On 11/28/05, Iavor Diatchki
Hello, Data.IntMap (distributed with GHC 6.4.1) contains the following instance: Ord a => Monoid (IntMap a) The instance is defined with mempty = empty, and mappend = union, neither of which requires the Ord instances on the values in the map, so the 'Ord a' constraint appears unneccessary.
You're right, I seem to be guilty of careless cut and paste from the corresponding code in Data.Map. I will fix that.
-Iavor PS: Isn't picking union for the monoid class a little arbitrary?
Indeed. The problem is that Monoid is very general and classes have global scope. Union seemed like a reasonable definition... If it doesn't fit the algorithm at hand, one can resort to "explicit dictionnary passing" for emulating local instances. Cheers, JP.