Hi, Café: I am not sure how to make the question, but the idea is to have a type operator that returns the same type. Let me show what I need. I have some type like this: data Example f t = Example (f t) So if I want a type variable that contains a list of integers, I can use Example [] Int and then treat the content like a list: f :: Example [] Int -> Int f (Example l) = length l Now, the problem is what can I do to have a type that contains a single Int? Is there a sort of id for types so that I can write f :: Example ?? Int -> Int f (Example i) = 2 * i ? Thanks in advance Juan Miguel Vilar -- Juan Miguel Vilar Torres Profesor titular de universidad Departamento de Lenguajes y Sistemas Informáticos Escuela Superior de Tecnología y Ciencias Experimentales Universitat Jaume I Av. de Vicent Sos Baynat s/n 12071 Castelló de la Plana (Spain) Tel: +34 964 72 8365 Fax: +34 964 72 8435 jvilar@uji.es