the first argument, 1 to the second, and so on.)  If the -a option is not used, this command does nothing; otherwise, the value of the argument will be set to the integer.  (This involves doing a reverse assignment if the argument is a non-variable expression, as usual, and causes a fatal error in the INTERCAL program if the reverse assignment is impossible or an attempt is made to assign a scalar to an array.)

X
     This is identical to C, except that it does a NEXT FROM rather than a COME FROM.

As with external calls to C, terminating any program involved (whether the INTERCAL program with GIVE UP, the Befunge program with @ or q, or a C program with exit()) causes all programs involved to terminate, and likewise a fatal error will end all programs with an error.

One final point which is probably worth mentioning is that flow control instructions only record the IP's position and direction, nothing else; so for instance, if the stack is modified in one part of the code, those modifications will remain even after a RESUME, for instance.

Miscellaneous External Calls
============================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.29+   no   no

It is possible to specify other information to the external calls system by using the filename list after all the options are given.  To be precise, certain filename patterns are recognised and used to change the options that are used to compile the externally-called files.

The .c99 extension is treated identically to .c, except that it causes the file with that extension to be preprocessed as C99 (the more modern version of the C standard, the older C89 is more common), and that all C files involved will be compiled and linked as C99. (This corresponds to -std=c99 in gcc.)

The .a extension indicates that an object-code library should be linked in to the final program.  This is most commonly used to link in the maths library libm.a and other such system libraries. If the filename is of the form lib*.a, then the file will be searched for in the standard directories for libraries on your system, and also where the C-INTERCAL libraries are stored (which may be the same place); otherwise, the current directory will be searched. (Specifying libm.a on the command line corresponds to passing -lm to gcc.)

Using External Calls
====================

Whatever language your source files are written in, when -e is used (see -e), the compiler will go through much the same steps.

First, the INTERCAL program specified is compiled into a C program that uses the INTERCAL external call conventions for its control flow operations.  The resulting .c file will be left behind in the same directory (even if -g isn't used); if you look at it, you'll see the #include <ick_ec.h> line, and the other hallmarks of an external call program (for instance, INTERCAL NEXTs will translate into slightly modified ick_nexts; the modification is simply to allow the correct line number to be displayed in case of error).

After that, the resulting files are preprocessed twice.  First, the C preprocessor is run on the files; then, a special C-INTERCAL `preprocessor' is run on the files.  (`Preprocessor' is a bit of a misnomer here, as it's near the end of the compilation process; `postprocessor' would likely be more accurate, or maybe `interprocessor'.)  Its job is to fix line labels between the gotos that are used to implement jumping into the middle of a C function, to assign unique numbers to things that need them, and to keep track of which functions need to be checked for line labels and for COME FROMs and NEXT FROMs.  The resulting file will have the extension .cio; it is almost human-readable, especially if you run it through a C code indenter, and consists of C code (or code in whatever language is being linked to the INTERCAL code, but so far only C is accepted) and instructions to gcc.  The .cio file will be left behind for you to look at, if you like.

Once the .cio files have been produced, gcc is used to compile all the .cio files and link them together into an executable; the executable will have the same name as the INTERCAL source, minus any extension (and on DJGPP, assuming that its version of gcc could handle the resulting command line (not necessarily guaranteed), a .exe extension is added), and will consist of all the C files linked together with the INTERCAL.  Any functions named main in the C files will be deleted; likewise, if there is a name clash between any two functions, the one in the file named earlier on the command line will be used.  There is presumably some use for this feature, although I haven't figured out what it is yet.

Extending this to other compiled languages is mostly a problem of determining how they fit into the INTERCAL control structure, which is not a trivial task, and of figuring out how to link them to C code, which in some cases is trivial (especially if the language is one that gcc can compile!) and in other cases is very difficult.  If anyone has any ideas of new languages that could be added to the external calls system, feel free to contact the current C-INTERCAL maintainer with suggestions or patches.

Expansion Libraries
===================

INTERCAL-72   C-INTERCAL   CLC-INTERCAL   J-INTERCAL
no   version 0.28+   no   no

The C-INTERCAL distribution comes with libraries that can be used to extend its capabilities; they are implemented using the external call mechanism, and are in effect standard files to include using that mechanism.  To use an expansion library, give the -e option to ick (note that this means you cannot use them with the debugger or profiler, nor with multithreaded or backtracking programs), and specify the expansion library's name at the end of the command line (or to be precise, anywhere after the initial INTERCAL file).  The libraries themselves are written in C and have a .c extension, and are human-readable; C-INTERCAL will look for them in the same places as it looks for the system library (including in the current directory, so you can test your own expansion libraries without having to install them).

Expansion libraries use C identifiers which start with the string ick_my_ (this is not used by the compiler, and is explicitly not affected by the prohibition on identifiers starting ick_ when writing an expansion library), and use line labels in the range (1600) to (1699).  (Most programs will be avoiding this range anyway, because it's inside the (1000) to (1999) range reserved for the system library, but the system library doesn't use it, in much the same way that the identifiers used are inside the range reserved for the compiler, but the compiler doesn't use them.)

