ambiguity problem

Hi, I'm using -- Hugs Version February 2001. hugs -98 reports ambiguity for the following program class Bar a b | a -> b where bar :: a -> b class Foo a b where foo :: Bar b c => a -> b Ambiguous type signature in class declaration *** ambiguous type : (Foo a b, Bar b c) => a -> b *** assigned to : foo This strange cause the functional dependency should uniquely determine c! Suprisingly, class Bar a b | a -> b where bar :: a -> b class Foo a b -- ok f :: (Foo a b, Bar b c) => a->b f = undefined is accepted. This seems like a bug. Martin