pretty ugly

pretty.py

#!/usr/bin/env python3

def word(w):
    def f(tokens):
        return tokens[1:] if len(tokens) > 0 and tokens[0] == w else False
    return f

def zero_or_more(parser):
    def f(tokens):
        if len(tokens) == 0:
            return []
        while True:
            result = parser(tokens)
            if not result:
                return tokens
            tokens = result
        return tokens
    return f

def then(parser1, parser2):
    def f(tokens):
        result = parser1(tokens)
        if not result:
            return False
        return parser2(result)
    return f

def either(parser1, parser2):
    def f(tokens):
        result = parser1(tokens)
        if result is not False:
            return result
        return parser2(tokens)
    return f

def ok(parser, s):
    return True if parser(s.split()) == [] else False

if __name__ == "__main__":
    pretty = word("pretty")
    its = word("it's")
    ugly = word("ugly")
    zero_or_more_pretty = zero_or_more(pretty)
    parser = then(then(its, zero_or_more_pretty), either(ugly, pretty))
    assert(ok(parser, "it's pretty ugly"))
    assert(ok(parser, "it's pretty pretty"))
    assert(ok(parser, "it's pretty"))
    assert(ok(parser, "it's pretty pretty pretty pretty"))
    assert(ok(parser, "it's ugly"))
    assert(not ok(parser, "it's bad"))
    assert(not ok(parser, "it's ugly pretty"))