Errors with binary search in Haskell

Question

I just learned about Maybe in Haskell, so I decided to try to use it with a binary search.

Here's the function:

binarySearch :: (Ord a, Int b) => [a] -> a -> b -> b -> Maybe b
binarySearch l e beg last
 | lookat < e = (binarySearch l e i last)
 | lookat > e = (binarySearch l e beg i)
 | lookat == e = Just i
 | otherwise = Nothing
 where i = floor ((beg+last)/2)
 lookat = l !! i

l is a list, e is the element of interest, beg is the start of the section of interest and end is the end of said section. The error I am getting is:

BinarySearch.hs:1:25:
 `Int' is applied to too many type arguments
 In the type signature for `binarySearch':
 binarySearch :: (Ord a, Int b) => [a] -> a -> b -> b -> Maybe b

I have tried a few other things including:

binarySearch :: Ord a => [a] -> a -> Int -> Int -> Maybe b

Yielding the error:

BinarySearch.hs:6:23:
 Could not deduce (b ~ Int)
 from the context (Ord a)
 bound by the type signature for
 binarySearch :: Ord a => [a] -> a -> Int -> Int -> Maybe b
 at BinarySearch.hs:(3,1)-(9,22)
 `b' is a rigid type variable bound by
 the type signature for
 binarySearch :: Ord a => [a] -> a -> Int -> Int -> Maybe b
 at BinarySearch.hs:3:1
 In the first argument of `Just', namely `i'
 In the expression: Just i
 In an equation for `binarySearch':
 binarySearch l e beg last
 | lookat < e = (binarySearch l e i last)
 | lookat > e = (binarySearch l e beg i)
 | lookat == e = Just i
 | otherwise = Nothing
 where
 i = floor ((beg + last) / 2)
 lookat = l !! i

I am not sure what I am doing wrong. Any corrections or comments on style or solution would be highly appreciated.

Answer 1

You don't want Int b, you just want Int:

Ord a => [a] -> a -> Int -> Int -> Maybe Int

• mjgpy3: Thank you! To make it fully work I also had to get rid of float and use quot instead of / when calculating i