The Observer pattern revisited

In Chapter 1, What is Reactive Programming?, we saw a brief overview of the Observer design pattern and a simple implementation of it in Clojure using watches. Here's how we did it:

(def numbers (atom []))

(defn adder [key ref old-state new-state]
  (print "Current sum is " (reduce + new-state)))

(add-watch numbers :adder adder) 

In the preceding example, our observable subject is the var, numbers. The observer is the adder watch. When the observable changes, it pushes its changes to the observer synchronously.

Now, contrast this to working with sequences:

(->> [1 2 3 4 5 6]
     (map inc)
     (filter even?)
     (reduce +))

This time around, the vector is the subject being observed and the functions processing it can be thought of as the observers. However, this works in a pull-based model. The vector doesn't push any elements down the sequence. Instead, map and friends ask the sequence for more elements. This is a synchronous operation...