Count the number of pairs in `nums` that total `target`. Sort is performed at the very beginning to allow for some assumptions to be made in the recursive routine. I want to figure out how to remove the sort; it wouldn't be needed in the declarative version.

countPairs.hs

import Data.List

countPairs :: (Integral a, Num b) => [a] -> a -> b
countPairs nums target = counter (sort nums) target
  where 
    counter :: (Integral a, Num b) => [a] -> a -> b
    counter [] _ = 0
    counter [_] _ = 0
    counter sn t
      | s == t    = 1 + counter (init (tail sn)) t
      | s <  t    = counter (tail sn) t
      | otherwise = counter (init sn) t
      where
        s  = head sn + last sn


-- countPairs [1,2,5,8,9] 10 == 2
-- countPairs [1,9,1,8,1,2,9] 10 == 3