LongestSubArray.hs

{-# LANGUAGE MultiWayIf #-}

module Main where

main :: IO ()
main = print $ maxLength [10, 9, 2, 3, 4, 6] 500

maxLength :: [Int] -> Int -> Int
maxLength numbers aLimit = collector numbers aLimit []
  where
    collector :: [Int] -> Int -> [[Int]] -> Int
    collector [] _ subArrays = maximum [length sa | sa <- subArrays]
    collector (x:xs) k subArrays =
      if | x >  k    -> collector xs k subArrays
         | x == k    -> collector xs k $ subArrays ++ [[x], []]
         | otherwise ->
            let subArray = if null subArrays then [x] else last subArrays ++ [x]
            in if sum subArray > k
                  then collector xs k $ subArrays ++ [[x]]
                  else collector xs k $ replaceLastBy subArray subArrays
      where
        replaceLastBy sArray sArrays =
          if null sArrays then [sArray] else init sArrays ++ [sArray]

It's the easiest solution, without reading of stdin. I've checked it with different lists and limits - it works. In the real life I'd write some test (a-la QuickCheck) to prove it works.