Hi! How can I specify intermediate type variables to help the compiler do its job? The following, simple, code does not compile because the compiler cannot infer that the type variable a' is CInt; this makes sense, because finding that out would be quite hard. How can I tell the compiler that a' is CInt? {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} class C a a' where convert :: a -> a' class F a b where apply :: a -> b class S s a where select :: s -> a data CInt = CInt Int instance S (Int,String) Int where select (i,_) = i instance C Int CInt where convert = CInt instance F CInt Int where apply (CInt i) = i + 1 f :: forall s a a' b . (S s a, C a a', F a' b) => s -> b f s = let v = select s :: a v' = convert v :: a' y = apply v' :: b in y x :: Int x = f (10,"foo") Thanks