On 22 December 2011 01:58, <wagne...@seas.upenn.edu> wrote: > Quoting Bas van Dijk <v.dijk....@gmail.com>: > >> I'm playing a bit with the new ConstraintKinds feature in GHC >> 7.4.1-rc1. I'm trying to give the Functor class an associated >> constraint so that we can make Set an instance of Functor. The >> following code works but I wonder if the trick with: class Empty a; >> instance Empty a, is the recommended way to do this: > > > Maybe something like this? > > class Functor f where > type C f a :: Constraint > type C f a = () > > instance Functor Set where > type C Set a = Ord a > > ~d > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Yes I already tried that, but the following gives a type error: instance Functor [] where fmap = map testList = fmap (+1) [1,2,3] Could not deduce (C [] b0) arising from a use of `fmap' In the expression: fmap (+ 1) [1, 2, 3] _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users