parsers.py

class ParseError(Exception):
    pass

# Each parser function should take a list of characters and an index
# into that list, and return something that it parsed and a new index.

def parse_optionally(parser):
    """
    Parses or returns None
    """
    def _parse(chars, index):
        try:
            return parser(chars, index)
        except ParseError:
            return None, index
    return _parse

def parse_0_or_more(parser):
    """
    Takes a parser and returns a new parser which
    parses 0 or more of the object that the parser
    parses.
    """
    def _parse(chars, index):
        results = []
        while True:
            try:
                item, new_index = parser(index)
                results.append(item)
            except ParseError:
                return results
            index = new_index
    return _parse

def parse_at_least_one(parser):
    """
    Takes a parser and returns a new parser which
    parses one or more of the object that the parser
    parses.
    """
    def _parse(chars, index):
        first, index2 = parser(chars, index)
        rest, index3 = parse_0_or_more(parser)(chars, index2)
        return [first] + rest, index3
    return _parse

def parse_one_of(char_set):
    """
    Given a set of characters, makes a parser which parses one
    of the characters in the set
    """
    def _parse(chars, index):
        if chars[index] not in char_set:
            raise ParseError('Expected one of ' + char_set)
        return chars[index], index + 1

parse_char = lambda char: parse_one_of(set([char]))
parse_digit = parse_one_of(set('0123456789'))
parse_lower_letter = parse_one_of(set(string.ascii_lowercase))
parse_upper_letter = parse_one_of(set(string.ascii_uppercase))

def parse_number(chars, index):
    """
    Parses an integer.
    """
    neg, index2 = parse_optionally(parse_char('-'))(chars, index)
    digits, index3 = parse_at_least_one(parse_digit)(chars, index2)
    num = int(digits)
    if neg is not None:
        num = -num
    return num, index3