Thanks all for your help. Here's another one. Seems like I could use a fold here, but am unsure how that would work. Also, if I pass in a search value that's too big, then the function blows up. (source at http://github.com/joevandyk/haskell/raw/master/pearls/binary_search/binary_s... -- Feel free to fork it.) -- A translation of http://en.wikipedia.org/wiki/Binary_search_algorithm#Recursive binary_find :: Ord a => [a] -> a -> Maybe Int binary_find [] elem = Nothing binary_find list elem = do_search list elem 0 (length list) where do_search list elem low high = if high < low then Nothing else if list !! mid > elem then do_search list elem low (mid - 1) else if list !! mid < elem then do_search list elem (mid + 1) high else Just mid where mid = low + (high - low) `div` 2 main = do print $ binary_find [1] 1 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] 100