Now that our trees balance themselves we can keep replicating the built-in set API. This week, we’ll answer two questions:

  1. Is an item in the set?
  2. How many items are in the set?

And we’re going to do it recursively!

Quick Recap

Our sets are AVL trees (a kind of binary search tree) under the covers. We’ve modeled this data structure in our Elm code like this:

type Set comparable
    = Tree Int comparable (Set comparable) (Set comparable)
    | Empty

We can create empty sets using empty, sets with a single item using singleton, and bigger sets by using insert or fromList.

Membership / Contains

It’s one thing to be able to store a set, but it’s not much use if we can’t tell what we’ve stored. What would we want that API to look like? Probably a function that takes an item and a set and returns a boolean value indicating if the value is in the set, right?

member 5 oneThroughFive == True

member 8 oneThroughFive == False

The core API for sets calls this function “member”, but it sometimes makes more sense to call it “contains”. Think of it however makes the most sense for you. Anyway, here’s how we’ll implement it:

member : comparable -> Set comparable -> Bool
member item set =
    case set of
        Empty ->

        Tree _ head left right ->
            if item < head then
                member item left
            else if item > head then
                member item right

A general piece of advice when working with recursive functions: start with the base case first. We’re doing that here with Empty. An empty set does not contain any values by definition, so we return False.

If we have a non-empty set, we do a couple of checks. First, if the item we’re looking for is less than the head, we recurse down the left tree. Otherwise, we recurse down the right tree. If neither of those are true, the item and the head are equal, so we return True.

Lost and Found

Let’s visualize how this works. Given oneThroughFive, we want to find if 5 is in the set:

A set containing the numbers 1 through 5.

A set containing the numbers 1 through 5.

We hit the Set case in our code, and examine the head. 3 and 5 are not equal, but 5 is greater than 3 so we recurse down the right subtree.

A set containing the numbers 4 and 5.

A set containing the numbers 4 and 5.

We again hit the Set case in our code. 4 and 5 are not equal. Surprise! We’re closer, but 5 is greater than 4, so we recurse down the right subtree again.

A set containing only 5.

A set containing only 5.

This time, the head is 5 and our item is 5. They’re equal, so our code returns True. Since we’re not modifying the value on it’s way back up the tree, we return True from the top level.

Lost and Lost

What happens when we search for a value we don’t have? How about 8? We’d take exactly the same path as before, except where we returned True we’d see that 5 and 8 are not equal so we’d recurse down the right tree. When we got there, we’d find that the subtree was Empty.

An empty set.

An empty set.

Our code would return False, and we’d get that back out the top.

Count on Me

Let’s finish off by figuring out how many items are in our set. In oneThroughFive, the answer is 5. We’ll call the new function size.

size : Set comparable -> Int
size set =
    case set of
        Empty ->

        Tree _ _ left right ->
            1 + size left + size right

So what are we doing here? Remember how in member we only ever returned a value, and never modified it on the way back from our recursive calls? Well, we don’t always have to do that! In fact, it’s necessary in this case.

Starting with our base case again, an Empty set contains no items. That’s easy, we’ll return 0.

When we have a non-empty set, we know it at least has one item in it. So we’ll say “I have one, plus however big my left and right subtrees are.” The recursive calls add up!

If we do fromList [8, 15], we’ll get a set that looks like this:

Tree 2 8 Empty (Tree 1 15 Empty Empty)

When we call size eightAndFifteen, we can substitute the subtrees for our addition:

1 + size Empty + size (Tree 1 15 Empty Empty)

The left subtree will come back with 0, so we’re left with:

1 + 0 + size (Tree 1 15 Empty Empty)

And replacing the second size call:

1 + 0 + (1 + size Empty + size Empty)

Again, we know that empty sets have size 0, so:

1 + 0 + (1 + 0 + 0)

When we add all these numbers together, we get 2, which is the correct answer. Yay! When you’re working with recursive functions, it can help to write out the values one step at a time. You’ll see exactly where your logic might be breaking down. It’s a very handy debugging tactic!


So now we have two more tools in our Set toolbox: member and size. And you have two more tools in your recursive function debugging toolbox:

  • Start with the base case first
  • Write out and substitute calls step-by-step to debug.