I think that Haskell's standard library implementation of a binary search tree (Data.Map) had long had a balancing bug... It probably doesn't anymore, but I gu

I think that Haskell's standard library implementation of a binary search tree (Data.Map) had long had a balancing bug... It probably doesn't anymore, but I guess it probably makes it a practical win to get type-level guarantees. Also, guarantees about correctness are easier to get from tools like QuickCheck. Balance errors will be harder to spot.