On Mon, Jan 12, 2009 at 04:26:16PM +0100, Wolfgang Jeltsch wrote:
And we should try to make Applicative a superclass of Monad and drop MonadPlus in favor of Alternative + Monad. It would be probably even better to have somethink like Alternative/MonadPlus for functors (since functors are the most general concept at this point) and drop Alternative and MonadPlus (or use the name 'Alternative' for this new class). It's terrible to explain all this legacy (Applicative not being superclass of Monad, two different classes for monoid functors) to students and to mention Applicative in a class context which already mentions Monad.
The legacy hierarchy is a problem with transformers. Some of them are Applicative transformers: instance (Monoid w, Applicative m) => Applicative (WriterT w m) instance (Applicative m) => Applicative (ReaderT r m) so they're also defined as Functor transformers: instance (Functor m) => Functor (WriterT w m) instance (Functor m) => Functor (ReaderT r m) So far so good. But some others require the argument to be a Monad: instance (Monad m) => Applicative (StateT s m) instance (Monad m) => Applicative (ErrorT e m) but to be Applicative they must also be Functor, so we keep the old instances: instance (Monad m) => Functor (StateT s m) instance (Monad m) => Functor (ErrorT e m) We could change these to assume Functor, but then we'd have to add a Functor constraint to the Applicative instances. Hmm, on second thoughts that's probably the right way to go.