Brandon Moore wrote:
I don't quite understand the problem, but maybe an example involving an explicit recursion operator will help.
-- Maybe he's looking for something like... {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} import List type Var = String type Const = String data MonoType mt = TyVar Var | TyConst Const [mt] deriving (Eq, Show) newtype Fix f = In { out :: f (Fix f) } class Types a where freeVars :: a -> [Var] instance Types (a (Fix a)) => Types (Fix a) where freeVars (In x) = freeVars x instance Types a => Types (MonoType a) where freeVars (TyVar x) = [x] freeVars (TyConst _ xs) = nub . concatMap freeVars $ xs main = do print $ freeVars (TyConst "foo" [(In (TyVar "abc")), (In (TyVar "123")), (In (TyConst "bar" [(In (TyVar "www"))]))]))