[Haskell-cafe] Thinking about an unlistN

[Robert Greayer]

--- On Sun, 8/10/08, Michael Feathers <mfeathers at mindspring.com> wrote:

> I wrote this function the other day, and I was wondering if
> I'm missing 
> something.. whether there is already a function or idiom
> around to do this.
> 
> 
> unlist3 :: (a -> a -> a -> b) -> [a] -> b
> unlist3 f (x:y:z:xs) = f x y z
> 
> 
> I was also wondering whether the function can be
> generalized to N or 
> whether this is just one of those edges in the type system
> that you 
> can't abstract over.
> 

Well, there's always haskell's Swiss Army Knife, TH:

> module Unlist(unlistN) where
> 
> import Language.Haskell.TH
> 
> unlistN n = do
>     f <- newName "f"
>     xs <- sequence (replicate n (newName "x"))
>     lamE [varP f,(foldr ((flip infixP) '(:)) (wildP) (map varP xs))] 
>          (foldl appE (varE f) (map varE xs))

> {-# OPTIONS_GHC -XTemplateHaskell #-}
> module UseUnlist where
> 
> import Unlist
> 
> f i0 i1 i2 i3 = i0 + i1 + i2 + i3
> 
> x = $(unlistN 4) f [1,2,3,4,5,6,7]

rcg