binary search on a list and BST with Haskell

bsearch.hs

-- GHCi, version 8.8.4
-- explain video https://www.youtube.com/watch?v=LxbJS9L531g&t=798s
import Data.Maybe
import PrettyT
-- data Btree a = Empty | Node a (Btree a) (Btree a) deriving Show

data LR = Lm | Rm deriving Eq

b :: Ord a => [a] -> a -> LR -> Maybe Int
b [] _ _ = Nothing
b xs e lr = go Nothing 0 xs where
                    go c _ [] = c
                    go c cb [a] = if a == e then Just cb else c
                    go c cb x | (me == e) && (lr == Rm) = go (Just $ cb+mi) (cb+(length xl)+1) xr
                              | (me == e) && (lr == Lm) = go (Just $ cb+mi) cb xl
                              | e > me = go c (cb+(length xl)+1) xr
                              | e < me = go c cb xl
                       where
                          (xl,(me:xr)) = splitAt mi x
                          mi = ((length x) `div` 2)

data D = L | R deriving Show
-- binary search in a BST
bs :: Ord a => Btree a -> a -> Maybe [D]
bs t e = reverse <$> go [] t where
                               go f Empty = Nothing
                               go f (Node a l r) | a == e = Just $ f
                                                 | e > a = go (R:f) r
                                                 | e < a = go (L:f) l

build :: Ord a => [a] -> Btree a
build l = foldr ins Empty l

ins :: Ord a => a -> Btree a -> Btree a
ins e Empty = Node e Empty Empty
ins e (Node a l r) | e >= a = Node a l (ins e r)
                   | e < a = Node a (ins e l) r

explain video https://www.youtube.com/watch?v=LxbJS9L531g&t=798s