Consider this: module Test3 where class C a b | a -> b where m :: a -> b data D a = forall b. (C a b) => MkD a f :: (C a b) => D a -> b f (MkD a) = m a This compiles fine under GHC 5.02.2. But under 5.03, it gives an error: Model/Test3.hs:9: Inferred type is less polymorphic than expected Quantified type variable `b' escapes When checking an existential match that binds and whose type is D a -> b1 In the definition of `f': f (MkD a) = m a I consider that the 5.02.2 behaviour is preferable, and that this is a perfectly good program. 'b' does not escape because it is fundep on 'a', which is specified in the type-signature. There can be only one. What was changed in 5.03 and why? -- Ashley Yakeley, Seattle WA