[Haskell-cafe] Type signature for function where function parameter is used twice?

Lets say I've got the following data type: data D c a b = D (c a b) (c b a) And I define a function to manipulate it: apply :: (c a b -> c a2 b2) -> (c b a -> c b2 a2) -> D c a b -> D c a2 b2 apply f1 f2 (D x y) = D (f1 x) (f2 y) This is all fine. But I want a shorter function if (f1 = f2). So I write: applyBoth f = apply f f I originally thought that if "apply f f" is valid, then logically "applyBoth f" should also be valid. But it seems that type inference results in applyBoth only working for functions "c a a -> c a2 a2". Is there a way to type "applyBoth" so it works for all functions that would work simply by repeating them twice in "apply"?