Hi, as discussed, I started to work basing the newtype coercions on classes instead of types. I poked around the code, especially TcDeriv.lhs, and I have a few questions: For my current design I wanted to have class IsNT a where type Concrete a coercion :: a ~# Concrete a but that does not work, as methods seem to need to have a boxed type. Using "a ~ Concrete a" does not work as well, as that has kind Constraint. So I wrapped ~# myself and created: data NT a b = NT ((~#) a b) class IsNT a where type Concrete a coercion :: NT a (Concrete a) with only IsNT (I’ll think more about the name ;-)) and probably Concrete exposed; not or coercion. The interface would then be functions castNT :: IsNT a => Concrete a -> a uncastNT :: IsNT a => a -> Concrete a Now we want the user to be able to specify deriving instance IsNT Age -- with Concrete Age = Int deriving instance IsNT a => IsNT [a] -- with Concrete [a] = [Concrete a] deriving instance IsNT v => IsNT (Map k v) -- dito and probably also deriving instance IsNT Int -- with Concrete Int = Int for all non-newtypes. After adding the definitions to ghc-prim and extending PrelNames I tried to make the deriving code, starting with the last examle (IsNT Int). There, the generated code should be something like instance IsNT Int where type Concrete Int = Int coercion = NT (refl Int) But here I am stuck: The deriving code mechanism expects me to give the method definitions as HsExpr, but I’d like to generate Core here. How can I achieve that, or what other options do I have? (Of course this will need further adjustments when Richard’s role code is in.) (If there is a reason to go back to the two parameter type class "NT a b", no problem, but the problem of HsExpr vs. CoreExpr would still remain.) Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org