实现一个更好的Lisp解释器

class Symbol(str): pass

def Sym(s, symbol_table={}):
    "Find or create unique Symbol entry for str s in symbol table."
    if s not in symbol_table: symbol_table[s] = Symbol(s)
    return symbol_table[s]

_quote, _if, _set, _define, _lambda, _begin, _definemacro, = map(Sym, 
"quote   if   set!  define   lambda   begin   define-macro".split())

_quasiquote, _unquote, _unquotesplicing = map(Sym,
"quasiquote   unquote   unquote-splicing".split())

(define a 1)
(define b 2)

'(a b)                          ; => (a b) 
`(a b)                          ; => '(a b)

`(a ,b)                         ; => '(a 2), a 依旧是那个变量 a
(quasiquote (a (unquote b)))    ; => '(a 2), 和 `(a ,b) 等价

`(,a ,b)                        ; => '(1 2)

(define c '(3 4 5))
`(,b ,@c)                       ; => (2 3 4 5) 

def let(*args):
    args = list(args)
    x = cons(_let, args)
    require(x, len(args)>1)
    bindings, body = args[0], args[1:]
    require(x, all(isa(b, list) and len(b)==2 and isa(b[0], Symbol)
                   for b in bindings), "illegal binding list")
    vars, vals = zip(*bindings)
    return [[_lambda, list(vars)]+map(expand, body)] + map(expand, vals)

_append, _cons, _let = map(Sym("append cons let".split))

macro_table = {_let:let} ## More macros can go here

eval(parse("""(begin

(define-macro and (lambda args 
   (if (null? args) #t
       (if (= (length args) 1) (car args)
           `(if ,(car args) (and ,@(cdr args)) #f)))))

;; More macros can go here

)"""))

def eval(x, env=global_env):
    "Evaluate an expression in an environment."
    while True:
        if isa(x, Symbol):       # variable reference
            return env.find(x)[x]
        elif not isa(x, list):   # constant literal
            return x                
        elif x[0] is _quote:     # (quote exp)
            (_, exp) = x
            return exp
        elif x[0] is _if:        # (if test conseq alt)
            (_, test, conseq, alt) = x
            x = (conseq if eval(test, env) else alt) # 更新 x 
        elif x[0] is _set:       # (set! var exp)
            (_, var, exp) = x
            env.find(var)[var] = eval(exp, env)
            return None # 退出循环
        elif x[0] is _define:    # (define var exp)
            (_, var, exp) = x
            env[var] = eval(exp, env)
            return None # 退出循环
        elif x[0] is _lambda:    # (lambda (var*) exp)
            (_, vars, exp) = x
            return Procedure(vars, exp, env)
        elif x[0] is _begin:     # (begin exp+)
            for exp in x[1:-1]:
                eval(exp, env)
                x = x[-1]
        else:                    # (proc exp*)
            exps = [eval(exp, env) for exp in x]
            proc = exps.pop(0)
            if isa(proc, Procedure):
                x = proc.exp # 更新求值字符串为自定义过程体
                env = Env(proc.parms, exps, proc.env) # 更新环境体
            else:
                return proc(*exps)

class Procedure(object):
    "A user-defined Scheme procedure."
    def __init__(self, parms, exp, env):
        self.parms, self.exp, self.env = parms, exp, env
    def __call__(self, *args): 
        return eval(self.exp, Env(self.parms, args, self.env))

(call/cc (lambda (throw) 
           (+ 5 (* 10 (call/cc (lambda (escape) (* 100 (escape 3))))))))
;; => (+ 5 (* 10 3)) = 35  

(call/cc (lambda (throw) 
           (+ 5 (* 10 (call/cc (lambda (escape) (* 100 (throw 3))))))))
;; => 3 

def callcc(proc):
    "Call proc with current continuation; escape only"
    ball = RuntimeWarning("Sorry, can't continue this continuation any longer.")
    def throw(retval): ball.retval = retval; raise ball
    try:
        return proc(throw)
    except RuntimeWarning as w:
        if w is ball: return ball.retval
        else: raise w

class Env(dict):
    "An environment: a dict of {'var':val} pairs, with an outer Env."
    def __init__(self, parms=(), args=(), outer=None):
        # Bind parm list to corresponding args, or single parm to list of args
        self.outer = outer
        if isa(parms, Symbol): 
            self.update({parms:list(args)})
        else: 
            if len(args) != len(parms):
                raise TypeError('expected %s, given %s, ' 
                                % (to_string(parms), to_string(args)))
            self.update(zip(parms,args))
    def find(self, var):
        "Find the innermost Env where var appears."
        if var in self: return self
        elif self.outer is None: raise LookupError(var)
        else: return self.outer.find(var)

