sub_array_sum.hs

#!/usr/bin/env stack
-- stack --resolver lts-6.15 script
{-# LANGUAGE OverloadedStrings #-}
-- import qualified Data.ByteString.Lazy.Char8 as L8

-- targetSum -> currentSum -> subArray -> remainingNums -> subarray if it sums to targetSum OR [] otherwise
target_sum :: Integer -> Integer -> [Integer] -> [Integer] -> [Integer]
target_sum ts _ [] (n:ns) = target_sum ts n [n] ns {- our base case: there's nothing in the sub array -}
target_sum ts cs a _ {- yay! the current sum is the target sum! return the subarray -}
  | ts == cs = a
target_sum ts cs (a:as) n {- the current sum is larger than the target sum. pop the first element and try again -}
  | cs > ts = target_sum ts (cs-a) as n
target_sum ts cs (a:as) (n:ns) {- current sum less than target sum. increment sum and -}
  | cs < ts = target_sum ts (cs+n) ((a:as)++[n]) ns
target_sum ts cs a [] {- ok so you didn't match anything and theres nothing left to check. better return an empty array-}
  | cs /= ts = []


main :: IO ()
main = do
    let my_sumTarget = 10
    let nums = [1, 4, 4, 4, 9, 4, 5, 1, 5]
    -- let nums = [12]
    print $ target_sum my_sumTarget 0 [] nums