Traversing Scalaz Tree

Question

I'm trying to understand the scalaz tree structure and am having some difficulty!

First I've defined a tree:

val tree: Tree[Int] =
      1.node(
        2.leaf,
        3.node(
          4.leaf,
          5.leaf))

So far using TreeLoc I've worked out how to find the first element that matches some predicate. E.g. to find the first node where the value is 3:

tree.loc.find(x => x.getLabel == 3)

My next challenge was to try and find all nodes that match some predicate. For example I would like to find all leaf nodes (which should be pretty easy using TreeLoc and isLeaf). Unfortunately I can't for the life of me work out how to walk the tree to do this.

Edit: Sorry I don't think I was clear enough in my original question. To be clear I want to walk the tree in such a way that I have information about the Node available to me. Flatten, foldRight etc just allow me to operate on [Int] whereas I want to be able to operate on Tree[Int] (or TreeLoc[Int]).

How do you want the results? The most straightforward approach (if you don't have more specific requirements) would be to use `flatten` on the tree and then filter the resulting `Stream`. — Travis Brown, May 10 '17 at 07:03
What type of tree traversal do you want to achieve? It impacts the order of your output (if you actually care about it). — Federico Pellegatta, May 10 '17 at 11:50
@TravisBrown: my requirement (sorry I should have been more clear on this) is that I want to walk the nodes not the values. In this case flatten would give me a Stream of Ints, so you'd have (for example) no way of knowing if the int came from a Node or a Leaf. — d80tb7, May 11 '17 at 07:38
@FedericoPellegatta: At this point the type of traversal doesn't matter as I wouldn't care about the order. — d80tb7, May 11 '17 at 07:41
@d80tb7 Ah. Off the top of my head then I'd probably write something like `tree.cobind(_.foldNode(a => _.isEmpty.option(a))).flatten.flatten` but you might not want multiple traversals. — Travis Brown, May 11 '17 at 15:02

score 1 · Accepted Answer · answered May 15 '17 at 09:10

Having a look to how find is implemented in scalaz, my suggestion is to implement something like:

implicit class FilterTreeLoc[A](treeLoc: TreeLoc[A]){
  def filter(p: TreeLoc[A] => Boolean): Stream[TreeLoc[A]] =
    Cobind[TreeLoc].cojoin(treeLoc).tree.flatten.filter(p)
}

It behaves like the find but it gives you back instead a Stream[TreeLoc[A]] instead of an Option[TreeLoc[A]].

You can use this as tree.loc.filter(_.isLeaf) and tree.loc.filter(_.getLabel == 3).

Note: the use of an implicit class can be obviously avoided if you prefer to have this declared as a method instead.

Traversing Scalaz Tree

1 Answers1