[Haskell-cafe] Type Families: infinite compile process?

Hi guys, I have been experimenting some weird stuff (risky, yes I know) but the behaviour was certainly not the one I expected: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module Nest where data Nest a = Nil | Cons a (Nest (a,a)) type family F a x :: * type instance F (Nest a) x = Either () (a,F (Nest (a,a)) x) fnn :: F (Nest Int) (Nest Int) fnn = Left () The following module fails to compile (or better, compilation never ends). Maybe there is something very bad going on due to the undecidable-instances extension? Any clue? hugo