Expansion libraries are available from C-INTERCAL version 0.28; CLC-INTERCAL has a similar concept (that of `preloads'), but implemented a completely different way.

syslibc
-------

syslibc is an implementation of the base-2 INTERCAL system library in C (see System Library); using it in programs running in other bases is accepted by the compiler, but likely to produce unpredictable results.  When using this expansion library, you also need to give the -E option (see -E) so that the main system library is not included, or it will be used in preference to the expansion library.  All documented features of the INTERCAL base-2 system library are implemented, but most undocumented features are not, so INTERCAL programs which relied on them (dubious behaviour in any case) will not work with syslibc.  The main reason to use this library is to increase the speed of an INTERCAL program; however, note that the speed gains in arithmetic will be accompanied by the performance penalty of using the external calls infrastructure, unless you were already using it.

compunex
--------

As an example of using ick_create, a very simple expansion library is provided to enable a computed NEXT capability, by defining a new command COMPUNEX.  It is used as DO .1 COMPUNEX (allowing any expression in place of the .1), and is similar to an ordinary NEXT, but has two limitations: it takes up two NEXT stack entries, and the top one should not be RESUMEd past or forgotten (thus it isn't a particularly useful command, except maybe to produce the equivalent of something like function pointers).  By the way, note that C-INTERCAL avoids computed NEXT mainstream for much the same way that CLC-INTERCAL avoids NEXT altogether; it makes things too easy.  This example is provided mostly just to demonstrate the syntax, and the care that needs to be taken with implementing flow control operators.

compunex is double-deprecated; an alternative is the following sequence of commands involving computed CREATE:

     DO CREATE .1 ABC
     DO ABC

This sequence emulates all features of NEXT (although it has different gerunds and is two statements, not one), making it much more useful for simulating computed NEXT than COMPUNEX is. (There's no need to avoid forgetting the return value; although this skips the CREATE cleanup, none is required because the created statement ABC (any other statement would do just as well) takes no arguments.)

Differences to Other Compilers
******************************

The C-INTERCAL compiler exists in a world of several other compilers.

Differences to the Princeton compiler
-------------------------------------

The Princeton compiler was the first INTERCAL compiler available, and compiled INTERCAL-72.  Using convickt (see convickt) to translate its programs from the original EBCDIC to Latin-1 or Atari-syntax ASCII is required to run them under the C-INTERCAL compiler, but apart from that there should be no problems; everything that that compiler can do can be reproduced by C-INTERCAL, even including some of its bugs.  The only potential problems may be where constructs were nonportable or dubious to begin with (such as the IGNORE/RETRIEVE interaction), or where commands intended to be syntax errors were used in the program but have a meaning in C-INTERCAL.  For extra portability, it's possible to use the -t compiler option to ick (see -t) to tell it to interpret the program as INTERCAL-72, but as C-INTERCAL's dialect of INTERCAL is basically backward-compatible anyway this mostly serves to check newer programs for compatibility with older compilers.

Differences to the Atari compiler
---------------------------------

The Atari compiler was another implementation of INTERCAL-72, which was basically identical to the Princeton compiler apart from its use of ASCII and Atari syntax.  Everything said under the previous section applies, except that as it uses the same syntax as C-INTERCAL anyway (C-INTERCAL's syntax was based on the Atari compiler's), there is no need to use convickt.

Differences to J-INTERCAL
-------------------------

The J-INTERCAL compiler is an implementation of INTERCAL written in Java that compiles INTERCAL into Java (and so has a similar relationship with Java to that of the C-INTERCAL compiler (which is written in C and compiles into C) with C).  J-INTERCAL has much the same feature set as older versions of C-INTERCAL, with a few changes (such as the addition of Esperanto and error messages coming up in different situations). J-INTERCAL programs should run fine on C-INTERCAL without trouble (as it is also an Atari syntax compiler), except in nonportable cases such as IGNORE/RETRIEVE interaction.

Differences to CLC-INTERCAL
---------------------------

The CLC-INTERCAL compiler is the most modern INTERCAL compiler apart from C-INTERCAL (both compilers are maintained and updated every now and then as of the time of writing, so which is more modern is normally a matter of when you happen to check).  Unlike the other three compilers mentioned above, it has a quite significant feature set, including many features not implemented or only partially implemented in C-INTERCAL, and is responsible for the origin of many of the features added in more recent versions of C-INTERCAL.  Generally speaking, a CLC-INTERCAL program that uses its advanced features is unlikely to run on C-INTERCAL, or vice versa, whatever you do (apart from completely rewriting the more advanced parts of the program.)

However, there are certain steps that can be taken to transfer less advanced programs from one compiler to the other.  First, translate the program to Latin-1 Princeton syntax (if translating from CLC-INTERCAL to C-INTERCAL) or Atari syntax (if translating from C-INTERCAL to CLC-INTERCAL), maybe using convickt, if necessary.  (Note that here the program is being translated to the syntax that is not default for the target compiler.)  Then use command-line arguments to switch the compiler into the correct emulation mode for the other compiler; C-INTERCAL uses the options -xX, and on CLC-INTERCAL this is done by selecting the appropriate preloads, or by changing the program's file extension to .ci.  In each case other options may be needed to turn on various extensions (maybe -m or -v if translating to C-INTERCAL, maybe the preload for gerund-based COME
FROM if translating to CLC-INTERCAL), and if translating to CLC-INTERCAL you need to append the system library to your program yourself because CLC-INTERCAL doesn't load it automatically.

In the case of very simple programs, or if you want to spend the effort in translating compiler-specific code from one compiler to another, you may be able to work without emulation options.  (This is a good target to aim for, in any case.)  In such a case, you would do nothing other than possibly edit the program to be more portable and a possible character set and syntax change using convickt.  If you need compiler-specific code, you may be able to detect the compiler in the code itself and adapt accordingly; making use of the IGNORE/RETRIEVE interaction is one way to do this, as it differs between C-INTERCAL, J-INTERCAL, and CLC-INTERCAL.  The other things to watch out for when doing this are that CLC-INTERCAL needs an explicit option to enable the use of NEXT, that CLC-INTERCAL doesn't load the system library itself (you need to manually append it to the end of the program) and that you probably shouldn't number a line (666) unless you know what you're doing, because that line number has a special meaning in CLC-INTERCAL.

