foo.py

class ParsePair:
    def __init__(self, result, rest):
        self.result = result
        self.rest = rest


_DIGITS = frozenset('0123456789')


def digits_parser(string):
    if len(string) < 1:
        return []
    digits = []
    charstack = list(reversed(string))
    while charstack:
        c = charstack.pop()
        if c in _DIGITS:
            digits.append(c)
        else:
            charstack.append(c)
            break
    if not digits:
        return []
    return [ ParsePair(''.join(digits), ''.join(reversed(charstack))) ]


def char(c):
    def char_parser(string):
        if len(string) < 1:
            return []
        c_candidate, rest = string[0], string[1:]
        if c_candidate == c:
            return [ ParsePair(c_candidate, rest) ]
        return []

    return char_parser


def sequence(first_parser, second_parser):
    def combined_parser(string):
        return [second_parser(pp.rest) for pp in first_parser(string)]
    return combined_parser


def bind(parser, kf):
    def bound_parser(string):
        pairs = parser(string)
        next_pairs = [kf(pp.result)(pp.rest) for pp in pairs]
        return next_pairs
    return bound_parser


def pure(ready_result):
    return lambda string: [ ParsePair(ready_result, string) ]


def item_parser(string):
    if len(string) < 1:
        return []
    return [ ParsePair(string[0], string[1:]) ]