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

[Henning Thielemann]

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 .