[Haskell-beginners] Confused by type constraints

I have some code resembling class Foo f where foo :: a -> f a data Bar f a = Foo f => Bar {bar :: f a} instance Foo (Bar f) where foo a = Bar $ foo a GHC insists on Foo f => in the instance declaration. However, the definition of Bar guarantees that this will always be the case. Why do I have to state this explicitly?