On Sat, Jul 18, 2009 at 6:14 AM, Conor McBride
Until that joyous day, I'd like to hope we might consider ensuring that MonadPlus m, Alternative m, and Monoid (m a) functionalities do at least coincide.
I'm thinking in particular of Maybe, which behaves splendidly as an implementation of an exception monad, until you start using foldMap as a control operator.
Yeah, the Monoid instance for Maybe is somewhat unfortunate. I can see where they were going as Maybe does provide the natural extension of a semigroup into a monoid by adding a unit element, but it doesn't have a Semigroup class to build on, and so has to require Monoid and in the end you get a definition that conflicts with the MonadPlus/Alternative instances for Maybe, and only really helps if you have broken Monoid instances around that are secretly just Semigroups. -Edward Kmett