Character Sets
**************

The following table explains the equivalences between the various character sets used for INTERCAL: 7-bit ASCII Atari syntax, 5-bit Baudot Princeton syntax, 8-bit EBCDIC Princeton syntax, and 8-bit Latin-1 Princeton syntax.  (The Baudot and EBCDIC are the CLC-INTERCAL versions, which are used by INTERCAL compilers but basically nowhere else.)  The characters themselves are not shown in the table below, because they would have to be shown in some syntax, which would be misleading.  (Atari syntax is used throughout this manual; you could convert from that, assuming you have an ASCII table handy.)  You can also use the convickt command-line tool to translate INTERCAL programs from one format to another (see convickt).  Note that Baudot has more than one 'shift state'; the shift state (1, 2, 3, or 4) is written before the hexadecimal code for each character, and * represents a character available in every shift state.  To change from one shift state to another, use character 1f to change from shift states 3 or 4 to 1, or from 1 or 2 to 2, and character 1b to change from shift states 1 or 2 to 3, or from 3 or 4 to 4.

Atari   Baudot   EBCDIC   Latin-1
09   N/A   09   09
0a   * 02   0a   0a
0d   * 08   0d   0d
20   * 04   40   20
21   3 0d   4f   21
22   3 11   7f   22
23   4 06   7b   23
24   4 01   4a   a2
25   4 1c   6c   25
26   3 1a   50   26
27   3 0b   7d   27
28   3 0f   4d   28
29   3 12   5d   29
2a   4 09   5c   2a
2b   4 03   4e   2b
2c   3 0c   6b   2c
2d   3 03   60   2d
2e   3 1c   4b   2e
2f   3 1d   61   2f
30   3 16   f0   30
31   3 17   f1   31
32   3 13   f2   32
33   3 01   f3   33
34   3 0a   f4   34
35   3 10   f5   35
36   3 15   f6   36
37   3 07   f7   37
38   3 06   f8   38
39   3 18   f9   39
3a   3 0e   7a   3a
3b   3 1e   5e   3b
3c   4 0f   4c   3c
3d   4 07   7e   3d
3e   4 12   6e   3e
3f   4 0c   65   a5
40   3 19   6f   3f
41   1 03   c1   41
42   1 19   c2   42
43   1 0e   c3   43
44   1 09   c4   44
45   1 01   c5   45
46   1 0d   c6   46
47   1 1a   c7   47
48   1 14   c8   48
49   1 06   c9   49
4a   1 0b   d1   4a
4b   1 0f   d2   4b
4c   1 13   d3   4c
4d   1 1c   d4   4d
4e   1 0c   d5   4e
4f   1 18   d6   4f
50   1 16   d7   50
51   1 17   d8   51
52   1 0a   d9   52
53   1 05   e2   53
54   1 10   e3   54
55   1 07   e4   55
56   1 1e   e5   56
57   1 12   e6   57
58   1 1d   e7   58
59   1 15   e8   59
5a   1 11   e9   5a
5b   4 10   9e   5b
5c   4 05   N/A   5c
5d   4 13   5a   5d
5e   4 0d   6a   7c
5f   4 15   7c   40
60   N/A   N/A   60
61   2 03   81   61
62   2 19   82   62
63   2 0e   83   63
64   2 09   84   64
65   2 01   85   65
66   2 0d   86   66
67   2 1a   87   67
68   2 14   88   68
69   2 06   89   69
6a   2 0b   91   6a
6b   2 0f   92   6b
6c   2 13   93   6c
6d   2 1c   94   6d
6e   2 0c   95   6e
6f   2 18   96   6f
70   2 16   97   70
71   2 17   98   71
72   2 0a   99   72
73   2 05   a2   73
74   2 10   a3   74
75   2 07   a4   75
76   2 1e   a5   76
77   2 12   a6   77
78   2 1d   a7   78
79   2 15   a8   79
7a   2 11   a9   7a
7b   4 0a   9c   7b
7c   4 1e   fe   N/A
7d   4 11   dc   7d
7e   4 0b   a1   7e

convickt
********

A variety of character sets have historically been used to represent INTERCAL programs.  Atari syntax was designed specifically for use with ASCII-7, and all Atari-syntax-based INTERCAL compilers accept that character set as possible input.  (C-INTERCAL also accepts Latin-1 and UTF-8.)  However, the story is more complicated with Princeton syntax; the original Princeton compiler was designed to work with EBCDIC, but because modern computers are often not designed to work with this character set other character sets are often used to represent it, particularly Latin-1.  The CLC-INTERCAL compiler accepts Latin-1, a custom dialect of EBCDIC, Baudot, and a punched-card format as input; C-INTERCAL can cope with Latin-1 Princeton syntax, but for the other character sets, for other compilers, or just for getting something human-readable, it's useful to have a conversion program. convickt is an INTERCAL character set conversion program designed with these needs in mind.

The syntax for using convickt is

     convickt INPUTSET OUTPUTSET [PADDING]

(that is, the input and output character sets are compulsory, but the parameter specifying what sort of padding to use is optional).

