updated lis.py examples

02-array-seq/lispy/original/lis.py

@@ -0,0 +1,135 @@
+################ Lispy: Scheme Interpreter in Python 3.3+
+
+## (c) Peter Norvig, 2010-18; See http://norvig.com/lispy.html
+
+################ Imports and Types
+
+import math
+import operator as op
+from collections import ChainMap as Environment
+
+Symbol = str          # A Lisp Symbol is implemented as a Python str
+List   = list         # A Lisp List   is implemented as a Python list
+Number = (int, float) # A Lisp Number is implemented as a Python int or float
+
+class Procedure(object):
+    "A user-defined Scheme procedure."
+    def __init__(self, parms, body, env):
+        self.parms, self.body, self.env = parms, body, env
+    def __call__(self, *args):
+        env =  Environment(dict(zip(self.parms, args)), self.env)
+        for exp in self.body:
+            result = eval(exp, env)
+        return result
+
+
+################ Global Environment
+
+def standard_env():
+    "An environment with some Scheme standard procedures."
+    env = {}
+    env.update(vars(math)) # sin, cos, sqrt, pi, ...
+    env.update({
+        '+':op.add, '-':op.sub, '*':op.mul, '/':op.truediv,
+        '>':op.gt, '<':op.lt, '>=':op.ge, '<=':op.le, '=':op.eq,
+        'abs':     abs,
+        'append':  op.add,
+        'apply':   lambda proc, args: proc(*args),
+        'begin':   lambda *x: x[-1],
+        'car':     lambda x: x[0],
+        'cdr':     lambda x: x[1:],
+        'cons':    lambda x,y: [x] + y,
+        'eq?':     op.is_,
+        'equal?':  op.eq,
+        'length':  len,
+        'list':    lambda *x: list(x),
+        'list?':   lambda x: isinstance(x,list),
+        'map':     lambda *args: list(map(*args)),
+        'max':     max,
+        'min':     min,
+        'not':     op.not_,
+        'null?':   lambda x: x == [],
+        'number?': lambda x: isinstance(x, Number),
+        'procedure?': callable,
+        'round':   round,
+        'symbol?': lambda x: isinstance(x, Symbol),
+    })
+    return env
+
+global_env = standard_env()
+
+################ Parsing: parse, tokenize, and read_from_tokens
+
+def parse(program):
+    "Read a Scheme expression from a string."
+    return read_from_tokens(tokenize(program))
+
+def tokenize(s):
+    "Convert a string into a list of tokens."
+    return s.replace('(',' ( ').replace(')',' ) ').split()
+
+def read_from_tokens(tokens):
+    "Read an expression from a sequence of tokens."
+    if len(tokens) == 0:
+        raise SyntaxError('unexpected EOF while reading')
+    token = tokens.pop(0)
+    if '(' == token:
+        L = []
+        while tokens[0] != ')':
+            L.append(read_from_tokens(tokens))
+        tokens.pop(0) # pop off ')'
+        return L
+    elif ')' == token:
+        raise SyntaxError('unexpected )')
+    else:
+        return atom(token)
+
+def atom(token):
+    "Numbers become numbers; every other token is a symbol."
+    try: return int(token)
+    except ValueError:
+        try: return float(token)
+        except ValueError:
+            return Symbol(token)
+
+################ Interaction: A REPL
+
+def repl(prompt='lis.py> '):
+    "A prompt-read-eval-print loop."
+    while True:
+        val = eval(parse(input(prompt)))
+        if val is not None:
+            print(lispstr(val))
+
+def lispstr(exp):
+    "Convert a Python object back into a Lisp-readable string."
+    if isinstance(exp, List):
+        return '(' + ' '.join(map(lispstr, exp)) + ')'
+    else:
+        return str(exp)
+
+################ eval
+
+def eval(x, env=global_env):
+    "Evaluate an expression in an environment."
+    if isinstance(x, Symbol):      # variable reference
+        return env[x]
+    elif not isinstance(x, List):  # constant literal
+        return x
+    elif x[0] == 'quote':          # (quote exp)
+        (_, exp) = x
+        return exp
+    elif x[0] == 'if':             # (if test conseq alt)
+        (_, test, conseq, alt) = x
+        exp = (conseq if eval(test, env) else alt)
+        return eval(exp, env)
+    elif x[0] == 'define':         # (define var exp)
+        (_, var, exp) = x
+        env[var] = eval(exp, env)
+    elif x[0] == 'lambda':         # (lambda (var...) body)
+        (_, parms, *body) = x
+        return Procedure(parms, body, env)
+    else:                          # (proc arg...)
+        proc = eval(x[0], env)
+        args = [eval(exp, env) for exp in x[1:]]
+        return proc(*args)