Still on the same subject, I solved one problem but don't really understand why it didn't work in the first place ! Although it seems to be a difference between "newtype" and "data" ! I have a newtype defined as : data Fct s a = Fct (s -> [a]) and a function defined by : plus :: Fct a b -> Fct a b -> Fct a b plus (Fct f1) (Fct f2) = Fct ( \ a -> (f1 a) ++ (f2 a) ) For some reason, this function does not use leazy evaluation ! I can test it using : test_fct :: Fct Int Int test_fct = Fct( \ i -> [i] ) value = head $ apply (foldr plus test_fct $ repeat test_fct) 12 ... trying to get "value" does not terminate ! But if I change the type declaration into : newtype Fct s a = Fct (s -> [a]) ... it works ! The "plus" function uses leazy evaluation and "value" can be computed. Now, I didn't really understood the difference between newtype and data. Can anyone explain that behaviour ? Thank you, Pierre -- Pierre Barbier de Reuille INRA - UMR Cirad/Inra/Cnrs/Univ.MontpellierII AMAP Botanique et Bio-informatique de l'Architecture des Plantes TA40/PSII, Boulevard de la Lironde 34398 MONTPELLIER CEDEX 5, France tel : (33) 4 67 61 65 77 fax : (33) 4 67 61 56 68