https://norvig.com/lispy2.html with its test suite updated to work on Python 3

lispy.py

"""
Scheme Interpreter in Python

(c) Peter Norvig, 2010; See http://norvig.com/lispy2.html

Fixes to run on Python 3 and minor style tweaks from Warren Henning, 2022 <[email protected]>
"""

import cmath
import math
import operator as op
import re
import sys
from io import StringIO, IOBase


class Symbol(str):
    pass


def make_symbol(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(make_symbol,
"quote   if   set!  define   lambda   begin   define-macro".split())

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


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 lispy_eval(self.exp, Env(self.parms, args, self.env))


# parse, read, and user interaction

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(inport))
    return expand(read(inport), toplevel=True)


eof_object = Symbol('#<eof-object>')  # Note: uninterned; can't be read


class InPort(object):
    """An input port. Retains a line of chars."""
    tokenizer = r"""\s*(,@|[('`,)]|"(?:[\\].|[^\\"])*"|;.*|[^\s('"`,;)]*)(.*)"""

    def __init__(self, file):
        self.file = file
        self.line = ''

    def next_token(self):
        """Return the next token, reading new text into line buffer if needed."""
        while True:
            if self.line == '':
                self.line = self.file.readline()
            if self.line == '':
                return eof_object
            token, self.line = re.match(InPort.tokenizer, self.line).groups()
            if token != '' and not token.startswith(';'):
                return token


def readchar(inport):
    """Read the next character from an input port."""
    if inport.line != '':
        ch, inport.line = inport.line[0], inport.line[1:]
        return ch
    else:
        return inport.file.read(1) or eof_object


def read(inport):
    """Read a Scheme expression from an input port."""

    def read_ahead(token):
        if '(' == token:
            L = []
            while True:
                token = inport.next_token()
                if token == ')':
                    return L
                else:
                    L.append(read_ahead(token))
        elif ')' == token:
            raise SyntaxError('unexpected )')
        elif token in quotes:
            return [quotes[token], read(inport)]
        elif token is eof_object:
            raise SyntaxError('unexpected EOF in list')
        else:
            return atom(token)

    # body of read:
    token1 = inport.next_token()
    return eof_object if token1 is eof_object else read_ahead(token1)


quotes = {"'": _quote, "`": _quasiquote, ",": _unquote, ",@": _unquotesplicing}


def atom(token):
    """Numbers become numbers; #t and #f are booleans; "..." string; otherwise Symbol."""
    if token == '#t':
        return True
    elif token == '#f':
        return False
    elif token[0] == '"':
        return bytes(token[1:-1], encoding='utf-8').decode('unicode_escape')
    try:
        return int(token)
    except ValueError:
        try:
            return float(token)
        except ValueError:
            try:
                return complex(token.replace('i', 'j', 1))
            except ValueError:
                return make_symbol(token)


def to_string(x):
    """Convert a Python object back into a Lisp-readable string."""
    if x is True:
        return "#t"
    elif x is False:
        return "#f"
    elif isa(x, Symbol):
        return x
    elif isa(x, str):
        return '"%s"' % x.encode('string_escape').replace('"', r'\"')
    elif isa(x, list):
        return '(' + ' '.join(map(to_string, x)) + ')'
    elif isa(x, complex):
        return str(x).replace('j', 'i')
    else:
        return str(x)


def load(filename):
    """Eval every expression from a file."""
    repl(None, InPort(open(filename)), None)


def repl(prompt='lispy> ', inport=InPort(sys.stdin), out=sys.stderr):
    """A prompt-read-eval-print loop."""
    sys.stderr.write("Lispy version 2.0\n")
    while True:
        try:
            if prompt:
                sys.stderr.write(prompt)
            x = parse(inport)
            if x is eof_object:
                return
            val = lispy_eval(x)
            if val is not None and out:
                print(to_string(val), file=out)
        except Exception as e:
            print(type(e).__name__, ': ', e)


# Environment class

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)


def is_pair(x): return x != [] and isa(x, list)


def cons(x, y): return [x] + y


def callcc(proc):
    """Call proc with current continuation; escape only"""
    ball = RuntimeWarning("Sorry, can't continue this continuation any longer.")
    ball.retval = None

    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


def add_globals(self):
    """Add some Scheme standard procedures."""

    self.update(vars(math))
    self.update(vars(cmath))
    self.update({
        '+': op.add, '-': op.sub, '*': op.mul, '/': op.truediv, 'not': op.not_,
        '>': op.gt, '<': op.lt, '>=': op.ge, '<=': op.le, '=': op.eq,
        'equal?': op.eq, 'eq?': op.is_, 'length': len, 'cons': cons,
        '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, IOBase), 'apply': lambda proc, l: proc(*l),
        'eval': lambda x: lispy_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


isa = isinstance

global_env = add_globals(Env())


