Re: [Haskell-cafe] Byte Histogram

On Tue, Feb 8, 2011 at 12:33 PM, Ivan Lazar Miljenovic < ivan.miljenovic@gmail.com> wrote:

On 8 February 2011 23:25, John Lato wrote:

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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