Re: ConstraintKinds and default associated empty constraints

On 22 December 2011 01:58, wrote:

Quoting Bas van Dijk :

...

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

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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]