printLeaves.hs

import Control.Concurrent.Async (mapConcurrently_)
import Control.Monad (unless)
import Data.IORef (atomicModifyIORef, newIORef)
import qualified Data.Set as Set

data Tree a
  = Leaf a
  | Node (Tree a) (Tree a)
  deriving (Eq, Ord)

printLeaves :: (Ord a, Show a) => Tree a -> IO ()
printLeaves tree = do
  visited <- newIORef Set.empty
  go visited tree
  where
    go visited element = do
      isVisitedBefore <- atomicModifyIORef visited \visited ->
        (Set.insert element visited, Set.member element visited)
      unless isVisitedBefore do
        case element of
          Leaf x -> print x
          Node l r -> mapConcurrently_ (go visited) [l, r]

main :: IO ()
main =
  printLeaves $
    let x = Node (Leaf 1) (Leaf 2)
     in Node (Node x (Leaf 3)) x