Christophe Poucet wrote:
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?
{-# OPTIONS -fglasgow-exts #-} -- Multiparameter type classes? import List type Var = String type Const = String data MonoType mt = TyVar Var | TyConst Const [mt] deriving (Eq, Show) data PolyType mt = TyPoly [Var] mt deriving (Show) class Type a b where toType :: b -> a b fromType :: a b -> b freeVars :: a b -> [Var] occurs :: Var -> a b -> Bool instance Type MonoType Int -- yada, yada, yada... instance Type MonoType (MonoType Int) 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