From ebda2935b1ea9fed4c299c0141f9c40586bd4f9c Mon Sep 17 00:00:00 2001
From: Peter Norvig <peter@norvig.com>
Date: Wed, 24 Oct 2018 15:09:42 -0700
Subject: [PATCH] Update lis.py

---
 py/lis.py | 120 +++++++++++++++++++++++-------------------------------
 1 file changed, 51 insertions(+), 69 deletions(-)

diff --git a/py/lis.py b/py/lis.py
index 065eab4..f81376a 100644
--- a/py/lis.py
+++ b/py/lis.py
@@ -1,22 +1,60 @@
-################ Lispy: Scheme Interpreter in Python
+################ Lispy: Scheme Interpreter in Python 3.3+
 
-## (c) Peter Norvig, 2010-16; See http://norvig.com/lispy.html
+## (c) Peter Norvig, 2010-18; See http://norvig.com/lispy.html
+
+################ Imports and Types
 
-from __future__ import division
 import math
 import operator as op
-
-try:
-    raw_input          # Python 2
-except NameError:
-    raw_input = input  # Python 3
-
-################ Types
+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
+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)
+        return eval(self.body, env)
+
+################ 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):
@@ -51,56 +89,12 @@ def atom(token):
         except ValueError:
             return Symbol(token)
 
-################ Environments
-
-def standard_env():
-    "An environment with some Scheme standard procedures."
-    env = 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':     map,
-        '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
-
-class Env(dict):
-    "An environment: a dict of {'var':val} pairs, with an outer Env."
-    def __init__(self, parms=(), args=(), outer=None):
-        self.update(zip(parms, args))
-        self.outer = outer
-    def find(self, var):
-        "Find the innermost Env where var appears."
-        return self if (var in self) else self.outer.find(var)
-
-global_env = standard_env()
-
 ################ Interaction: A REPL
 
 def repl(prompt='lis.py> '):
     "A prompt-read-eval-print loop."
     while True:
-        val = eval(parse(raw_input(prompt)))
+        val = eval(parse(input(prompt)))
         if val is not None:
             print(lispstr(val))
 
@@ -111,21 +105,12 @@ def lispstr(exp):
     else:
         return str(exp)
 
-################ Procedures
-
-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): 
-        return eval(self.body, Env(self.parms, args, self.env))
-
 ################ eval
 
 def eval(x, env=global_env):
     "Evaluate an expression in an environment."
     if isinstance(x, Symbol):      # variable reference
-        return env.find(x)[x]
+        return env[x]
     elif not isinstance(x, List):  # constant literal
         return x                
     elif x[0] == 'quote':          # (quote exp)
@@ -138,9 +123,6 @@ def eval(x, env=global_env):
     elif x[0] == 'define':         # (define var exp)
         (_, var, exp) = x
         env[var] = eval(exp, env)
-    elif x[0] == 'set!':           # (set! var exp)
-        (_, var, exp) = x
-        env.find(var)[var] = eval(exp, env)
     elif x[0] == 'lambda':         # (lambda (var...) body)
         (_, parms, body) = x
         return Procedure(parms, body, env)