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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
