How to make Data.Set.Set a Functor

Lajos Nagy lnagy at
Tue Oct 11 12:16:23 EDT 2005


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 = f s

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

More information about the Glasgow-haskell-users mailing list