trying to find first element with duplicates (doesn't work)

BinarySearch.hs

{-# LANGUAGE ScopedTypeVariables #-}

module BinarySearch where

import Data.Array

binarySearch :: (Ord a1) => Array Int a1 -> a1 -> Int -> Int -> Maybe Int
binarySearch haystack needle lo hi
  | hi < lo = Nothing
  | pivot > needle  = binarySearch haystack needle lo (mid-1)
  | pivot < needle && pivot' < needle  = binarySearch haystack needle mid' hi
  | pivot == needle && pivot' == needle = binarySearch haystack needle mid' hi
  | pivot < needle && pivot' == needle = Just mid'
  | otherwise   = Just mid
  where
    mid = lo + (hi-lo) `div` 2
    mid' = mid + 1
    pivot   = haystack!mid
    pivot' = haystack!mid'

main :: Int -> IO ()
main divisor = do
    let size = 30
        a    = array (0, size) [(i, i `div` divisor) | i <- [0..size]]
    putStrLn $ "hi: " ++ show a
    putStrLn $ "hi: " ++ show (binarySearch a 4 0 size)

-- main 5
--hi: array (0,30) [(0,0),(1,0),(2,0),(3,0),(4,0),(5,1),(6,1),(7,1),(8,1),(9,1),(10,2),(11,2),(12,2),(13,2),(14,2),(15,3),(16,3),(17,3),(18,3),(19,3),(20,4),(21,4),(22,4),(23,4),(24,4),(25,5),(26,5),(27,5),(28,5),(29,5),(30,6)]
--hi: Just 24