Hi Joe, I think you wanted (length list - 1) where you call do_search. The version below is my rewrite with guards, this change, and "midVal" which keeps "list !! mid" from being evaluated twice per recursion. Unfortunately, I have no idea how to work a fold into this. Good luck! --Tim binary_find :: Ord a => [a] -> a -> Maybe Int binary_find [] elem = Nothing binary_find list elem = do_search list elem 0 (length list -1) where do_search list elem low high | high < low = Nothing | midVal > elem = do_search list elem low (mid - 1) | midVal < elem = do_search list elem (mid + 1) high | otherwise = Just mid where midVal = list !! mid mid = low + (high - low) `div` 2 main = do print $ binary_find [1] 2 print $ binary_find [1,3] 1 print $ binary_find [1,3,4] 3 print $ binary_find [1,3,4] 4 print $ binary_find [1,2,4,6,8,9,12,15,17,20] 17 print $ binary_find "hello" 'l' print $ binary_find [0.0, 1.5, 3.0] 3.0 print $ binary_find [] 1 print $ binary_find [1,3] 2 print $ binary_find [1,4,6,8,9,12,15,17,20] 2 -- boom? print $ binary_find [1,4,6,8,9,12,15,17,20] 19