Hi, Am Donnerstag, den 14.10.2010, 09:34 +0200 schrieb Jacek Generowicz:
Another example:
Let's say I need an Int -> String. Both
(fnA2 :: Banana -> String) . (fnA1:: Int -> Banana)
and
(fnB2 :: Onion -> String) . (fnB1 :: Int -> Onion)
will do. So please allow me to store (fnA1, fnA2) and (fnB1, fnB2) in the same place. The program can tell that it can combine them with (.) because the type of
let (fn1, fn2) = pair in fn2 . fn1
is always
Int -> String.
This is possible: {-# LANGUAGE ExistentialQuantification #-} data SplitFun a b = forall x. SplitFun (a -> x, x -> b) splitFuns :: [SplitFun Int String] splitFuns = [SplitFun (\n -> replicate n "hi", concat) ,SplitFun (show, id)] main = mapM_ putStrLn $ map (\(SplitFun (f1,f2)) -> f2 (f1 2)) splitFuns This prints: *Main> main hihi 2 Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail@joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata@debian.org