[Haskell-cafe] Lifting strictness to types

I've just read the post "Destroying Performance with Strictness" by Neil Mitchell [1]. One of the comments from an Anonymous says: How hard would it be to lift strictness annotations to type-level? E.g. instead of f :: Int -> Int f !x = x + 1 write f :: !Int -> Int f x = x + 1 which would have the same effect. At least it would be transparent to the developer using a particular function. The problem I see with this approach is on type classes, as it would be impossible to declare a type instance with strict implementation to a type class that used lazy types. Is this a real problem? Is it the only one? [1] http://neilmitchell.blogspot.ru/2013/08/destroying-performance-with-strictne...