How to implement the Observer Design Pattern in a pure functional way?

Question

Let\'s say I want to implement an event bus using a OO programming language. I could do this (pseudocode):

class EventBus

    listeners = []

    public registe         


        

Answer 1

If the Observer pattern is essentially about publishers and subscribers then Clojure has a couple of functions that you could use:

• add-watch
• remove-watch

The add-watch function takes three arguments: a reference, a watch function key, and a watch function that is called when the reference changes state.

Clearly, because of the changes in mutable state, this is not purely functional (as you clearly requested), but add-watcher will give you a way to react to events, if that's the effect you were seeking, like so:

(def number-cats (ref 3))

(defn updated-cat-count [k r o n]
  ;; Takes a function key, reference, old value and new value
  (println (str "Number of cats was " o))
  (println (str "Number of cats is now " n)))

(add-watch number-cats :cat-count-watcher updated-cat-count)

(dosync (alter number-cats inc))

Output:

Number of cats was 3
Number of cats is now 4
4