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
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