At Tue, 14 Jun 2011 19:52:00 -0700 (PDT),
o...@okmij.org wrote:
>
>
> Dan Doel wrote:
> > class C a b | a -> b where
> > foo :: a -> b
> > foo = error "Yo dawg."
> >
> > instance C a b where
>
> The instance 'C a b' blatantly violates functional dependency and
> should not have been accepted. The fact that it was is a known bug in
> GHC. The bug keeps getting mentioned on Haskell mailing lists
> about every year. Alas, it is still not fixed. Here is one of the
> earlier messages about it:
>
> http://www.haskell.org/pipermail/haskell-cafe/2007-March/023916.html
But Oleg, isn't what you are complaining about *exactly* the lifting
of the coverage condition, which is one of the explicit points of
-XUndecidableInstances? Are you advocating two separate switches for
lifting Paterson vs. Coverage?
What about the following code--do you think this should be illegal,
too?
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE UndecidableInstances #-}
class C a b c | a -> b where
instance C (Maybe a) (Maybe b) (Maybe b) where
David
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