[Haskell-cafe] Client-extensible heterogeneous types (Duck-typed variadic functions?)

[Joachim Breitner]

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 at joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C
  JID: nomeata at joachim-breitner.de | http://www.joachim-breitner.de/
  Debian Developer: nomeata at debian.org
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