Imperative tree traversal sample

traverse.hs

{-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Control.Applicative
import Data.Foldable
import Data.Traversable
import Data.IORef

data BinTree a = Leaf a | Node (BinTree a) (BinTree a)
    deriving (Functor, Foldable, Traversable, Show)

exTree :: BinTree Int
exTree = (Leaf 1 `Node` (Leaf 50 `Node` Leaf 20)) `Node` (Leaf 100)


main :: IO ()
main = do
    sum :: IORef Int <- newIORef 0
    flip traverse exTree $ \x -> do
        modifyIORef sum (+x)
        print x
    putStr "Total: "
    print =<< readIORef sum