binsearch :: (Ord a) => [a] -> a -> Int -> Int -> Maybe Int — list, value, low, high, return int
binsearch xs value low high
| high value = binsearch xs value low (mid-1)
| xs!!mid < value = binsearch xs value (mid+1) high
| otherwise = Just mid
where
mid = low + ((high – low) `div` 2)