Daniel Fischer
Maybe it is because deleteBy is defined wrongly? i.e. it is not logical, doesn't follow the common sense user might expect. It accepts any predicate but narrows requirements only in docs.
Unfortunately, you can't easily encode the requirement that it ought to be an equivalence relation via types.
This is just another way of saying the same thing :-) An important point of a powerful type system is to model your program so that only sensible code is legal. This makes me wonder why deleteBy is defined so loosely, instead of e.g. deleteBy :: (a -> Bool) -> [a] -> [a] I don't think there would be any doubt what 'deleteBy (<= 5) [1..10]' would do. And I just don't see what the requirement for an equivalence relation buys you. -k -- If I haven't seen further, it is by standing in the footprints of giants