(define f (lambda (x) (set! 3 x)))
(define g (lambda (3) (if (x = 0))))
(define h (lambda (x) (if (x = 0) 1 2 3)))

def parse(inport):
    "Parse a program: read and expand/error-check it."
    # Backwards compatibility: given a str, convert it to an InPort
    if isinstance(inport, str): inport = InPort(StringIO.StringIO(inport))
    return expand(read(inport), toplevel=True)

def expand(x, toplevel=False):
    "Walk tree of x, making optimizations/fixes, and signaling SyntaxError."
    require(x, x!=[])                    # () => Error
    if not isa(x, list):                 # constant => unchanged
        return x
    elif x[0] is _quote:                 # (quote exp)
        require(x, len(x)==2)
        return x
    elif x[0] is _if:                    
        if len(x)==3: x = x + [None]     # (if t c) => (if t c None)
        require(x, len(x)==4)
        return map(expand, x)
    elif x[0] is _set:                   
        require(x, len(x)==3); 
        var = x[1]                       # (set! non-var exp) => Error
        require(x, isa(var, Symbol), "can set! only a symbol")
        return [_set, var, expand(x[2])]
    elif x[0] is _define or x[0] is _definemacro: 
        require(x, len(x)>=3)            
        _def, v, body = x[0], x[1], x[2:]
        if isa(v, list) and v:           # (define (f args) body)
            f, args = v[0], v[1:]        #  => (define f (lambda (args) body))
            return expand([_def, f, [_lambda, args]+body])
        else:
            require(x, len(x)==3)        # (define non-var/list exp) => Error
            require(x, isa(v, Symbol), "can define only a symbol")
            exp = expand(x[2])
            if _def is _definemacro:     
                require(x, toplevel, "define-macro only allowed at top level")
                proc = eval(exp)       
                require(x, callable(proc), "macro must be a procedure")
                macro_table[v] = proc    # (define-macro v proc)
                return None              #  => None; add v:proc to macro_table
            return [_define, v, exp]
    elif x[0] is _begin:
        if len(x)==1: return None        # (begin) => None
        else: return [expand(xi, toplevel) for xi in x]
    elif x[0] is _lambda:                # (lambda (x) e1 e2) 
        require(x, len(x)>=3)            #  => (lambda (x) (begin e1 e2))
        vars, body = x[1], x[2:]
        require(x, (isa(vars, list) and all(isa(v, Symbol) for v in vars))
                or isa(vars, Symbol), "illegal lambda argument list")
        exp = body[0] if len(body) == 1 else [_begin] + body
        return [_lambda, vars, expand(exp)]   
    elif x[0] is _quasiquote:            # `x => expand_quasiquote(x)
        require(x, len(x)==2)
        return expand_quasiquote(x[1])
    elif isa(x[0], Symbol) and x[0] in macro_table:
        return expand(macro_table[x[0]](*x[1:]), toplevel) # (m arg...) 
    else:                                #        => macroexpand if m isa macro
        return map(expand, x)            # (f arg...) => expand each

def require(x, predicate, msg="wrong length"):
    "Signal a syntax error if predicate is false."
    if not predicate: raise SyntaxError(to_string(x)+': '+msg)

def expand_quasiquote(x):
    """Expand `x => 'x; `,x => x; `(,@x y) => (append x y) """
    if not is_pair(x):
        return [_quote, x]
    require(x, x[0] is not _unquotesplicing, "can't splice here")
    if x[0] is _unquote:
        require(x, len(x)==2)
        return x[1]
    elif is_pair(x[0]) and x[0][0] is _unquotesplicing:
        require(x[0], len(x[0])==2)
        return [_append, x[0][1], expand_quasiquote(x[1:])]
    else:
        return [_cons, expand_quasiquote(x[0]), expand_quasiquote(x[1:])]

def add_globals(self):
    "Add some Scheme standard procedures."
    import math, cmath, operator as op
    self.update(vars(math))
    self.update(vars(cmath))
    self.update({
     '+':op.add, '-':op.sub, '*':op.mul, '/':op.div, 'not':op.not_, 
     '>':op.gt, '<':op.lt, '>=':op.ge, '<=':op.le, '=':op.eq, 
     'equal?':op.eq, 'eq?':op.is_, 'length':len, 'cons':lambda x,y:[x]+list(y), 
     'car':lambda x:x[0], 'cdr':lambda x:x[1:], 'append':op.add,  
     'list':lambda *x:list(x), 'list?': lambda x:isa(x,list),
     'null?':lambda x:x==[], 'symbol?':lambda x: isa(x, Symbol),
     'boolean?':lambda x: isa(x, bool), 'pair?':is_pair, 
     'port?': lambda x:isa(x,file), 'apply':lambda proc,l: proc(*l), 
     'eval':lambda x: eval(expand(x)), 'load':lambda fn: load(fn), 'call/cc':callcc,
     'open-input-file':open,'close-input-port':lambda p: p.file.close(), 
     'open-output-file':lambda f:open(f,'w'), 'close-output-port':lambda p: p.close(),
     'eof-object?':lambda x:x is eof_object, 'read-char':readchar,
     'read':read, 'write':lambda x,port=sys.stdout:port.write(to_string(x)),
     'display':lambda x,port=sys.stdout:port.write(x if isa(x,str) else to_string(x))})
    return self

global_env = add_globals(Env())