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Luciano Ramalho
33
02-array-seq/lispy/py3.9-no-hints/README.md
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33
02-array-seq/lispy/py3.9-no-hints/README.md
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# Changes from the original
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While adapting Peter Norvig's [lis.py](https://github.com/norvig/pytudes/blob/705c0a335c1811a203e79587d7d41865cf7f41c7/py/lis.py) for
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use in _Fluent Python, Second Edition_, I made a few changes for didactic reasons.
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_Luciano Ramalho_
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## Major changes
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* Make the `lambda` form accept more than one expression as the body. This is consistent with _Scheme_ syntax, and provides a useful example for the book. To implement this:
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* In `Procedure.__call__`: evaluate `self.body` as a list of expressions, instead of a single expression. Return the value of the last expression.
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* In `evaluate()`: when processing `lambda`, unpack expression into `(_, parms, *body)`, to accept a list of expressions as the body.
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* Remove the `global_env` global `dict`. It is only used as a default value for the `env` parameter in `evaluate()`, but it is unsafe to use mutable data structures as parameter default values. To implement this:
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* In `repl()`: create local variable `global_env` and pass it as the `env` paramater of `evaluate()`.
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* In `evaluate()`, remove `global_env` default value for `env`.
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* Rewrite the custom test script
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[lispytest.py](https://github.com/norvig/pytudes/blob/705c0a335c1811a203e79587d7d41865cf7f41c7/py/lispytest.py) as
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[lis_test.py](https://github.com/fluentpython/example-code-2e/blob/master/02-array-seq/lispy/py3.9/lis_test.py):
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a standard [pytest](https://docs.pytest.org) test suite including new test cases, preserving all Norvig's the test cases for
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[lis.py](https://github.com/norvig/pytudes/blob/705c0a335c1811a203e79587d7d41865cf7f41c7/py/lis.py)
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but removing the test cases for the features implemented only in
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[lispy.py](https://github.com/norvig/pytudes/blob/705c0a335c1811a203e79587d7d41865cf7f41c7/py/lispy.py)
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## Minor changes
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Cosmetic changes to make the code look more familiar to
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Python programmers, the audience of _Fluent Python_.
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* Rename `eval()` to `evaluate()`, to avoid confusion with Python's `eval` built-in function.
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* Refer to the list class as `list` instead of aliasing as `List`, to avoid confusion with `typing.List` which is often imported as `List`.
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* Import `collections.ChainMap` as `ChainMap` instead of `Environment`.
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142
02-array-seq/lispy/py3.9-no-hints/lis.py
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142
02-array-seq/lispy/py3.9-no-hints/lis.py
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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 typing import Any
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Symbol = str # A Lisp Symbol is implemented as a Python str
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Number = (int, float) # A Lisp Number is implemented as a Python int or float
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class Procedure:
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"A user-defined Scheme procedure."
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def __init__(self, parms, body, env):
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self.parms, self.body, self.env = parms, body, env
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def __call__(self, *args):
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env = ChainMap(dict(zip(self.parms, args)), 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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################ Global Environment
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def standard_env():
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"An environment with some Scheme standard procedures."
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env = {}
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env.update(vars(math)) # sin, cos, sqrt, pi, ...
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env.update({
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'+':op.add, '-':op.sub, '*':op.mul, '/':op.truediv,
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'>':op.gt, '<':op.lt, '>=':op.ge, '<=':op.le, '=':op.eq,
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'abs': abs,
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'append': op.add,
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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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'eq?': op.is_,
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'equal?': op.eq,
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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, Number),
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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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################ Parsing: parse, tokenize, and read_from_tokens
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def parse(program):
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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):
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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):
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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):
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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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################ Interaction: A REPL
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def repl(prompt: str = 'lis.py> ') -> None:
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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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val = evaluate(parse(input(prompt)), 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):
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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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################ eval
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def evaluate(x, env):
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"Evaluate an expression in an environment."
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if isinstance(x, Symbol): # variable reference
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return env[x]
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elif not isinstance(x, list): # constant literal
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return x
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elif x[0] == 'quote': # (quote exp)
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(_, exp) = x
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return exp
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elif x[0] == 'if': # (if test conseq alt)
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(_, test, conseq, alt) = x
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exp = (conseq if evaluate(test, env) else alt)
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return evaluate(exp, env)
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elif x[0] == 'define': # (define var exp)
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(_, var, exp) = x
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env[var] = evaluate(exp, env)
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elif x[0] == 'lambda': # (lambda (var...) body)
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(_, parms, *body) = x
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return Procedure(parms, body, env)
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else: # (proc arg...)
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proc = evaluate(x[0], env)
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args = [evaluate(exp, env) for exp in x[1:]]
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return proc(*args)
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166
02-array-seq/lispy/py3.9-no-hints/lis_test.py
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166
02-array-seq/lispy/py3.9-no-hints/lis_test.py
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from pytest import mark, fixture
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from lis import parse, evaluate, standard_env
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############################################################# tests for parse
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@mark.parametrize( 'source, expected', [
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('7', 7),
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('x', 'x'),
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('(sum 1 2 3)', ['sum', 1, 2, 3]),
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('(+ (* 2 100) (* 1 10))', ['+', ['*', 2, 100], ['*', 1, 10]]),
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('99 100', 99), # parse stops at the first complete expression
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('(a)(b)', ['a']),
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])
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def test_parse(source: str, expected):
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got = parse(source)
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assert got == expected
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########################################################## tests for evaluate
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# Norvig's tests are not isolated: they assume the
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# same environment from first to last test.
