--- On Sun, 8/10/08, Michael Feathers
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