On Fri, 11 May 2001, Lauri Alanko wrote:
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 instance is illegal anyway. One of types in the instance head must be a type constructor applied to something (type variables in Haskell 98, anything with -fglasgow-exts). Even if it was legal, it would overlap with instance Monad m => MonadState s (StateT s m) Also MonadReader and MonadWriter can't have such generic instances anyway because their methods have values of type 'm a' as arguments. OTOH without the fundep there are ambiguities. For example: class ParsingState s where stateInput :: s -> String stateSkip :: Int -> s -> s instance ParsingState String where ... instance ParsingState s => ParsingState (s, Pos) where ... input :: (ParsingState s, MonadState s m) => m String -- Ambiguous without the fundep. input = gets stateInput -- Marcin 'Qrczak' Kowalczyk