simple_combinators.py

# primitives

def literal(lit):
  def parse(input):
      if len(input) > 0 and input[0] == lit:
          return input[1:]
      else:
          return False
  return parse

def success():
    return lambda input: input

def failure():
    return lambda input: False

# first-order combinators

def sequence(a, b):
    def parse(input):
        a_result = a(input)
        if isinstance(a_result, str):
            return b(a_result)
        else:
            return False
    return parse

def choice(a, b):
    def parse(input):
        a_result = a(input)
        if isinstance(a_result, str):
            return a_result
        else:
            b_result = b(input)
            return b_result
    return parse

# second-order combinators (use first order combinators)

def choice_of(parsers_list):
    return reduce(choice, parsers_list)

def sequence_of(parsers_list):
    return reduce(sequence, parsers_list)

# conveniences

def word(string):
    return sequence_of(map(literal, string))

def digit(string):
    return choice_of(map(literal, '0123456789'))