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global_env_for_first_test = standard_env()
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@mark.parametrize( 'source, expected', [
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("(quote (testing 1 (2.0) -3.14e159))", ['testing', 1, [2.0], -3.14e159]),
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("(+ 2 2)", 4),
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("(+ (* 2 100) (* 1 10))", 210),
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("(if (> 6 5) (+ 1 1) (+ 2 2))", 2),
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("(if (< 6 5) (+ 1 1) (+ 2 2))", 4),
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("(define x 3)", None),
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("x", 3),
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("(+ x x)", 6),
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("((lambda (x) (+ x x)) 5)", 10),
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("(define twice (lambda (x) (* 2 x)))", None),
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("(twice 5)", 10),
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("(define compose (lambda (f g) (lambda (x) (f (g x)))))", None),
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("((compose list twice) 5)", [10]),
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("(define repeat (lambda (f) (compose f f)))", None),
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("((repeat twice) 5)", 20),
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("((repeat (repeat twice)) 5)", 80),
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("(define fact (lambda (n) (if (<= n 1) 1 (* n (fact (- n 1))))))", None),
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("(fact 3)", 6),
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("(fact 50)", 30414093201713378043612608166064768844377641568960512000000000000),
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("(define abs (lambda (n) ((if (> n 0) + -) 0 n)))", None),
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("(list (abs -3) (abs 0) (abs 3))", [3, 0, 3]),
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("""(define combine (lambda (f)
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(lambda (x y)
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(if (null? x) (quote ())
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(f (list (car x) (car y))
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((combine f) (cdr x) (cdr y)))))))""", None),
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("(define zip (combine cons))", None),
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("(zip (list 1 2 3 4) (list 5 6 7 8))", [[1, 5], [2, 6], [3, 7], [4, 8]]),
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("""(define riff-shuffle (lambda (deck)
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(begin
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(define take (lambda (n seq) (if (<= n 0) (quote ()) (cons (car seq) (take (- n 1) (cdr seq))))))
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(define drop (lambda (n seq) (if (<= n 0) seq (drop (- n 1) (cdr seq)))))
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(define mid (lambda (seq) (/ (length seq) 2)))
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((combine append) (take (mid deck) deck) (drop (mid deck) deck)))))""", None),
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("(riff-shuffle (list 1 2 3 4 5 6 7 8))", [1, 5, 2, 6, 3, 7, 4, 8]),
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("((repeat riff-shuffle) (list 1 2 3 4 5 6 7 8))", [1, 3, 5, 7, 2, 4, 6, 8]),
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("(riff-shuffle (riff-shuffle (riff-shuffle (list 1 2 3 4 5 6 7 8))))", [1,2,3,4,5,6,7,8]),
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])
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def test_evaluate(source, expected):
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got = evaluate(parse(source), global_env_for_first_test)
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assert got == expected
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@fixture
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def std_env():
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return standard_env()
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# tests for each of the cases in evaluate
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def test_evaluate_variable():
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env = dict(x=10)
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source = 'x'
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expected = 10
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got = evaluate(parse(source), env)
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assert got == expected
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def test_evaluate_literal(std_env):
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source = '3.3'
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expected = 3.3
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got = evaluate(parse(source), std_env)
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assert got == expected
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def test_evaluate_quote(std_env):
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source = '(quote (1.1 is not 1))'
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expected = [1.1, 'is', 'not', 1]
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got = evaluate(parse(source), std_env)
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assert got == expected
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def test_evaluate_if_true(std_env) -> None:
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source = '(if 1 10 no-such-thing)'
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expected = 10
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got = evaluate(parse(source), std_env)
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assert got == expected
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def test_evaluate_if_false(std_env) -> None:
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source = '(if 0 no-such-thing 20)'
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expected = 20
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got = evaluate(parse(source), std_env)
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assert got == expected
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def test_define(std_env) -> None:
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source = '(define answer (* 6 7))'
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got = evaluate(parse(source), std_env)
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assert got is None
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assert std_env['answer'] == 42
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def test_lambda(std_env) -> None:
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source = '(lambda (a b) (if (>= a b) a b))'
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func = evaluate(parse(source), std_env)
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assert func.parms == ['a', 'b']
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assert func.body == [['if', ['>=', 'a', 'b'], 'a', 'b']]
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assert func.env is std_env
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assert func(1, 2) == 2
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assert func(3, 2) == 3
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def test_begin(std_env) -> None:
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source = """
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(begin
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(define x (* 2 3))
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(* x 7)
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)
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"""
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got = evaluate(parse(source), std_env)
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assert got == 42
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def test_invocation_builtin_car(std_env) -> None:
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source = '(car (quote (11 22 33)))'
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got = evaluate(parse(source), std_env)
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assert got == 11
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def test_invocation_builtin_append(std_env) -> None:
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source = '(append (quote (a b)) (quote (c d)))'
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got = evaluate(parse(source), std_env)
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assert got == ['a', 'b', 'c', 'd']
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def test_invocation_builtin_map(std_env) -> None:
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source = '(map (lambda (x) (* x 2)) (quote (1 2 3))))'
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got = evaluate(parse(source), std_env)
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assert got == [2, 4, 6]
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def test_invocation_user_procedure(std_env):
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source = """
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(begin
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(define max (lambda (a b) (if (>= a b) a b)))
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(max 22 11)
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)
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"""
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got = evaluate(parse(source), std_env)
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assert got == 22
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