On Thu, May 10, 2001 at 09:24:36PM +0000, Marcin 'Qrczak' Kowalczyk wrote:
MonadReader, MonadWriter and MonadState classes have fundeps from the monad type to the environment / output / state type (I added them a few months ago).
Why? This makes composing and "subtyping" impossible: instance (MonadTrans t, MonadState s m, Monad (t m)) => MonadState s (t m) where get = lift get put = lift . put This is really useful, because you can thread multiple states (or read multiple environments, or write multiple outputs, or whatnot), and _directly_ use functions that operate on a particular state, without having to do explicit lifting (to the correct level) all the time. Fundeps add convenience, yes, but they restrict what you can do with the monads. A bad tradeoff, IMHO. Ah well, I'm kind of used to having to use customized versions of all the common libs... Lauri Alanko la@iki.fi