How to make Data.Set.Set a Functor

[Lajos Nagy]

Hi,

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


GHCi:
     Could not deduce (Ord a, Ord b) from the context (Functor 
Data.Set.Set)
       arising from use of `Data.Set.map' at ...
     Probable fix: add (Ord a, Ord b) to the class or instance method 
`fmap'
     In the definition of `fmap': fmap f s = Data.Set.map f s
     In the definition for method `fmap'
     In the instance declaration for `Functor Data.Set.Set'

On the other hand, it seems intuitively natural to make Set an instance of 
fmap.  Any ideas on how to do it?

Thanks and Regards,

Lajos Nagy