An Env Comonad example

SearchTreeWithComonad.hs

import Control.Comonad.Env

data Bin a = Node a (Bin a) (Bin a) | Leaf a
    deriving Show


bintree_ex :: Bin Int
bintree_ex = Node 5 (Leaf 3) (Node 8 (Leaf 7) (Leaf 10))


main :: IO ()
main = do
    (_e, v) <- return $ runEnv $
        (env bintree_ex ()) =>> selectW 3
        
    print v
    

selectW :: Int -> Env (Bin Int) a -> Maybe Int
selectW n w =
    extract $
    w =>>
    ask =>> \wbt ->
    case extract wbt of
        Leaf a
            | a == n -> Just a
            | otherwise -> Nothing
        Node a l r
            | n == a -> Just a
            | n > a  -> selectW n (env r ())
            | otherwise  -> selectW n (env l ())