simple lisp interpreter

simple_lisp_interpreter.py

# coding: utf-8

Env = dict
Symbol = str
List = list
Number = (int, float)

def tokenize(chars):
    return chars.replace('(', ' ( ').replace(')', ' ) ').split()

def parse(program):
    return read_from_tokens(tokenize(program))


def read_from_tokens(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))
            #print(L)
        tokens.pop(0) # pop off ')'
        return L
    elif token == ')': raise SyntaxError('unexpected')
    else: return atom(token)
    
def atom(token):
    try: return int(token)
    except ValueError:
        try: return float(token)
        except ValueError:
            return Symbol(token)
    
def starndard_env():
    import operator as op
    env = Env()
    env.update({
    '+': op.add,
    '-': op.sub,
    '*': op.mul,
    '/': op.truediv,
    '>': op.gt,
    '<':op.lt,
    '>=': op.ge,
    '<=': op.le,
    '=': op.eq,
    })
    return env
global_env = starndard_env()
global_env


def eval(x, env=global_env):
    if isinstance(x, Symbol): return env.find(x)[x]
    elif not isinstance(x, List): return x
    else:
        proc = eval(x[0], env)
        args = [eval(arg, env) for arg in x[1:]]
        return proc(*args)


def repl(prompt="(scheme)> "):
    while True:
        val = eval(parse(input(prompt)))
        if val is not None:
            print(val)