On Monday 16 July 2007, Tony Morris wrote:
...it seems to be a special case of Set? Does Data.Map add anything more useful than Map' below?
Why does Data.Set exist when it's just a special case of Data.Map? import Data.Map type Set a = Map a () And, in fact, I think going this way doesn't lose any functionality, whereas implementing Map in terms of Set loses you stuff like unionWith (at least, barring your taking time to re-implement it specifically), which may or may not be a big deal to you (I think I've used it before, though). The answer is, I suppose, that the interface is subtly different (and the semantics may be, too; are you sure that your insert using Set behaves the same way as insert on Map?), and when you're doing Set stuff, you don't want to be bugged by the fact that you're using a Map of ()s, and vice versa (although you could probably finesse things to the point where it wouldn't be noticeable). The real question is why there's Data.Map and Data.IntMap, when the compiler should really be able to detect that we're using a certain key type, and automatically use the optimized Map for that key type without our having to do anything. And the answer to that is that maybe, in the future, that will be the case, once associated types/data families are widely available. :) Cheers, Dan Doel