Dear, I am writing a compiler (as you may have gathered from some previous messages). Anyways I am stuck with a small monadic issue. I mostly use indirect composite as this gives me the most flexibility with regards taking out parts of the AST, or decorating at whim. Basically the question regards how to implement a certain instance. Currently I have the code that can be seen below. What I would like to do is combine HasVars and Type (mostly because in my framework the two concepts shouldn't be divided from a design perspective) into one type class to clean it up a bit. However I fail to see how I would implement toType and fromType for the given instance. Is this feasible without resorting to ugly hacks? --- Types data MonoType mt = TyVar Var | TyConst Const [mt] deriving (Eq, Show) data PolyType mt = TyPoly [Var] mt deriving (Show) class Type mt where toType :: mt -> MonoType mt fromType :: MonoType mt -> mt class HasVars a where freeVars :: a -> [Var] occurs :: Var -> a -> Bool toPoly :: (HasVars a) => a -> PolyType a toPoly x = TyPoly (freeVars x) x instance HasVars a => HasVars (MonoType a) where freeVars (TyVar x) = [x] freeVars (TyConst _ xs) = nub . concatMap freeVars $ xs occurs x (TyVar y) = x == y occurs x (TyConst _ xs) = or . map (occurs x) $ xs Cheers, Christophe(vincenz) -- Christophe Poucet Ph.D. Student Phone:+32 16 28 87 20 E-mail: Christophe (dot) Poucet (at) imec (dot) be Website: http://notvincenz.com/ IMEC vzw – Register of Legal Entities Leuven VAT BE 0425.260.668 – Kapeldreef 75, B-3001 Leuven, Belgium – www.imec.be *****DISCLAIMER***** This e-mail and/or its attachments may contain confidential information. It is intended solely for the intended addressee(s). Any use of the information contained herein by other persons is prohibited. IMEC vzw does not accept any liability for the contents of this e-mail and/or its attachments. **********