The following values for INPUTSET and OUTPUTSET are permissible:

latin1
     Latin-1, or to give it its official name ISO-8859-1, is the character set most commonly used for transmitting CLC-INTERCAL programs, and therefore nowadays the most popular character set for Princeton syntax programs.  Because it is identical to ASCII-7 in all codepoints that don't have the high bit set, most of the characters in it can be read by most modern editors and terminals.  It is also far more likely to be supported by modern editors than EBCDIC, Baudot, or punched cards, all of which have fallen into relative disuse since 1972.  It is also the only input character set that C-INTERCAL supports for Princeton syntax programs.  It uses 8 bit characters.

ebcdic
     EBCDIC is an 8-bit character set that was an alternative to ASCII in 1972, and is the character set used by the original Princeton compiler.  Unfortunately, there is no single standard version; the version of EBCDIC used by convickt is the one that CLC-INTERCAL uses.  It is the default input character set that CLC-INTERCAL uses (although more recent versions of CLC-INTERCAL instead try to guess the input character set based on the input program.)

baudot
     Baudot is a 5-bit character set with shift codes; therefore when storing it in a file on an 8-bit computer, padding is needed to fill in the remaining three bits.  The standard Baudot character set does not contain all the characters needed by INTERCAL; therefore, CLC-INTERCAL uses repeated shift codes to add two more sets of characters. convickt uses the CLC-INTERCAL version of Baudot, so as to be able to translate programs designed for that compiler; however, standard Baudot is also accepted in input if it contains no redundant shift codes, and if the input contains no characters not in standard Baudot, the output will be written so that it is both correct standard Baudot and correct CLC-INTERCAL Baudot for those characters.

atari
     This option causes convickt to attempt a limited conversion to or from Atari syntax; this uses ASCII-7 as the character set, but also tries to translate between Atari and Princeton syntax at the character level, which is sometimes but not always effective.  For instance, ? is translated from Atari to Princeton as a yen sign, and from Princeton to Atari as a whirlpool (@); this sort of behaviour is often capable of translating expressions automatically, but will fail when characters outside ASCII-7 (Atari) or Latin-1 (Princeton) are used, and will not, for instance, translate a Princeton V, backspace, - into Atari ?, but instead leave it untouched.  ASCII-7 is a 7-bit character set, so on an 8 bit computer, there is one bit of padding that needs to be generated; note, however, that it is usual nowadays to clear the top bit when transmitting ASCII-7, which the `printable' and `zero' padding styles will do, but the `random' style may not do.

When using a character set where not all bits in each byte are specified, a third argument can be given to specify what sort of padding to use for the top bits of each character. There are three options for this:

Option   Meaning
printable
Keep the output in the range 32-126 where possible
zero
Zero the high bits in the output
random
Pad with random bits (avoiding all-zero bytes)

Note that not all conversions are possible.  If a character cannot be converted, it will normally be converted to a NUL byte (which is invalid in every character set); note that this will prevent round-tripping, because NUL is interpreted as end-of-input if given in the input.  There is one exception; if the character that could not be converted is a tab character, it will be converted to the other character set's representation of a space character, if possible, because the two characters have the same meaning in INTERCAL (the only difference is if the command is a syntax error that's printed as an error message).  (The exception exists to make it possible to translate existing INTERCAL source code into Baudot.)

Optimizer Idiom Language
************************

One file in the C-INTERCAL distribution (src/idiotism.oil) is written in Optimizer Idiom Language, a programming language designed especially for expressing optimizer idioms for INTERCAL in an easily editable form (well, at least it's easier than the unmaintainable set of idioms hard-coded in C that were used in previous versions of the INTERCAL compiler).

OIL Basics
==========

The structure of an OIL file consists of a sequence of idioms. An optimizer idiom looks for a certain pattern in an expression (which could be an INTERCAL expression, or an expression that has already been partly optimized and therefore contains some non-INTERCAL operators), and replaces it with a replacement that's `simpler' in some sense (in the case of C-INTERCAL, `simpler' is interpreted to mean `compiles into a faster or smaller executable when run through a C compiler').  When an OIL program acts on an input INTERCAL file, it keeps on matching idioms to simplify expressions, until none of the idioms act any more (and if a situation occurs where idioms can keep matching indefinitely, the compiler goes into an infinite loop; so don't allow that to happen); at present, the idioms are tried from left to right, from the leaves of an expression to its root, and from the start of the OIL file to the end; but don't rely on that, because it's subject to change (and gets confusing when you think about what happens when the program actually does a replacement).  Anyway, the point is that if an idiom can match an expression, and another idiom doesn't change it first, then the idiom will be matched against that part of the expression eventually, and the program won't end until there are no idioms that match the optimized expression.

At present, the only place that OIL is used in the C-INTERCAL compiler is when the -O option (see -O) is used in base 2.  (Syntax is planned to extend OIL to higher bases, and some of this is documented and even implemented, but there's no way to use it.)  The idioms are read from the file src/idiotism.oil during the compilation of the C-INTERCAL from sources; you can change the idioms, but you will then have to recompile the distribution (and if you are using the config.sh method, also reinstall, but that will be pretty fast.)

OIL Syntax
==========

An OIL file is encoded as an ASCII text file using no codepoints outside the range 0-127; using 10 for newline (as on a UNIX or Linux system) is always acceptable, but using 13 then 10 (as is common on Windows or DOS) for newline is acceptable only if your C compiler recognizes that as a newline.  I have no idea what happens if you use just 13 on an Apple computer on which that is the common newline convention.

