looks like the problem are the 'e's. you can see it from Could not deduce (TFunOut t1 e ~ TFunOut t1 e0) the e0 comes from the output of TFunIn/TFunOut and the instance does not know that it is the same as the e variable you use. I don't know exactly what your program does. but if e is just a phantom/security parameter you can do it like this. type family TFunIn t :: * -> * type instance TFunIn IdT = Signal type instance TFunIn (ComposeT t1 t2) = TFunIn t2 instance (SignalT t1 e, SignalT t2 e, TFunOut t2 ~ TFunIn t1) => SignalT (ComposeT t1 t2) e where transform _ x = (transform (undefined :: t1) . transform (undefined :: t2)) x Otherwise you will have to find a way to prove that the 'e's are the same Cheers Silvio