standard.ml

 functor BFS (structure Q: QUEUE) = (* after Okasaki, ICFP, 2000 *)
  struct 
     datatype 'a tree
      = E
      | T of 'a * 'a tree * 'a tree
    fun bfsQ (q  : 'a tree Q.queue)  : 'a list = 
      if Q.isEmpty q then []
      else let
         val (t, q') = Q.remove q
        in case t
          of E => bfsQ q'
           | T (x, l, r) => let
                val q'' = Q.insert (r, Q.insert (l, q'))
               in
                 x  :: bfsQ q''
                end
         end
     fun bfs t = bfsQ (Q.singleton t)
  end