Re: Instance declaration superclasses

[Simon L Peyton Jones]

> >> GHC complains about:
> >>
> >> > class (Monad m, {-, Monad (t m)-}) => MonadT t m where
> >> >   lift :: m a -> (t m) a
> >> >
> >> > instance (Monad m) => Monad (EnvT env m) where
> >> >   ...
> > instance Monad m => MonadT (EnvT env) m where ...
> >
> 
> Even when I use the "decent" definition you suggest, GHC duplicates the
> context:
> 
> 8 $d2 _:_ _forall_ [a b :: (* -> *)] {PrelBase.Monad b, PrelBase.Monad b} =>
> (MonadT (EnvT a) b) ;;

Aha!  A palpable bug!

GHC implements an optimisation suggested by Mark. Consider

        class S a => C a where ...

        instance D a => D [a] where ...

The instance declaration gives rise to a function

        $d :: D a -> D [a]

that takes a dictionary for (D a) into one for (D [a]).  The optimisation
is to make $d do this:

        $d :: D a -> S [a] -> D [a]

that is, to pass a dictionary for the superclass too.  That can increase
sharing of dictionaries.

Ihe single parameter case, it's impossible for D a and S [a] to be
the same.  But in this multi-parameter case they can, and the dups
aren't eliminiated.  Hence the error.


I don't see a workaround.  I'll just have to fix it...
How urgent is it?

Simon