Edd Mann Developer

Creating a IPersistentSet compatible Binary Tree in Clojure

Following on from my article on Binary Search Trees last week, I decided to see how I could use types and interfaces to implement the glue that is required to make the Binary Tree implementation compatible with the clojure.lang.IPersistentSet interface. In doing so, this would allow any library or function requiring an IPersistentSet to use the Binary Tree data-structure.


At the end of the previous article we were left with a handful of useful functions which worked with the Binary Search Tree data-structure we had developed. For a recap, below are the definitions for each of the functions.

(defrecord Node [el left right])

(defn insert [tree value] ...)
(defn remove [tree value] ...)
(defn contains? [tree value] ...)
(defn to-list [tree] ...)
(defn search [tree value] ...)

Implementing the IPersistentSet interface

In the code laid out below we are providing a wrapper around the functions that we have already developed - for the contract that is needed to become a IPersistentSet. Creating a new type called BinaryTree which holds a root tree node, allows us to easily handle the use-case of an empty set, along with logical proxying to the existing functions.

(deftype BinaryTree [tree]
  (cons [this value] (BinaryTree. (insert tree value)))
  (disjoin [this value] (BinaryTree. (remove tree value)))
  (empty [this] (BinaryTree. nil))
  (equiv [this that] (if (instance? BinaryTree that)
                       (= tree (.tree that))
  (seq [this] (to-list tree))
  (get [this value] (search tree value))
  (contains [this value] (contains? this value))
  (invoke [this value] (get this value)))


We are now able to supply a BinaryTree in any place that desires a IPersistentSet compatible type. When the function is invoked Clojure polymorphically dispatches to the correct types function implementation.

(def xs (into (BinaryTree. nil) (range 10))) ; #{0 1 2 3 4 5 6 7 8 9}
(disj xs 4) ; #{0 1 2 3 5 6 7 8 9}
(= xs xs) ; true
(= (disj xs 4) (disj xs 2)) ; false
(get xs 5) ; #user.Node{:el 5, :left nil, :right #user.Node...
(xs 5) ; #user.Node{:el 5, :left nil, :right #user.Node...