parse.py

'''
From Peter Novrig's "Lispy"

http://norvig.com/lispy.html

(I've written my own parsers, but it's a pain
in the ass and this one is at least as good
as any of mine.)
'''

# pylint: skip-file
# because I didn't write this!

# primary functions

def parse(program):
    "Read a Scheme expression from a string."
    return read_from_tokens(tokenize(program))

def schemify(exp):
    "Convert a Python object back into a Scheme-readable string."
    if isinstance(exp, List):
        return '(' + ' '.join(map(schemify, exp)) + ')'
    else:
        return str(exp)

# helpers

def tokenize(chars):
    "Convert a string of characters into a list of tokens."
    return chars.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)

Symbol = str          # A Scheme Symbol is implemented as a Python str
List   = list         # A Scheme List is implemented as a Python list
Number = (int, float) # A Scheme Number is implemented as a Python int or float