Adrian Hey
insertUsing :: (a -> a -> COrdering a) -> a -> Set a -> Set a
But as others have pointed out, this is probably the wrong thing to do as Sets should provide some kind of semantic guarantees
My gut reaction to this is that it's ugly - whether a Set use structural equality or just some possibly ill-defined equivalence, that should be a property of the Set *type*, not the insertion operator. And what if you change the equivalence while inserting? I guess it is impractical to use a phantom type to provide the ordering function? :-) How about something like: data SetEql a b = -- abstract, set of as, requires Ord b with empty :: Ord b => (a->b) -> SetEql a b (and similar for other set construction functions)? I'm still not convinced this solves any problem not solved better by a Map, though. And it still doesn't embed the comparison function in the type, so I guess you will have to define left or right bias for union, etc. (Or perhaps run both (a->b) functions on all keys to check for equality, with the alternative being a run time error?) -k -- If I haven't seen further, it is by standing in the footprints of giants