Lispy: Scheme Interpreter in Python

xiaochuan

################ Lispy: Scheme Interpreter in Python
## (c) Peter Norvig, 2010-14; See http://norvig.com/lispy.html
################ Types
from __future__ import division
Symbol = str          # A Lisp Symbol is implemented as a Python str
List   = list         # A Lisp List is implemented as a Python list
Number = (int, float) # A Lisp Number is implemented as a Python int or float
################ Parsing: parse, tokenize, and read_from_tokens
def parse(program):
"Read a Scheme expression from a string."
return read_from_tokens(tokenize(program))
def tokenize(s):
"Convert a string into a list of tokens."
return s.replace('(',' ( ').replace(')',' ) ').split()
def read_from_tokens(tokens):
"Read an expression from a sequence of tokens."
if len(tokens) == 0:
raise SyntaxError('unexpected EOF while reading')
token = tokens.pop(0)
if '(' == token:
L = []
while tokens[0] != ')':
L.append(read_from_tokens(tokens))
tokens.pop(0) # pop off ')'
return L
elif ')' == token:
raise SyntaxError('unexpected )')
else:
return atom(token)
def atom(token):
"Numbers become numbers; every other token is a symbol."
try: return int(token)
except ValueError:
try: return float(token)
except ValueError:
return Symbol(token)
################ Environments
def standard_env():
"An environment with some Scheme standard procedures."
import math, operator as op
env = Env()
env.update(vars(math)) # sin, cos, sqrt, pi, ...
env.update({
'+':op.add, '-':op.sub, '*':op.mul, '/':op.div,
'>':op.gt, '<':op.lt, '>=':op.ge, '<=':op.le, '=':op.eq,
'abs':     abs,
'append':  op.add,  
'apply':   apply,
'begin':   lambda *x: x[-1],
'car':     lambda x: x[0],
'cdr':     lambda x: x[1:],
'cons':    lambda x,y: [x] + y,
'eq?':     op.is_,
'equal?':  op.eq,
'length':  len,
'list':    lambda *x: list(x),
'list?':   lambda x: isinstance(x,list),
'map':     map,
'max':     max,
'min':     min,
'not':     op.not_,
'null?':   lambda x: x == [],
'number?': lambda x: isinstance(x, Number),   
'procedure?': callable,
'round':   round,
'symbol?': lambda x: isinstance(x, Symbol),
})
return env
class Env(dict):
"An environment: a dict of {'var':val} pairs, with an outer Env."
def __init__(self, parms=(), args=(), outer=None):
self.update(zip(parms, args))
self.outer = outer
def find(self, var):
"Find the innermost Env where var appears."
return self if (var in self) else self.outer.find(var)
global_env = standard_env()
################ Interaction: A REPL
def repl(prompt='lis.py> '):
"A prompt-read-eval-print loop."
while True:
val = eval(parse(raw_input(prompt)))
if val is not None:
print(lispstr(val))
def lispstr(exp):
"Convert a Python object back into a Lisp-readable string."
if  isinstance(exp, list):
return '(' + ' '.join(map(lispstr, exp)) + ')'
else:
return str(exp)
################ Procedures
class Procedure(object):
"A user-defined Scheme procedure."
def __init__(self, parms, body, env):
self.parms, self.body, self.env = parms, body, env
def __call__(self, *args):
return eval(self.body, Env(self.parms, args, self.env))
################ eval
def eval(x, env=global_env):
"Evaluate an expression in an environment."
if isinstance(x, Symbol):      # variable reference
return env.find(x)[x]
elif not isinstance(x, List):  # constant literal
return x               
elif x[0] == 'quote':          # (quote exp)
(_, exp) = x
return exp
elif x[0] == 'if':             # (if test conseq alt)
(_, test, conseq, alt) = x
exp = (conseq if eval(test, env) else alt)
return eval(exp, env)
elif x[0] == 'define':         # (define var exp)
(_, var, exp) = x
env[var] = eval(exp, env)
elif x[0] == 'set!':           # (set! var exp)
(_, var, exp) = x
env.find(var)[var] = eval(exp, env)
elif x[0] == 'lambda':         # (lambda (var...) body)
(_, parms, body) = x
return Procedure(parms, body, env)
else:                          # (proc arg...)
proc = eval(x[0], env)
args = [eval(exp, env) for exp in x[1:]]
return proc(*args)