title: Traversing a graph breadth first, marking visited nodes in Haskell
body: <p>So the question is simple. Given a graph (I hope that the structure of the graph doesn't matter much in this problem), how do I go about doing a BFS on it?</p>
  
  <p>I've recently asked a question about generating a list where each element appends many elements to the end of it. The answer should hopefully let me make a queue I need to do the BFS. But there's another key component that a search needs and it's marking the nodes as visited so we don't go over them again. This also needs to have no overhead on the execution of the algorithm. Neither marking or reading.</p>
  
  <p>Since Haskell doesn't let me change state, how would I go about doing that?</p>
  
  <p>(I'm not looking for a way to translate my imperative code to Haskell. An idiomatic Haskell solution would be great.)</p>