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1 Inspired by a September 14, 2006 Salon article "Why Johnny Can't Code" by
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2 David Brin (http://www.salon.com/tech/feature/2006/09/14/basic/index.html),
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3 I thought that a fully working BASIC interpreter might be an interesting,
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4 if not questionable, PLY example. Uh, okay, so maybe it's just a bad idea,
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5 but in any case, here it is.
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6
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7 In this example, you'll find a rough implementation of 1964 Dartmouth BASIC
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8 as described in the manual at:
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9
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10 http://www.bitsavers.org/pdf/dartmouth/BASIC_Oct64.pdf
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11
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12 See also:
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13
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14 http://en.wikipedia.org/wiki/Dartmouth_BASIC
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15
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16 This dialect is downright primitive---there are no string variables
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17 and no facilities for interactive input. Moreover, subroutines and functions
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18 are brain-dead even more than they usually are for BASIC. Of course,
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19 the GOTO statement is provided.
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20
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21 Nevertheless, there are a few interesting aspects of this example:
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22
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23 - It illustrates a fully working interpreter including lexing, parsing,
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24 and interpretation of instructions.
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25
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26 - The parser shows how to catch and report various kinds of parsing
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27 errors in a more graceful way.
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28
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29 - The example both parses files (supplied on command line) and
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30 interactive input entered line by line.
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31
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32 - It shows how you might represent parsed information. In this case,
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33 each BASIC statement is encoded into a Python tuple containing the
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34 statement type and parameters. These tuples are then stored in
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35 a dictionary indexed by program line numbers.
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36
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37 - Even though it's just BASIC, the parser contains more than 80
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38 rules and 150 parsing states. Thus, it's a little more meaty than
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39 the calculator example.
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40
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41 To use the example, run it as follows:
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42
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43 % python basic.py hello.bas
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44 HELLO WORLD
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45 %
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46
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47 or use it interactively:
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48
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49 % python basic.py
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50 [BASIC] 10 PRINT "HELLO WORLD"
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51 [BASIC] 20 END
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52 [BASIC] RUN
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53 HELLO WORLD
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54 [BASIC]
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55
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56 The following files are defined:
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57
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58 basic.py - High level script that controls everything
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59 basiclex.py - BASIC tokenizer
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60 basparse.py - BASIC parser
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61 basinterp.py - BASIC interpreter that runs parsed programs.
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62
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63 In addition, a number of sample BASIC programs (.bas suffix) are
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64 provided. These were taken out of the Dartmouth manual.
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65
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66 Disclaimer: I haven't spent a ton of time testing this and it's likely that
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67 I've skimped here and there on a few finer details (e.g., strictly enforcing
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68 variable naming rules). However, the interpreter seems to be able to run
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69 the examples in the BASIC manual.
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70
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71 Have fun!
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72
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73 -Dave
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74
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75
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76
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77
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78
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79
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