By chance, I noticed that the resolution of constraints in combination
with overlapping instances has changed in the `September 1999' version
of Hugs.
This code scrap used to run ...
> import Monad
> instance (Monad m) => Functor m where
> fmap f m = m >>= return . f
class (Monad m) => MonadPlus m where
mzero :: m a
mplus :: m a -> m a -> m a
> wrap :: (MonadPlus m) => m a -> m (Maybe a)
> wrap m = fmap Just m `mplus` return Nothing
Now, Hugs (hugs +o) reports
ERROR "Map.lhs" (line 7): Cannot justify constraints in explicitly typed binding
*** Expression : wrap
*** Type : MonadPlus a => a b -> a (Maybe b)
*** Given context : MonadPlus a
*** Constraints : Functor a
However, an instance of `MonadPlus' is also an instance of `Monad' and
the first instance declaration formalizes, that a `Monad' instance is
also an instance of `Functor' (of course, this instance overlaps with
all other instances).
Is this a deliberate change or a bug?
Cheers, Ralf
