[Haskell-cafe] Type Constraints on Data Constructors

{- continuing discussion from beginners@ -} I have code such as 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 that I put Foo f => on the instance declaration, even though the constructor for Bar implies this. Is there any reason why GHC cannot infer this constraint from the Bar constructor? One issue raised in the beginners thread is that undefined :: Bar f a is not Foo f, but as undefined cannot be evaluated, this would not appear to be a problem.