Clojure Unit testing : Check if a function was called

Question

I'm trying to write some unit tests for my clojure function (I'm using clojure.test, but I can switch to midje if necessary).

I have a function that reads like :

(defn GenerateNodes
   [is-sky-blue? hot-outside? name]
   (cond
     (is-sky-blue? name) (generate-sky-nodes)
     (hot-outside?) (generate-hot-nodes)))

when unit testing this function, I want to write the following test case :

(deftest when-sky-blue-then-generate-sky-nodes
   (let [is-sky-blue true]
       (GenerateNodes (fn[x] println "sky nodes generated."))
          (is (= true Was-generate-hot-nodes-called?))

How can I assert that the function generate-sky-nodes was called ? or not ? I would use a mocking framework in C# or java, but I don't know about clojure.

Answer 1

What you have already is not far from a working functional version. I changed things around a bit to be more idiomatic to Clojure.

The following assumes that generate-sky-nodes and generate-hot-nodes each return some value (this can be done in addition to any side effects they have), i.e.:

(defn generate-sky-nodes
   []
   (doseq [i (range 10)] (do-make-sky-node i))
   :sky-nodes)

then, your generate-nodes is adjusted as follows:

(defn generate-nodes
  [sky-blue? hot-outside? name]
  (cond
   (sky-blue? name) (generate-sky-nodes)
   (hot-outside?) (generate-hot-nodes)))

and finally, the functional version of the tests:

(deftest when-sky-blue-then-generate-sky-nodes
  (let [truthy (constantly true)
        falsey (constantly false)
        name nil]
  (is (= (generate-nodes truthy falsey name)
         :sky-nodes))
  (is (= (generate-nodes truthy truthy name)
         :sky-nodes))
  (is (not (= (generate-nodes falsey falsey name)
              :sky-nodes)))
  (is (not (= (generate-nodes falsey truthy name)
              :sky-nodes)))))

The general idea is that you don't test what it did, you test what it returns. Then you arrange your code such that (whenever possible) all that matters about a function call is what it returns.

An additional suggestion is to minimize the number of places where side effects happen by using generate-sky-nodes and generate-hot-nodes to return the side effect to be carried out:

(defn generate-sky-nodes
   []
   (fn []
    (doseq [i (range 10)] (do-make-sky-node i))
    :sky-nodes))

and your call of generate-nodes would look like the following:

(apply (generate-nodes blue-test hot-test name) [])

or more succinctly (though admittedly odd if you are less familiar with Clojure):

((generate-nodes blue-test hot-test name))

(mutatis mutandis in the above test code the tests will work with this version as well)