Comments can be given anywhere in the file by writing lines starting with semicolons (known as hybrids to INTERCAL programmers).  It's also possible to write a semicolon after part of a line to comment out the rest of the line.  Inside braced C expressions, comments can be given anywhere whitespace would be allowed by placing them between /* and */ (in such cases, the comments will be copied verbatim to the C temporary files used when building the C-INTERCAL compiler, where your C compiler will ignore them).  Whitespace is ignored nearly everywhere; the only places it isn't ignored are in the middle of a decimal constant, inside square brackets, immediately after one of the characters .:#_}, and anywhere that C doesn't allow it in quoted C code.  (This means that you can even place it inside operators like && if you like, as long as they're part of OIL code and not C code, although doing this is not recommended.)  If you use whitespace in a situation where it isn't ignored, that's almost certainly an error.

Idioms are grouped into groups of idioms by placing an identifier in square brackets before the group; this follows the rules for C identifiers, except that there's a maximum length of 30 characters. This identifier is the `name' of the group, which has no effect except on optimizer debug output; for that matter, the only effect a group has is that all idioms in the group look the same in optimizer debug output, because they have the same name.  It's recommended that idioms only have the same name if they are the same idiom, possibly written in several ways.  For example, a shift by 0 has no effect and may as well be removed from the output; the way to express this in OIL is:

     [nullshift]
     (_1 >> #0)->(_1)
     (_1 << #0)->(_1)

Here, nullshift is the name of the group of idioms, and two idioms are given; one which removes a null rightshift, and one which removes a null leftshift.

As the example above shows, the syntax of an idiom itself is

     (pattern)->(replacement)

The parentheses here are actually part of the pattern and/or replacement, and as such sparks (apostrophes) or rabbit-ears (double quotes) can be used instead; they're shown in the syntax because the outer layer of parenthesising is always required.  Both the pattern and replacement are OIL expressions, although they both have their own special syntax elements as well.

OIL Expressions
===============

An OIL expression is built around subexpressions connected by infix binary operators and/or preceded by prefix unary operators, the same way as in C or INTERCAL (although unary operators must be entirely before their argument; the one character later position is not allowed.) As in INTERCAL, there is no operator precedence; expressions must be very fully bracketed to show unambiguously what the precedences must be, and then more so; for instance, bracketing marks must be placed around the argument of a unary operator in most circumstances.  Bracketing of expressions can be done with parentheses, sparks (apostrophes) or rabbit-ears (double-quotes).

The following unary and binary operators are allowed in OIL expressions:

$   INTERCAL mingle
~   INTERCAL select
&16   INTERCAL unary AND (16-bit)
V16   INTERCAL unary OR (16-bit)
?16   INTERCAL unary XOR (16-bit)
^16   INTERCAL unary sharkfin (16-bit)
@16   INTERCAL unary whirlpool (16-bit)
@216..@516   INTERCAL unary generalised whirlpool (16-bit)
&32   INTERCAL unary AND (32-bit)
V32   INTERCAL unary OR (32-bit)
?32   INTERCAL unary XOR (32-bit)
^32   INTERCAL unary sharkfin (32-bit)
@32   INTERCAL unary whirlpool (32-bit)
@232..@532   INTERCAL unary generalised whirlpool (32-bit)
&   C binary bitwise AND
|   C binary bitwise OR
^   C binary bitwise XOR
+   C addition
-   C subtraction
*   C multiplication
/   C integer division
%   C modulus
>   C greater than
<   C less than
~   C unary bitwise complement
!=   C not equals operator
==   C equals operator
&&   C logical AND
||   C logical OR
>>   C bitwise rightshift
<<   C bitwise leftshift
!   C unary logical NOT

(Note that in some cases two operators are expressed the same way, but that this doesn't matter because one is unary and the other is binary so that there can't be any ambiguity, only confusion.  Also note that unlike INTERCAL unary logic operators, OIL unary logic operators must have a bitwidth stated.)

It hasn't yet been explained what operands these operators have to operate on; the syntax for those depends on whether it's a pattern or replacement that the expression is representing.

OIL Patterns
============

Patterns are simply OIL expressions; the expressions match either original INTERCAL input or expressions produced by earlier idioms. Each operator must match the same operator in the (possibly partially-optimised) input; the operands themselves are pattern templates specifying what operands in the input they can match.

One special simple form of match is possible: #NUMBER, where NUMBER is in decimal, matches a constant with that value. (Unlike in INTERCAL, this constant is not limited to being a onespot value; it is, however, limited to being at most twospot, as are all operands and intermediate values in OIL.)

Otherwise, an operand consists of the following parts, written in order:

 1. A character to specify the data type of what's being matched.  Usually, this will be _ to specify that any data type can be matched.  In a few cases, you may want to use . or : to specify that you only want to match a onespot or twospot value respectively (that is, 16- or 32-bit).  You can also use #; this specifies a value that can be any width, but must be known to be a constant with a known value at optimize time (either because it was hardcoded as a constant originally or because a constant was produced there by the optimizer, for instance via a constant folding optimization).

 2. Optionally, an expression in braces ({ and }).  This expression is written in C, not OIL (as are all expressions in braces), and puts an extra condition on whether the pattern matches. The exact meaning of this will be explained later.

 3. A reference number, which must be one decimal digit long.  A reference number of 0 causes the operand to be discarded immediately after matching; normally, you will want to specify a positive reference number.  Two operands with the same reference number must be exactly the same for the pattern to match (for instance, both references to the same variable, or identical subexpressions).  The reference number also allows the operand to be referenced by C expressions on other operands and by replacements.  Reference numbers must be unique within the idiom (unless two or reference numbers are deliberately the same so that the operands they reference have to be identical to produce a match), and they are scoped only as far as the containing idiom; they don't carry from one idiom to another.