# eval (tail recursive)

def lispy_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)
            (_, predicate, conseq, alt) = x
            x = (conseq if lispy_eval(predicate, env) else alt)
        elif x[0] is _set:  # (set! var exp)
            (_, var, exp) = x
            env.find(var)[var] = lispy_eval(exp, env)
            return None
        elif x[0] is _define:  # (define var exp)
            (_, var, exp) = x
            env[var] = lispy_eval(exp, env)
            return None
        elif x[0] is _lambda:  # (lambda (var*) exp)
            (_, lambda_vars, exp) = x
            return Procedure(lambda_vars, exp, env)
        elif x[0] is _begin:  # (begin exp+)
            for exp in x[1:-1]:
                lispy_eval(exp, env)
            x = x[-1]
        else:  # (proc exp*)
            exps = [lispy_eval(exp, env) for exp in x]
            proc = exps.pop(0)
            if isa(proc, Procedure):
                x = proc.exp
                env = Env(proc.parms, exps, proc.env)
            else:
                return proc(*exps)


# expand

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 list(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 = lispy_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))
        lambda_vars, body = x[1], x[2:]
        require(x, (isa(lambda_vars, list) and all(isa(v, Symbol) for v in lambda_vars))
                or isa(lambda_vars, Symbol), "illegal lambda argument list")
        exp = body[0] if len(body) == 1 else [_begin] + body
        return [_lambda, 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 list(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)


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


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 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)] + list(map(expand, body))] + list(map(expand, vals))


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

lispy_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 also go here

)"""))

lis_tests = [
    ("(quote (testing 1 (2.0) -3.14e159))", ['testing', 1, [2.0], -3.14e159]),
    ("(+ 2 2)", 4),
    ("(+ (* 2 100) (* 1 10))", 210),
    ("(if (> 6 5) (+ 1 1) (+ 2 2))", 2),
    ("(if (< 6 5) (+ 1 1) (+ 2 2))", 4),
    ("(define x 3)", None), ("x", 3), ("(+ x x)", 6),
    ("(begin (define x 1) (set! x (+ x 1)) (+ x 1))", 3),
    ("((lambda (x) (+ x x)) 5)", 10),
    ("(define twice (lambda (x) (* 2 x)))", None), ("(twice 5)", 10),
    ("(define compose (lambda (f g) (lambda (x) (f (g x)))))", None),
    ("((compose list twice) 5)", [10]),
    ("(define repeat (lambda (f) (compose f f)))", None),
    ("((repeat twice) 5)", 20), ("((repeat (repeat twice)) 5)", 80),
    ("(define fact (lambda (n) (if (<= n 1) 1 (* n (fact (- n 1))))))", None),
    ("(fact 3)", 6),
    ("(fact 50)", 30414093201713378043612608166064768844377641568960512000000000000),
    ("(define abs (lambda (n) ((if (> n 0) + -) 0 n)))", None),
    ("(list (abs -3) (abs 0) (abs 3))", [3, 0, 3]),
    ("""(define combine (lambda (f)
    (lambda (x y)
      (if (null? x) (quote ())
          (f (list (car x) (car y))
             ((combine f) (cdr x) (cdr y)))))))""", None),
    ("(define zip (combine cons))", None),
    ("(zip (list 1 2 3 4) (list 5 6 7 8))", [[1, 5], [2, 6], [3, 7], [4, 8]]),
    ("""(define riff-shuffle (lambda (deck) (begin
    (define take (lambda (n seq) (if (<= n 0) (quote ()) (cons (car seq) (take (- n 1) (cdr seq))))))
    (define drop (lambda (n seq) (if (<= n 0) seq (drop (- n 1) (cdr seq)))))
    (define mid (lambda (seq) (/ (length seq) 2)))
    ((combine append) (take (mid deck) deck) (drop (mid deck) deck)))))""", None),
    ("(riff-shuffle (list 1 2 3 4 5 6 7 8))", [1, 5, 2, 6, 3, 7, 4, 8]),
    ("((repeat riff-shuffle) (list 1 2 3 4 5 6 7 8))", [1, 3, 5, 7, 2, 4, 6, 8]),
    ("(riff-shuffle (riff-shuffle (riff-shuffle (list 1 2 3 4 5 6 7 8))))", [1, 2, 3, 4, 5, 6, 7, 8]),
]

