[Haskell-cafe] Predicativity?

Hello, I only have a vague understanding of predicativity/impredicativity, but cannot map this concept to practice. We know the type of id is forall a. a -> a. I thought id could not be applied to itself under predicative polymorphism. But Haksell and OCaml both type check (id id) with no problem. Is my understanding wrong? Can you show an example that doesn't type check under predicative polymorphism, but would type check under impredicative polymorphism? Thanks!