gistfile1.py

bind = function() # a -> m a
return_ = function() # m a -> (f a -> m b) -> m b

class Monad(TypeClass):
    __functions__ = (
        bind,
        return_,
    )

class Maybe(object): pass

class _Nothing(Maybe):
    unapply = lambda _: Nothing
Nothing = _Nothing()

class Just(Maybe):
    __init__ = lambda self, value: setattr(self, '_value', value)
    unapply = lambda self: (Just, self._value)
    __eq__ = lambda self, other: self._value == other._value


instance(Monad, Maybe)(
    (return_, lambda x: Just(x)),
    (bind, lambda s, f: match(s)(
        (Nothing, lambda _: Nothing),
        ((Just, undefined): lambda _, v: f(v))
    )
)

v = bind(Just(1), lambda x: Just(2 * x))
assert v == Just(2)

v = bind(Nothing, lambda x: Just(2 * x))
assert v == Nothing