Christophe Poucet wrote:
I'm not certain but I think this will still fail for exactly the piece that you ignored, which is the crux of the problem.
-- You're not looking for this solution, right? 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) newtype FMT = FMT (MonoType FMT) class Types a where freeVars :: a -> [Var] instance Types FMT where freeVars (FMT (TyVar x)) = [x] freeVars (FMT (TyConst _ xs)) = nub . concatMap freeVars $ xs main = print $ freeVars (FMT (TyConst "foo" [(FMT (TyVar "abc")), (FMT (TyVar "123")), (FMT (TyConst "bar" [(FMT (TyVar "www"))]))]))