How to make Data.Set.Set a Functor

[Simon Marlow]

On 11 October 2005 17:16, Lajos Nagy wrote:

> While working on a toy compiler I realized that Data.Set.Set (Set) is
> not an instance of the Functor class.  In other words: 'fmap' is not
> defined on it.  I tried various ways of defining an instance but I
> failed.  The reason is quite interesting: Set is a type constructor
> (* -> *) so it should qualify it for being a Functor.  (In a sense it
> is very similar to a Map or a list.)  However, most Set functions
> require the elements of the Set to be an instance of Ord.  The
> problem is that this constraint cannot be deduced from the instance
> declaration for Functor: 
> 
> instance Functor Data.Set.Set where
>     fmap f s = Data.Set.map f s

A nice way to do this would be:

  data FSet a where
     FSet :: Ord a => Set a -> FSet a

  instance Functor FSet where 
     ...

if only GADTs worked with type classes :-)

Cheers,
	Simon