Binary Search to find first occurrence with duplicates

BinarySearch.hs

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 = binarySearch haystack needle (mid+1) hi
  | lo /= mid = binarySearch haystack needle lo mid
  | otherwise   = Just mid
  where
    mid = lo + (hi-lo) `div` 2
    pivot   = haystack!mid


main :: Int -> IO ()
main divisor = do
    let size = 30
        a    = array (0, size) [(i, i `div` divisor) | i <- [0..size]]
    putStrLn $ "array: " ++ show a
    putStrLn $ "bs: " ++ show (binarySearch a 4 0 size)
    
main 5