On Mon, Apr 13, 2009 at 03:11:42PM -0700, Michael Mossey wrote:
I know Maybe is both a functor and a monad, and I was thinking: what's the difference? They are both wrappers on types. Then I realized, the difference is: they have different class definitions.
In fact, every monad should be a functor, but not every functor is a monad. Being a monad is a much stronger condition than being a functor.
class Functor f where fmap :: (a->b) -> f a -> f b
(Note how fussy this definition would be in C++. It would be a kind of template, but would probably look a lot more complex and would require lengthy declarations.)
class Monad m where a >>= b :: m a -> (a -> m b) -> m b
Don't forget return :: a -> m a ! That's the other key method in the Monad class. (There are also >> and 'fail' but those are unimportant---the first is just a specialization of >>=, and fail is a hack). -Brent