> Why is this so hard in Haskell? The challenge is composition. Adding Timestamped to something in a static, type-checked way, but not modifying the source co

> Why is this so hard in Haskell? The challenge is composition. Adding Timestamped to something in a static, type-checked way, but not modifying the source code of that something. > It doesn't have interface polymorphism? This is besides the point, but its interface polymorphism is static, not dynamic. If you have a List which is IMappable, and you do some IMappable operations on it, in most OOP languages you get back an IMappable, but in Haskell you get back List.