Re: [Haskell-cafe] Instances for Set of Functor, Traversable?

On Mon, 26 Jul 2010, Gregory Crosswhite wrote:

Is there a specific reason why Set doesn't have instances for Functor and Traversable?

Sure, fmap needs an Ord restriction for the element type, which is not possible for the plain Functor constructor class. E.g. in fmap (const 'a') set all result elements will coincide. If you need such an fmap, try http://hackage.haskell.org/package/rmonad .