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Luciano Ramalho
209
02-array-seq/lispy/py3.9/lis.py
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209
02-array-seq/lispy/py3.9/lis.py
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#!/usr/bin/env python
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################ Lispy: Scheme Interpreter in Python 3.9
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## (c) Peter Norvig, 2010-18; See http://norvig.com/lispy.html
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## Minor edits for Fluent Python, Second Edition (O'Reilly, 2021)
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## by Luciano Ramalho, adding type hints and pattern matching.
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################ Imports and Types
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import math
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import operator as op
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from collections import ChainMap
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from itertools import chain
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from typing import Any, Union, NoReturn
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Symbol = str
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Atom = Union[float, int, Symbol]
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Expression = Union[Atom, list]
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################ Parsing: parse, tokenize, and read_from_tokens
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def parse(program: str) -> Expression:
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"Read a Scheme expression from a string."
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return read_from_tokens(tokenize(program))
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def tokenize(s: str) -> list[str]:
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"Convert a string into a list of tokens."
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return s.replace('(', ' ( ').replace(')', ' ) ').split()
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def read_from_tokens(tokens: list[str]) -> Expression:
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"Read an expression from a sequence of tokens."
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if len(tokens) == 0:
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raise SyntaxError('unexpected EOF while reading')
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token = tokens.pop(0)
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if '(' == token:
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exp = []
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while tokens[0] != ')':
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exp.append(read_from_tokens(tokens))
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tokens.pop(0) # discard ')'
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return exp
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elif ')' == token:
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raise SyntaxError('unexpected )')
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else:
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return parse_atom(token)
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def parse_atom(token: str) -> Atom:
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"Numbers become numbers; every other token is a symbol."
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try:
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return int(token)
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except ValueError:
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try:
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return float(token)
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except ValueError:
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return Symbol(token)
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################ Global Environment
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class Environment(ChainMap[Symbol, Any]):
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"A ChainMap that allows changing an item in-place."
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def change(self, key: Symbol, value: object) -> None:
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"Find where key is defined and change the value there."
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for map in self.maps:
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if key in map:
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map[key] = value # type: ignore[index]
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return
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raise KeyError(key)
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def standard_env() -> Environment:
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"An environment with some Scheme standard procedures."
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env = Environment()
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env.update(vars(math)) # sin, cos, sqrt, pi, ...
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env.update({
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'+': op.add,
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'-': op.sub,
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'*': op.mul,
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'/': op.truediv,
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'//': op.floordiv,
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'>': op.gt,
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'<': op.lt,
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'>=': op.ge,
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'<=': op.le,
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'=': op.eq,
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'abs': abs,
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'append': lambda *args: list(chain(*args)),
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'apply': lambda proc, args: proc(*args),
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'begin': lambda *x: x[-1],
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'car': lambda x: x[0],
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'cdr': lambda x: x[1:],
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'cons': lambda x, y: [x] + y,
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'display': lambda x: print(lispstr(x)),
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'eq?': op.is_,
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'equal?': op.eq,
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'filter': lambda *args: list(filter(*args)),
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'length': len,
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'list': lambda *x: list(x),
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'list?': lambda x: isinstance(x, list),
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'map': lambda *args: list(map(*args)),
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'max': max,
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'min': min,
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'not': op.not_,
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'null?': lambda x: x == [],
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'number?': lambda x: isinstance(x, (int, float)),
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'procedure?': callable,
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'round': round,
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'symbol?': lambda x: isinstance(x, Symbol),
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})
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return env
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################ Interaction: A REPL
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def repl(prompt: str = 'lis.py> ') -> NoReturn:
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"A prompt-read-eval-print loop."
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global_env = standard_env()
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while True:
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ast = parse(input(prompt))
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val = evaluate(ast, global_env)
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if val is not None:
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print(lispstr(val))
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def lispstr(exp: object) -> str:
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"Convert a Python object back into a Lisp-readable string."
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if isinstance(exp, list):
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return '(' + ' '.join(map(lispstr, exp)) + ')'
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else:
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return str(exp)
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################ Evaluator
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# tag::EVAL_IF_TOP[]
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def evaluate(exp: Expression, env: Environment) -> Any:
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"Evaluate an expression in an environment."
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if isinstance(exp, Symbol): # variable reference
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return env[exp]
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# end::EVAL_IF_TOP[]
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elif not isinstance(exp, list): # constant literal
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return exp
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# tag::EVAL_IF_MIDDLE[]
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elif exp[0] == 'quote': # (quote exp)
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(_, x) = exp
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return x
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elif exp[0] == 'if': # (if test conseq alt)
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(_, test, consequence, alternative) = exp
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if evaluate(test, env):
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return evaluate(consequence, env)
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else:
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return evaluate(alternative, env)
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elif exp[0] == 'lambda': # (lambda (parm…) body…)
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(_, parms, *body) = exp
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return Procedure(parms, body, env)
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elif exp[0] == 'define':
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(_, name, value_exp) = exp
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env[name] = evaluate(value_exp, env)
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# end::EVAL_IF_MIDDLE[]
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elif exp[0] == 'set!':
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(_, name, value_exp) = exp
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env.change(name, evaluate(value_exp, env))
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else: # (proc arg…)
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(func_exp, *args) = exp
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proc = evaluate(func_exp, env)
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args = [evaluate(arg, env) for arg in args]
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return proc(*args)
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class Procedure:
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"A user-defined Scheme procedure."
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def __init__(
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self, parms: list[Symbol], body: list[Expression], env: Environment
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):
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self.parms = parms
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self.body = body
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self.env = env
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def __call__(self, *args: Expression) -> Any:
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local_env = dict(zip(self.parms, args))
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env = Environment(local_env, self.env)
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for exp in self.body:
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result = evaluate(exp, env)
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return result
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################ command-line interface
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def run(source: str) -> Any:
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global_env = standard_env()
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tokens = tokenize(source)
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while tokens:
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exp = read_from_tokens(tokens)
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result = evaluate(exp, global_env)
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return result
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def main(args: list[str]) -> None:
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if len(args) == 1:
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with open(args[0]) as fp:
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run(fp.read())
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else:
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repl()
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if __name__ == '__main__':
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import sys
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main(sys.argv[1:])
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