Note that syntax like #2 is ambiguous given what comes so far; the first interpretation is the one that is taken in practice, and if the second interpretation is wanted the operand should be expressed as #{1}2, using a no-op braced expression to tell them apart. This particular no-op is recommended because it's detected and optimized by the OIL compiler.

Braced expressions, which must be written purely in C, add extra conditions; they must return nonzero to allow a possible match or zero to prevent one.  They can reference the following variables and functions:

c
cNUMBER

     This accesses a calculation made automatically by the compiled OIL program to identify which bits of the operand can possibly be set, and which ones cannot be.  c by itself refers to the operand to which the braced expression is attached; if a number is given, it refers to another node (the number is interpreted as a reference number).  The actual value of c is a 32-bit unsigned integer, each bit of which is true, or 1, if there is any chance that the corresponding bit of the operand might be 1, and false, or 0, if it's known for certain that the corresponding bit of the operand is 0.

     For instance:

          _{!(c&4294901760LU)}1

     The constant given here is FFFF0000 when expressed in hexadecimal; the point is that the expression matches any operand that is known to have a value no greater than 65535.  Unless the operand is the argument to a unary AND, this check generally makes more sense than explicitly specifying . rather than _, because it will identify both 16- and 32-bit values as long as they're small enough to fit into a onespot variable.  This code could, for instance, be used to check that an argument to a mingle must be small enough before optimising it (this is important because an optimisation shouldn't optimise an error -- in this case, an overflowing mingle -- into a non-error).

x
xNUMBER

     x is like c, and refers to operands in the same way, except that it can only refer to an operand marked with #.  It holds the value of that constant (a 32-bit unsigned integer), which will be known to the optimizer at optimize time.  One common use of this is to detect whether a constant happens to be a power of 2, although there are many other possibilities that may be useful.

r

     When inside a loop, r is the value of the loop counter.  (It's almost certainly a mistake if you have a loop but don't reference the loop counter at least once, and usually at least twice, within the loop.)  See OIL Loops.

and16
and32
or16
or32
xor16
xor32
iselect
mingle

     These are all functions with one argument (apart from iselect and mingle, which each take two arguments); they exist so that INTERCAL operators can be used by C expressions.  They all take unsigned longs as input and output, even if they are onespot operators.  Note that it's entirely possible for these to cause a compile-time error if used on invalid arguments (such as mingling with an operand over 65535), or to silently truncate an invalid argument down to the right number of bits; both of these should be avoided if possible, so the optimiser should check first to make sure that it doesn't use any of these functions on invalid arguments.

xselx

     This function returns its argument selected with itself; so xselx(c) is shorthand for iselect(c,c).  When the argument is very complicated, this can save a lot of space in the original OIL program.

setbitcount

     This function simply returns the number of bits with value 1 in its argument.  This is sometimes useful with respect to various select-related optimisations, and can be a useful alternative to having to take logarithms in various situations.

smudgeright
smudgeleft

     The smudgeright function returns its argument but with all the bits less significant than the most significant bit with value 1 set to 1; likewise, smudgeleft returns its argument with all the bits more significant than the least significant bit with value 1 set to 1.

Note that all OIL character is done internally using unsigned 32-bit numbers, and C expressions you write should do the same.  The practical upshot of this is that you should write LU after any constant you write in C code; if you don't do this, you are reasonably likely to get compiler warnings, and the resulting program may not work reliably, although the OIL compiler itself will not complain.

Here's a more complicated example of an optimizer operand:

     #{!(x&2863311530LU)&&iselect(x,1431655765LU)==
       xselx(iselect(x,1431655765LU))}3

It helps to understand this if you know that 2863311530 in hexadecimal is AAAAAAAA and 1431655765 in hexadecimal is 55555555.  (It's worth putting a comment with some frequently-used decimal constants in an OIL input file to help explain what these numbers mean and make the code more maintainable.)  The operand matches any constant integer which has no bits in common with AAAAAAAA, and for which if any bit in common with 55555555 is set, all less significant bits in common with that number are also set.

OIL Replacements
================

Replacements have much the same syntax as patterns.  The expressions are parsed in much the same way; however, one peculiarity of replacements is that bitwidths must be specified.  INTERCAL has a typecaster that figures out whether each expression is 16 bits or 32 bits wide, but it runs before the optimizer, and as the optimizer can produce expressions whose bitwidths don't obey INTERCAL's rules, this information needs to be inserted somehow in a replacement.  In C-INTERCAL, it usually doesn't matter what the bitwidth is, and in cases where it doesn't matter the normal operators ($, ~, and so on) can be used.  (Note that the bit width of the entire replacement is always set to the same bit width as the bit width of the expression matched by the pattern; so you don't have to worry about the effect on unary logical operators that might be operating on the expression being optimized.  This is an exception to the normal bitwidth rules for a replacement.)  In cases where it does matter (due to C-INTERCAL's lenient interpretation of bitwidth on mingle inputs, the only place it matters is in the input to INTERCAL unary logical operators), both the bitwidth of the operator and the argument on which it operates must be explicitly given, and given as the same value; to set the bitwidth of an operator's result, simply write the bitwidth (16 or 32 for onespot and twospot respectively) immediately after the operator; for instance, !=32 will generate a not-equals operation with a 32-bit bitwidth.  If an operator's width is set to 16, and during the course of execution of the optimized program, a value that doesn't fit into 16 bits is encountered, that's undefined behaviour and anything might happen (most likely, though, the program will just act as though its width had been set to 32 bits instead); this error condition is not detected.  Also note that operators like &32 already have a bitwidth specified, so specifying &3232 (or worse, &3216) is not allowed.

