Binary tree search using lenses

LensyBinarySearch.hs

-- Define a simple binary tree
data BT a
    = BT { _leftTree  :: BT a
         , _val   :: a
         , _rightTree :: BT a
         }
    | Leaf
    deriving (Show, Eq, Functor, Foldable, Traversable)
-- Generate traversals for the (partial) fields
makeLenses ''BT

-- Branch into one traversal or the other based on a predicate
branchBy :: (a -> Bool) -> Traversal' a b -> Traversal' a b -> Traversal' a b
branchBy p ta tb f s =
    if p s then ta f s
           else tb f s

-- Look for an element in a binary tree
binSearch :: Int -> BT Int -> Bool
binSearch n = has (deepOf (branchBy (anyOf val (> n)) leftTree rightTree) (val . only n))

--     4
--    / \
--  /     \
-- 2       7
--  \     /
--   3   6
tree :: BT Int
tree =
    BT (BT Leaf 2 (BT Leaf 3 Leaf))
       4
       (BT (BT Leaf 6 Leaf) 7 Leaf)

>>> binSearch 2 tree
True
>>> binSearch 3 tree
True
>>> binSearch 6 tree
True
>>> binSearch 20 tree
False