tree.hs

markTree :: Tree () -> State Integer (Tree Integer)
markTree (Leaf _) = do    
    n <- get
    put (n+1)
    return (Leaf n)
markTree (Fork l _ r) = do
    l' <- markTree l
    n  <- get
    put (n+1)
    r' <- markTree r
    return (Fork l' n r')
    
    
numberTree :: Tree () -> Tree Integer
numberTree tree = evalState (markTree tree) 1