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