[Haskell-cafe] Factoring into type classes

[Holger Siegel]

Am Montag, den 19.01.2009, 14:47 +0100 schrieb Daniel Fischer:
> Am Montag, 19. Januar 2009 14:31 schrieb Antoine Latter:
> > 2009/1/19 Luke Palmer <lrpalmer at gmail.com>:
> > > As a side curiosity, I would love to see an example of any data structure
> > > which has more than one Functor instance.  Especially those which have
> > > more than one useful functor instance.
> >
> > (,)  ?
> >
> > -Antoine
> 
> Wrong kind. And 
> (,) a
> has only one useful instance.


What about

instance Functor ((,) a) where
        fmap f (x,y) = (x, f y)

and

instance Functor (, a) where
        fmap f (x, y) = (f x, y)

? Of course, the latter is not legal Haskell. But if it was, then it
might be useful.

Is there any way to declare this Functor instance, possibly with some
GHC extensions?