I started with the following: {-# LANGUAGE TypeFamilies #-} class DoC a where type A2 a op :: a -> A2 a data Con x = InCon (x (Con x)) type FCon x = x (Con x) foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t) doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) doCon (InCon x) = op x fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) fCon = foldDoC op I then changed the rank of op, but forgot to update the foldDoC accordingly---see below. Attempting to compile this causes GHC to run forever using 100% cpu. The corrected definition of foldDoC works fine. Should the GHC (6.12.1) behavior in the face of my foolishness be reported as a bug or is this a legitimate infinite recursion of type deduction? {-# LANGUAGE TypeFamilies #-} class DoC a where type A2 a type A3 a op :: a -> A2 a -> A3 a data Con x = InCon (x (Con x)) type FCon x = x (Con x) -- should have been changed to this, which works -- foldDoC :: Functor f => (f a -> a) -> A2 (FCon f) -> Con f -> a -- foldDoC f i (InCon t) = f (fmap (foldDoC f i) t) -- this original version causes GHC to hang foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t) doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) doCon (InCon x) = op x -- note that if this is commented out then there's no hang: presumably because GHC doesn't have to perform type deduction for foldDoC. fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) fCon = foldDoC op -- -KQ