monad.py

from __future__ import print_function

from operator import add, mul
from functools import partial


class Monad(object):
    def __init__(self, value):
        self._value = value

    def bind(self, fn):
        raise NotImplementedError('Must implement bind')

    @classmethod
    def lift(cls, fn):
        def wrapper(value):
            return cls(cls(value).bind(fn))
        return wrapper


class Identity(Monad):
    def bind(self, fn):
        return fn(self._value)


class Maybe(Monad):
    def bind(self, fn):
        if self._value == None:
            return self
        else:
            return fn(self._value)


if __name__ == '__main__':
    mprint = Identity.lift(print)
    Identity('hello').bind(mprint)

    minc = Maybe.lift(partial(add, 1))
    mdouble = Maybe.lift(partial(mul, 2))
    mnone = Maybe.lift(lambda x: None)
    mprint = Maybe.lift(print)

    Maybe(1).bind(minc).bind(mdouble).bind(mprint)
    Maybe(1).bind(mnone).bind(minc).bind(mprint)