Replacement operands are simpler than pattern operands, because there are only a few forms they can take.

_NUMBER
.NUMBER
:NUMBER

     This tells the optimiser to copy the operand or expression with reference number NUMBER to this point in the replacement used for the expression matched by the pattern.  The three forms are identical; the last two are provided for aesthetic reasons (it can look better and be clearer to match .1 in the pattern with .1 in the replacement, for instance).  You cannot use #NUMBER here to copy in a constant from the left-hand side, though, nor #{1}NUMBER, because the first means something else and the second is undefined behaviour (that is, no behaviour for the second case has been specifically implemented in the compiler and therefore its behaviour is unpredictable and subject to change in future versions); use _NUMBER to copy over a constant with an unknown at optimizer compile time (but known at optimize time) value from the left hand side, as you can do with any other operand being copied.

#NUMBER

     Insert a constant with the literal value NUMBER here.

#{EXPRESSION}0

     Calculate the value of EXPRESSION (a C expression, which can reference the same variables and functions as a C expression in a pattern can; see C functions in OIL) and insert a constant with the calculated value here.  (That is, a value is calculated at optimise-time and the resulting value is therefore constant at runtime.)

As an example, here's an idiom that moves C bitwise AND operations inside leftshifts.  (This is useful because if the optimizer has generated a large sequence of mixed ANDs and bitshifts, moving all the ANDs to one end allows them to be clumped together and optimized down to one AND, whilst the shifts can all be combined into one large shift.)

     ((_1 << #{1}2) & #{1}3)->((_1 & #{x3>>x2}0) << _2)

OIL Loops
=========

When writing idioms, sometimes instead of using very complicated expressions to try to match multiple situations at once it's easier to have a separate idiom for each possible situation; for instance, it's easier to write idioms for right-shift by 1, right-shift by 2, right-shift by 3, etc., rather than a general idiom to rightshift by any amount.  When the idioms follow a pattern, as they will do in basically every case of this sort, it's possible to automatically generate them using a loop.  For instance, idioms to optimize a one-bit rightshift and a two-bit rightshift are:

     (_1~#{xselx(x)<<1==x&&x}2)->((_1&_2)>>#1)
     (_1~#{xselx(x)<<2==x&&x}2)->((_1&_2)>>#2)

Adding a loop to automatically generate the idioms, and placing a name for the group of idioms at the start, produces the following code:

     [rshift]
     <#1-#31
     (_1~#{xselx(x)<<r==x&&x}2)->((_1&_2)>>#{r}0)
     >

That's 31 different idioms, generated with a loop.  As the above example shows, a loop starts with <#NUMBER-#NUMBER and ends with >; a different idiom is generated for each possible value of the loop counter r in the range given by the opening line of the loop.  Loops must be placed around idioms, but inside a group of idioms.  Note the use of #{r}0 to generate a constant whose value is equal to the value of the loop counter.

OIL Tips
========

Here are some tips for the best use of OIL:

  * The OIL compiler has a few deficiencies, such as error messages that don't give you much of an idea of what you did wrong (to compensate for this, it does give a reasonably accurate line and character number where the error happened), limits on things like how deep expressions can be nested and how many idioms are allowed (if you hit the first, you should really break it up into smaller idioms if possible, and if you hit the second, increase MAXTOFREE in oil.y; this isn't a limit on the number of idioms but on the number of strings that are allocated internally to process the idioms), and lack of error checking (invalid OIL may produce errors in the OIL compiler, or cause the output C code to contain errors or warnings, or may even appear to work).

  * When you have symmetrical operators in an idiom (like C binary logical operators), you need to write the idiom both ways round (in the same group of idioms, preferably), or write another idiom to standardise the format first; the first method can get very tedious when there are many symmetrical operators in an idiom, and the second is prone to optimizer loops and subtle errors (both methods have been used in the provided idiotism.oil file.)

  * Idioms should preferably be as general as possible, broken down into many separate idioms rather than all as one big idiom, and match the smallest part of the expression being optimized that is necessary for the idiom to be correct in all circumstances; all of these help to improve the performance of the optimizer in terms of what it can optimize.

  * If a program's optimizing incorrectly, or you just want to see how the optimizer deals with a particular program, it's possible to debug the optimizer by giving the -h, -H, or -hH (see -h) switch to the C-INTERCAL compiler, which will cause debug information to be output on stderr.  -h will show the initial and final expressions for each expression that was given as input to the optimizer, as well as a list of all optimizations used, in a language that's a sort of mix of C, INTERCAL, and OIL; -H will do the same, and also show the intermediate stages in the optimization. -hH is like -H, but produces its output entirely in C, so that the various intermediate stages can be tried with a C compiler, or for people who are fed up of reading INTERCAL.  -H can be a useful learning resource for people who want to understand how a particular INTERCAL expression works.

  * Make sure that the optimizer doesn't change a statement from a run-time error into something that silently succeeds!  Checking which bits can and can't be 1 is one good way to avoid this, especially with mingles. (At present, there are some idioms that can turn run-time errors into compile-time errors, such as with the expression ''#65535$#65535'$#0'; this could be avoided, but for the time being it isn't.)

