Select random element from a set, faster than linear time (Haskell)

Question

I\'d like to create this function, which selects a random element from a Set:

randElem :: (RandomGen g) => Set a -> g -> (a, g)

Si

Answer 1

As far as I know, the proper solution would be to use an indexed set -- i.e. an IntMap. You just need to store the total number of elements added along with the map. Every time you add an element, you add it with a key one higher than previously. Deleting an element is fine -- just don't alter the total elements counter. If, on looking up a keyed element, that element no longer exists, then generate a new random number and try again. This works until the total number of deletions dominates the number of active elements in the set. If that's a problem, you can keep a separate set of deleted keys to draw from when inserting new elements.