On Sun, Jan 21, 2007 at 12:37:02PM -0600, Nicolas Frisby wrote:
I have another small mtl complaint: ReaderT, for example, requires the base type to be a Monad in order to make it a Functor. So it's Monad m => Functor (ReaderT r m) instead of Functor f => Functor (ReaderT r f), which is what's actually necessary for the instance. Remember, I did say _small_ complaint.
I realize this has a bit to do with the class heirarchy and that's a sensitive issue. Regarding backwards compatibility and interface changes, note that the Monad m => Monad (ReaderT r m) instance would be unaffected by this... it's only the Functor => Functor instance. If someone wants to treat a transformed monad as a functor, then they're probably savvy enough to specify the base monad as a functor (especially with the @fmap f = (>>= return . f)@ recipe). They won't even see a difference unless they are rolling their own monad, since all mtl monads are also functors.
I agree. I'd like to do the same thing with Applicative.