Sorry I forgot to put a subject ________________________________________ De : Beginners [beginners-bounces@haskell.org] de la part de PICCA Frederic-Emmanuel Envoyé : mardi 17 janvier 2017 11:15 À : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Objet : [Haskell-beginners] (no subject) Hello, Here a reduction of my problem values :: IO [IO (Maybe Int)] values = do let v = [Just 1, Just 2, Just 3, Nothing, Just 5, Nothing, Just 7] :: [Maybe Int] return $ map return v main :: IO () main = do vs <- values nvs <- mapM_ go vs print nvs where go :: IO (Maybe Int) -> IO Int go v' = do Just v <- v' return v when I run this script, I get a runtime error picca@diffabs6:~/tmp$ runhaskell test.hs test.hs: user error (Pattern match failure in do expression at test.hs:13:10-15) What I want is a go method which skip silently the (IO Nothing) values. so when used in the mapM_ it return only the values which are returned by the IO (Maybe Int) (stored in the values) Thanks for your help Frédéric Indeed _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners