The article doesn't cover this but haskell's return type polymorphism is strictly more powerful because it can be used in cases where you don't know the type at

The article doesn't cover this but haskell's return type polymorphism is strictly more powerful because it can be used in cases where you don't know the type at compile time. Such a situation can occur with recursion on nested data types or with higher rank types. Now, i bet with some hackery one could emulate this too, but I don't expect it to be legible.