On 8 February 2011 23:25, John Lato <jwlato@gmail.com> wrote:
>> class Container c where
>> type Elem c :: *
>>
>> class (Container cIn, Container cOut) => CMap cIn cOut where
>> cmap :: (Elem cIn -> Elem cOut) -> cIn -> cOut
>>
>> instance (a ~ Elem (c a), b ~ Elem (c b), Functor c, Container (c a),
>> Container (c b)) => CMap (c a) (c b) where
>> cmap = fmap

I'm not sure if that will work for types like Set, as you're not
explicitly bringing the constraint in.

It won't work for Set, but Set's not a functor. In this case just write this instance instead:

> instance (Ord a, Ord b) => CMap (Set a) (Set b) where

> cmap = Set.map

John