{-# LANGUAGE UnicodeSyntax #-}
module StateBind where



import Control.Monad.State

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

labelTree ∷ Tree a → Tree Int
labelTree t = fst $ runState (labelTreeW t) 0 -- evalState = fst . runState

labelTreeW ∷ Tree a → State Int (Tree Int)
labelTreeW Leaf = return Leaf
labelTreeW (Node l _ r) =
	labelTreeW l       >>= \l'       →
	get                >>= \counterN →
	put (counterN + 1) >>= \_        →
	labelTreeW r       >>= \r'       →
	return $ Node l' counterN r'