Nice asymptotics.

I love that it's concise and clear. I don't know if I'd give it Best Practices though, as it's quadratic, whereas an n Log(n) solution isn't too much harder.

I don't think you need to worry about the general case for this problem. In fact, I don't think you even can implement foldr for the general Foldable a. You need to know the value constructors for your foldable type t a, so that you can pattern match against them when you go to write your specific Foldable a instance for t a.

I like your approach, because it's straightforward functional style. Very idiomatic for Haskell.

I'm just surprised because I've been having a lot of trouble getting the straightforward approaches to finish in under 6 seconds, not just in this kata, but in lots of kata. I'm probably just bad :-p

I'm surprised this completes within the 6-second limit.

I think you might be hitting the maximum precision of doubles on that one. I think I remember that one giving me trouble, too.

Clever, but doesn't work for very close points.