[Haskell-cafe] context deduction in GHC

Why does ghci 7.6.3 not like this example, which section 4.3.2 of the language definition says is valid Haskell? class Foo a class Foo a => Bar a instance (Eq a, Show a) => Foo [a] instance Num a => Bar [a] Ghci complains, "Could not deduce (Show a) arising from the superclasses of an instance declaration from the context (Num a)". Yet Show is a superclass of Num. Hugs accepts the code. Doug McIlroy