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996
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1 from parser import *
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2
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3 class EvaluatorException(Exception):
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4 pass
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5
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6 def eval(exp, env):
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7 # symbols are names to be left unevaluated.
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8 if isinstance(exp, NameExp):
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9 raise EvaluatorException("unbound variable: " + exp.body)
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10 if isinstance(exp, LambdaExp):
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11 # no change
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12 return exp
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13 if isinstance(exp, ApplyExp):
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14 # We're lazy, so we apply before evaluating the argument.
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15 # We do of course need to evaluate the function.
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16 rator = eval(exp.rator, env)
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17 rand = exp.rand
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18 return apply(rator, rand, env)
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19 if isinstance(exp, StringExp):
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20 import sys
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21 sys.stdout.write(exp.line)
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22 return LambdaExp('x', NameExp('x')) # i.e. the identity function
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23 if isinstance(exp, SpecialExp):
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24 if exp.body == '#list':
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25 # list all definitions in the environment
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26 import sys
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27 defs = env.keys()
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28 defs.sort()
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29 sys.stdout.write('[' + ' '.join(defs) + ']')
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30 return exp
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31 raise EvaluatorException("Unknown expression type for evaluation")
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32
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33 def substitute(exp, name, value):
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34 if isinstance(exp, StringExp):
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35 return exp
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36 if isinstance(exp, NameExp):
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37 if exp.body == name:
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38 return value
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39 return exp
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40 if isinstance(exp, ApplyExp):
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41 return ApplyExp(substitute(exp.rator, name, value), substitute(exp.rand, name, value))
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42 if isinstance(exp, SpecialExp):
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43 return exp
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44 if isinstance(exp, LambdaExp):
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45 # lambda can shadow the name
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46 if name == exp.arg:
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47 return exp
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48 return LambdaExp(exp.arg, substitute(exp.body, name, value))
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49 print exp
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50 raise EvaluatorException("Unknown expression type for substitution")
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51
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52 def apply(rator, rand, env):
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53 # since we're lazy, rand is unevaluated, so it might be anything
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54 # Rator, on the other hand ought to be a lambda or a special word
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55 if isinstance(rator, LambdaExp):
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56 # okay, now we actually have to do stuff
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57 # first we substitute all instances of the first named parameter
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58 # of the lambda with the rand.
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59 body = substitute(rator.body, rator.arg, rand)
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60 return eval(body, env)
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61 if isinstance(rator, SpecialExp):
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62 if rator.body == '#show':
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63 # first evaluates the operand, then shows it, returns the result of the evaluation
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64 import sys
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65 rand = eval(rand, env)
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66 sys.stdout.write(str(rand))
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67 return rand
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68 else:
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69 raise EvaluatorException("Unknown special word: " + rator.body)
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70 raise EvaluatorException("Trying to apply something that isn't a lambda")
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