And this works, and I don't know why:
mostly because you have pushed more possible conflicts till later - you could also have
added Monad b, Monad (t b) to the context for the second instance in your previous
version; shifting the constraints from the class to the method delays the point at which
they are checked (definition vs use).
if the Monad instances are really missing when you try to use that method, it
will be
picked up later; also, there is a potential instance overlap, as the more eager Hugs
will tell you for the definitions, while GHC will also report that later, only if it occurs
in use.
claus
class MonadTrans' i o where
lift' :: (Monad i, Monad o) => i a -> o a
instance Monad m => MonadTrans' m m where
lift' = id
instance (MonadTrans t, Monad b, MonadTrans' a b) => MonadTrans' a (t b) where
lift' = lift . lift'
Help?
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