On 2010 Oct 14, at 09:54, Joachim Breitner wrote:
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 #-}
Existential Quantification yet again! I see that its status in Haskell Prime is "None". Anybody care to hazard a guess as to the odds of its acceptance? Which implementations support it today ?
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)]
And x might be a function type (with any number of arguments), so we get some variadicity for free! I hadn't thought of that. That's brilliant.
main = mapM_ putStrLn $ map (\(SplitFun (f1,f2)) -> f2 (f1 2)) splitFuns
This prints: *Main> main hihi 2
Brilliant. Thanks.