lispy_tests = [
    ("()", SyntaxError), ("(set! x)", SyntaxError),
    ("(define 3 4)", SyntaxError),
    ("(quote 1 2)", SyntaxError), ("(if 1 2 3 4)", SyntaxError),
    ("(lambda 3 3)", SyntaxError), ("(lambda (x))", SyntaxError),
    ("""(if (= 1 2) (define-macro a 'a) 
     (define-macro a 'b))""", SyntaxError),
    ("(define (twice x) (* 2 x))", None), ("(twice 2)", 4),
    ("(twice 2 2)", TypeError),
    ("(define lyst (lambda items items))", None),
    ("(lyst 1 2 3 (+ 2 2))", [1, 2, 3, 4]),
    ("(if 1 2)", 2),
    ("(if (= 3 4) 2)", None),
    ("(define ((account bal) amt) (set! bal (+ bal amt)) bal)", None),
    ("(define a1 (account 100))", None),
    ("(a1 0)", 100), ("(a1 10)", 110), ("(a1 10)", 120),
    ("""(define (newton guess function derivative epsilon)
    (define guess2 (- guess (/ (function guess) (derivative guess))))
    (if (< (abs (- guess guess2)) epsilon) guess2
        (newton guess2 function derivative epsilon)))""", None),
    ("""(define (square-root a)
    (newton 1 (lambda (x) (- (* x x) a)) (lambda (x) (* 2 x)) 1e-8))""", None),
    ("(> (square-root 200.) 14.14213)", True),
    ("(< (square-root 200.) 14.14215)", True),
    ("(= (square-root 200.) (sqrt 200.))", True),
    ("""(define (sum-squares-range start end)
         (define (sumsq-acc start end acc)
            (if (> start end) acc (sumsq-acc (+ start 1) end (+ (* start start) acc))))
         (sumsq-acc start end 0))""", None),
    ("(sum-squares-range 1 3000)", 9004500500),  # Tests tail recursion
    ("(call/cc (lambda (throw) (+ 5 (* 10 (throw 1))))) ;; throw", 1),
    ("(call/cc (lambda (throw) (+ 5 (* 10 1)))) ;; do not throw", 15),
    ("""(call/cc (lambda (throw) 
         (+ 5 (* 10 (call/cc (lambda (escape) (* 100 (escape 3)))))))) ; 1 level""", 35),
    ("""(call/cc (lambda (throw) 
         (+ 5 (* 10 (call/cc (lambda (escape) (* 100 (throw 3)))))))) ; 2 levels""", 3),
    ("""(call/cc (lambda (throw) 
         (+ 5 (* 10 (call/cc (lambda (escape) (* 100 1))))))) ; 0 levels""", 1005),
    ("(* 1i 1i)", -1), ("(sqrt -1)", 1j),
    ("(let ((a 1) (b 2)) (+ a b))", 3),
    ("(let ((a 1) (b 2 3)) (+ a b))", SyntaxError),
    ("(and 1 2 3)", 3), ("(and (> 2 1) 2 3)", 3), ("(and)", True),
    ("(and (> 2 1) (> 2 3))", False),
    ("(define-macro unless (lambda args `(if (not ,(car args)) (begin ,@(cdr args))))) ; test `", None),
    ("(unless (= 2 (+ 1 1)) (display 2) 3 4)", None),
    (r'(unless (= 4 (+ 1 1)) (display 2) (display "\n") 3 4)', 4),
    ("(quote x)", 'x'),
    ("(quote (1 2 three))", [1, 2, 'three']),
    ("'x", 'x'),
    ("'(one 2 3)", ['one', 2, 3]),
    ("(define L (list 1 2 3))", None),
    ("`(testing ,@L testing)", ['testing', 1, 2, 3, 'testing']),
    ("`(testing ,L testing)", ['testing', [1, 2, 3], 'testing']),
    ("`,@L", SyntaxError),
    ("""'(1 ;test comments '
     ;skip this line
     2 ; more ; comments ; ) )
     3) ; final comment""", [1, 2, 3]),
]


def run_lispy_tests(tests, print_case=False):
    """For each (exp, expected) test case, see if eval(parse(exp)) == expected."""
    print('Running tests...', file=sys.stderr)
    fails = 0
    for (x, expected) in tests:
        try:
            result = lispy_eval(parse(x))
            if print_case:
                print(x, '=>', to_string(result))
            ok = (result == expected)
        except Exception as e:
            if print_case:
                print(x, '=raises=>', type(e).__name__, e)
            ok = issubclass(expected, Exception) and isinstance(e, expected)
        if not ok:
            fails += 1
            print('FAIL!!!  Expected', expected, file=sys.stderr)
    if fails != 0:
        print(fails, 'out of', len(tests), 'tests fail.', file=sys.stderr)
        sys.exit(1)
    else:
        print('All', len(tests), 'tests pass.', file=sys.stderr)


if __name__ == '__main__':
    run_lispy_tests(lis_tests + lispy_tests, print_case=False)

    repl()