  * You can help!  If you find an idiom that the optimizer isn't optimizing correctly, feel free to add it, or multiple idioms that come to the same thing, to idiotism.oil.  In such a case, it would help other users if you would submit your new optimizer idiom to the project (see Reporting Bugs); this will help other users and future releases of INTERCAL, and also has a chance of allowing other users to check your new idioms to see if they cause problems or could be written better.

OIL Example
===========

To finish off this appendix, here's an example of the power of OIL; this is the optimization of an idiom from the INTERCAL-72 system library, as shown with -H; this should give a good idea of how OIL programs work.  (All the relevant idioms are in idiotism.oil as of the time of writing.)  Note how the expression is reduced one small step at a time; the smallness of the steps makes the optimizer more general, because if the original expression had been slightly different, the optimizer wouldn't have come to the same result but could have optimized it quite a bit of the way, up to the point where the optimizations were no longer valid; in an older version of INTERCAL, this idiom was simply hardcoded as a special case and so slight variations of it weren't optimized at all.  If you look at the idioms themselves, it'll also be apparent how c (the record of which bits of an expression can be 1 and which bits can't be) is important information in being able to apply an optimization more aggressively.

     .3 <- ((((((((.3 $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [minglefold]

     .3 <- ((((((((.3 $ 0x0) ~ 0x2aaaaaab) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [lshift16]

     .3 <- ((((((((((.3 >> 0x0) & 0x7fff) << 0x1) | 0x0) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [noopor]

     .3 <- (((((((((.3 >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ (0x7fff $ 0x1)) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [minglefold]

     .3 <- (((((((((.3 >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ 0x2aaaaaab) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [lshift16]

     .3 <- (((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) | 0x0) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [noopor]

     .3 <- ((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ (0x3fff $ 0x3)) $ 0x0) ~ (0xfff $ 0xf))

     [minglefold]

     .3 <- ((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) $ 0x0) ~ 0xaaaaaaf) $ 0x0) ~ (0xfff $ 0xf))

     [lshift16]

     .3 <- ((((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) | 0x0) $ 0x0) ~ (0xfff $ 0xf))

     [noopor]

     .3 <- (((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) $ 0x0) ~ (0xfff $ 0xf))

     [minglefold]

     .3 <- (((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) $ 0x0) ~ 0xaaaaff)

     [lshift16]

     .3 <- (((((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7) | 0x0)

     [noopor]

     .3 <- ((((((((((((.3 >> 0x0) & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [nullshift]

     .3 <- (((((((((((.3 & 0x7fff) << 0x1) >> 0x0) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [combinelrshift]

     .3 <- ((((((((((.3 & 0x7fff) << 0x1) & 0x7fff) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [andintolshift]

     .3 <- ((((((((((.3 & 0x7fff) & 0x3fff) << 0x1) << 0x1) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [combinellshift]

     .3 <- (((((((((.3 & 0x7fff) & 0x3fff) << 0x2) >> 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [combinelrshift]

     .3 <- ((((((((.3 & 0x7fff) & 0x3fff) << 0x1) & 0x1fff) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [andintolshift]

     .3 <- ((((((((.3 & 0x7fff) & 0x3fff) & 0xfff) << 0x1) << 0x3) >> 0x3) & 0x1ff) << 0x7)

     [combinellshift]

     .3 <- (((((((.3 & 0x7fff) & 0x3fff) & 0xfff) << 0x4) >> 0x3) & 0x1ff) << 0x7)

     [combinelrshift]

     .3 <- ((((((.3 & 0x7fff) & 0x3fff) & 0xfff) << 0x1) & 0x1ff) << 0x7)

     [andintolshift]

     .3 <- ((((((.3 & 0x7fff) & 0x3fff) & 0xfff) & 0xff) << 0x1) << 0x7)

     [combinellshift]

     .3 <- (((((.3 & 0x7fff) & 0x3fff) & 0xfff) & 0xff) << 0x8)

     [combineand]

     .3 <- ((((.3 & 0x3fff) & 0xfff) & 0xff) << 0x8)

     [combineand]

     .3 <- (((.3 & 0xfff) & 0xff) << 0x8)

     [combineand]

     .3 <- ((.3 & 0xff) << 0x8)

Copying
*******

The majority of the files in the C-INTERCAL distribution are licensed under the GNU General Public License (version 2 or later), but with some exceptions.  The files ick-wrap.c and pickwrap.c are licensed under a license that allows them to be used for any purpose and redistributed at will, and are explicitly not GPL'd.  This means that C source code generated by the compiler has the same copyright conditions as the original INTERCAL source.  (Note that the libraries libick.a and libickmt.a are GPL, though, so you cannot redistribute an executable produced by ick or by linking a C file to either of those libraries unless the original INTERCAL source was GPL.)  For similar reasons, the expansion libraries syslibc.c and compunex.c (currently the only ones in the distribution) is explicitly public domain.  Also, this manual, and the files that are the source code for creating it, are licensed under the GNU Free Documentation License rather than the GPL, and the licenses themselves (fdl-1-2.txi and COPYING.txt) are licensed under a license that allows verbatim redistribution but not creation of derivative works.  All other files, though (including the man pages, which are not part of this manual), are licensed under the GPL.  For the full text of the GPL, see the file COPYING.txt in the distribution.

GNU Free Documentation License
==============================

     [file fdl-1-2.txi not included]

Index
*****

This is the index of everything in this manual.  (Note that in some versions of the manual this is called Main Index to prevent it transforming into a page called index.html in the HTML version of the manual.  The complications that that caused were really odd.)