leetcode 40. Combination Sum2

import Data.List

cal :: [Int] -> Int -> [[Int]]

cal xs 0 = [[]]

cal xs s = if sum xs < s then [] else nub $ map sort $ concat [ map (x:) $ cal (delete i ys) (s - x) | (x, i) <- zip ys [0..] ]

where ys = [ x | x <- xs, x <= s ]

main = do

putStrLn $ show $ cal [10, 1, 2, 7, 6, 1, 5] 8

putStrLn $ show $ cal [2, 5, 2, 1, 2] 5

putStrLn $ show $ cal [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1] 27