Python实现Scheme

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

def parse(tokens):
    def read_from_tokens():
        if len(tokens) == 0:
            raise SyntaxError("意外结束")
        token = tokens.pop(0)
        if token == '(':
            L = []
            while tokens[0] != ')':
                L.append(read_from_tokens())
            tokens.pop(0)  # 消耗 ')'
            return L
        elif token == ')':
            raise SyntaxError("括号不匹配")
        else:
            return atom(token)
<pre class="brush:php;toolbar:false;"><pre class="brush:php;toolbar:false;">def atom(token):
    try: return int(token)
    except ValueError:
        try: return float(token)
        except ValueError:
            return token  # 符号如 x, +, lambda

return read_from_tokens()

class Env(dict):
    def __init__(self, parms=(), args=(), outer=None):
        self.update(zip(parms, args))
        self.outer = outer
    def find(self, var):
        return self if var in self else self.outer.find(var) if self.outer else None

import math
import operator as op
<p>def standard_env():
env = Env()
env.update({
'+': op.add, '-': op.sub, '<em>': op.mul, '/': op.truediv,
'>': op.gt, '<': op.lt, '>=': op.ge, '<=': op.le, '=': op.eq,
'abs': abs,
'pi': math.pi,
'sin': math.sin,
'cos': math.cos,
'sqrt': math.sqrt,
'pow': pow,
'begin': lambda </em>x: x[-1],
})
return env

def eval(x, env):
    if isinstance(x, str):           # 变量引用
        return env.find(x)[x]
    elif not isinstance(x, list):    # 字面量：数字
        return x
    elif x[0] == 'if':               # 条件
        _, test, conseq, alt = x
        exp = conseq if eval(test, env) else alt
        return eval(exp, env)
    elif x[0] == 'define':           # 变量定义
        _, var, expr = x
        env[var] = eval(expr, env)
    elif x[0] == 'lambda':           # 匿名函数
        _, parms, body = x
        return lambda *args: eval(body, Env(parms, args, env))
    else:                            # 函数调用
        proc = eval(x[0], env)
        args = [eval(arg, env) for arg in x[1:]]
        return proc(*args)

def repl(prompt='lis.py> '):
    env = standard_env()
    while True:
        try:
            source = input(prompt)
            if source.strip() == "quit":
                break
            val = eval(parse(tokenize(source)), env)
            if val is not None:
                print(schemestr(val))
        except Exception as e:
            print(f"错误: {e}")
<p>def schemestr(exp):
if isinstance(exp, list):
return '(' + ' '.join(map(schemestr, exp)) + ')'
return str(exp)

lis.py> (+ 2 3)
5
lis.py> (define x 10)
lis.py> (* x x)
100
lis.py> ((lambda (x) (* x x)) 5)
25
lis.py> (if (> 3 2) 42 0)
42