Attempting Knuth-Morris-Pratt in Haskell for Hackerrank. But - time out ?

main.hs

module Main where

-- | Return a Knuth-Morris-Pratt index for a given string

stringIndex :: String -> [Int]
stringIndex "" = []
stringIndex txt = indexr [0] 0 1 where

  indexr :: [Int] -> Int -> Int -> [Int]
  indexr rsf subPos chrPos
    | length rsf == length txt = rsf

    | txt !! subPos == txt !! chrPos
    = indexr (rsf ++ [subPos + 1]) (subPos + 1) (chrPos + 1)

    | subPos > 1 = indexr rsf (rsf !! (subPos - 1)) chrPos

    | otherwise = indexr (rsf ++ [0]) 0 (chrPos + 1)

-- | Tells us if given text contains given string.  Relies on isSubstring for
-- | recursive searching.

stringHaz :: String -> String -> Bool

stringHaz text term = isSubstring 0 0 where

  termTable = stringIndex term
  jump 0 = 1
  jump distance = 1 + (termTable !! distance)

  resume 0 = 0
  resume distance = max (distance - (jump distance)) 0

  isSubstring :: Int -> Int -> Bool
  isSubstring textPos termPos
    | length term <= termPos = True
    | length text <= textPos - 1 + length term  = False
    | term !! termPos == text !! (textPos + termPos) = isSubstring textPos (termPos + 1)
    | otherwise = isSubstring (textPos + (jump termPos)) (resume termPos)

answer :: Bool -> String
answer True = "YES"
answer False = "NO"

stuplefy :: IO (String, String)
stuplefy = do
  a <- getLine
  b <- getLine
  return (a, b)

main = do
  init <- getLine
  let reps = read init :: Int
  hackerrankTests <- sequence $ replicate reps stuplefy
  let results = map (answer . (\cs -> stringHaz (fst cs) (snd cs))) hackerrankTests
  let output = foldl (\rsf x -> rsf ++ "\n" ++ x) (head results) (tail results)
  putStrLn output