[Haskell-cafe] Help wanted getting old GADT code to compile

Here is a legacy file that does not compile with GHC 8.0.1: {-# LANGUAGE RankNTypes, GADTs, KindSignatures #-} module Bookgadt where data TExp :: * -> * where TLit :: (forall a . (a -> (TExp a))) TLit' :: (a -> (TExp a)) The compiler rejects value constructor TLit with this error message: • Data constructor ‘TLit’ returns type ‘forall a. a -> TExp a’ instead of an instance of its parent type ‘TExp a’ I really want to keep the explicit "forall," as I'm trying to make a point for readers not familiar with Haskell. Does anyone know if there is some option or other incantation that would enable this code to compile? Norman