[Haskell-cafe] Adding Ord constraint to instance Monad Set?

[Wolfgang Jeltsch]

Am Mittwoch, 31. März 2004 03:11 schrieb Benjamin Franksen:
> >    instance Monad Set where
> >       m >>= k = concatSets (mapSet k m)
> >       return x = unitSet x
> >       fail s = emptySet
> >
> >    concatSets sets = foldl union emptySet (setToList sets)
> >    instance (Eq b,Ord b) => Ord (Set b) where
> >       compare set1 set2 = compare (setToList set1) (setToList set2)> [...]

> [...]

> Now, as i think a little more about it, i believe what you want to do makes
> no sense. The monad operation '>>=' works on monads over *different*
> 'element' (i.e. argument) types (look at the type of '>>='). Your
> implementation only works if argument types are the same. I can't see how
> this can be generalized to different argument types even if both are
> instances of class Ord.

I disagree.  AFAICS, his implementation also works with different element 
types.  Am I overlooking something?

> [...]

> Ben

Wolfgang