The following code builds and appears to work great (assuming one allows undecidable instances). I have tried both a natural translation into type synonym families, and also the mechanical transformation in http://www.cse.unsw.edu.au/~chak/papers/tyfuns.pdf -- but I can't get either to work. The two fishy things are: 1. There is a functional dependency (a -> t) in the original. Actually, this isn't always true since t is sometimes not uniquely determined by a; but GHC always seems to do the right thing anyway. There doesn't seem to be any way to "cheat" like this with type families. 2. In the second and fourth instances, the type variable x appears twice in the parameters of the type function built from the fundep (a b -> c). This causes an error. If I try adding (x ~ x') to the context and making one of them x' instead of x, I get different errors. I'm starting to this this is an example of using functional dependencies that has no translation to type families. Can anyone free me of such thoughts? Code with fundeps is: data Var a = Var Int (Maybe String) data RHS a b = RHS a b class Action t a b c | a -> t, a b -> c, a c -> b instance Action t () y y instance Action t (Var x) (x -> y) y instance Action t [t] y y instance (Action t a b c) => Action t (RHS (Var x) a) (x -> b) c instance (Action t a b c) => Action t (RHS [t] a) b c -- Chris Smith