At first glance it appeared to be something nice, but even the documentation states that "...it is almost always preferable to do this directly now..." - with an anonymous function.

(map #(.charAt %1 %2) ["fred" "ethel" "lucy"] [1 2 3])
-> (\r \h \y)

-- clojure.org, again

I pondered memfn. If it's almost always preferable to use an anonymous function, when is it preferable to use memfn? Nothing came to mind, so I moved on and never really gave memfn another thought.

Then the day came where I needed to test some Clojure code that called some very ugly and complex Java.

In production we have an object that is created in Java and passed directly to Clojure. Interacting with this object is easy (in production); however, creating an instance of that class (while testing) is an entirely different task. My interaction with the instance is minimal, only one method call, but it's an important method call. It needs to work perfectly today and every day forward.

I tried to construct the object myself. I wanted to test my interaction with this object from Clojure, but creating an instance turned out to be quite a significant task. After failing to easily create an instance after 15 minutes I decided to see if memfn could provide a solution. I'd never actually used memfn, but the documentation seemed promising.

In order to verify the behavior I was looking for, all I'll I needed was a function that I could rebind to return an expected value. The memfn macro provided exactly what I needed.

As a (contrived) example, let's assume you want to create a new order with a sequence id generated by incrementAndGet on AtomicLong. In production you'll use an actual AtomicLong and you might see something like the example below.

(def sequence-generator (AtomicLong.))
(defn new-order []
  (hash-map :id (.incrementAndGet sequence-generator)))

(println (new-order)) ; => {:id 1}
(println (new-order)) ; => {:id 2}

While that might be exactly what you need in production, it's generally preferable to use something more explicit while testing. I haven't found an easy way to rebind a Java method (.incrementAndGet in our example); however, if I use memfn I can create a first-class function that is easily rebound.

(def sequence-generator (AtomicLong.))
(def inc&get (memfn incrementAndGet))
(defn new-order []
  (hash-map :id (inc&get sequence-generator)))

(println (new-order)) ; => {:id 1}
(println (new-order)) ; => {:id 2}

At this point we can see that memfn is calling our AtomicLong and our results haven't been altered in anyway. The final example shows a version that uses binding to ensure that inc&get always returns 10.

(def sequence-generator (AtomicLong.))
(def inc&get (memfn incrementAndGet))
(defn new-order []
  (hash-map :id (inc&get sequence-generator)))

(println (new-order)) ; => 1
(println (new-order)) ; => 2
(binding [inc&get (fn [_] 10)]
  (println (new-order)) ; => 10
  (println (new-order))) ; => 10

With inc&get being constant, we can now easily test our new-order function.