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Expecting Side Effects

Sometimes the code we are testing calls out to other functions to cause side effects. When we are testing, we don't always want side effects to happen but we often want to make sure the code being tested still calls those functions.

Expectations provides a side-effects macro that lets you run the code under test with those side-effecting functions mocked out, and returns a vector of all of the argument lists passed in calls to those functions.

(defn my-fn [x] (cond x        (println "x" x)
                      (nil? x) (println "no value")))

(defexpect my-fn-test
  (expect [["x" 42]]
          (side-effects [println] (my-fn 42)))
  (expect [["no value"]]
          (side-effects [println] (my-fn nil)))
  (expect empty?
          (side-effects [println] (my-fn false))))

Our function under test calls println conditionally. We want to verify the logic so we expect println to be called with "x" and the (truthy) value passed to my-fn, "no value" when nil is passed to my-fn, and for there to be no calls to println if other values are passed (i.e., if false is passed).

By default, mocked out calls return nil, but sometimes your code under test will expect other values back, in order for you to correctly test paths through that code. You can specify the mocked function as a pair of its name and its return value in that case.

(defn my-pred [x] (= 42 x))
(defn my-code [x] (if (my-pred x) (println "good") (println "bad")))

(defexpect my-code-test-1
  (expect [[99] ["bad"]]
          (side-effects [my-pred println] (my-code 99)))
  ;; this will fail because the mocked my-pred returns nil
  (expect [[42] ["good"]]
          (side-effects [my-pred println] (my-code 42))))

(defexpect my-code-test-2
  (expect [[99] ["bad"]]
          (side-effects [my-pred println] (my-code 99)))
  (expect [[42] ["good"]]
          (side-effects [[my-pred true] println] (my-code 42))))

Here's the output of running those two tests:

user=> (my-code-test-1)

FAIL in (my-code-test-1) (...:...)

expected: (=? [[42] ["good"]] (side-effects [my-pred println] (my-code 42)))
  actual: (not (=? [[42] ["good"]] [(42) ("bad")]))
nil
user=> (my-code-test-2)
nil

As we can see, the second expectation in my-code-test-1 fails because my-code calls println with "bad" -- because the mocked my-pred returns nil and so my-code executes the non-truthy path.

In my-code-test-2, we mock my-pred to return true and so my-code executes the truthy path and we can verify that calls println as expected.

Because side-effects can return multiple results, and each result is a sequence of values, it often helps to combine side-effects with more-of:

(defn my-pred [x] (= 42 x))
(defn my-code [x] (if (my-pred x) (println "good") (println "bad")))

(defexpect my-code-test
  (expect (more-of [[x] [msg] :as all]
                   2     (count all) ; check there were just two calls
                   99    x
                   "bad" msg)
          (side-effects [my-pred println] (my-code 99)))
  (expect (more-of [[x] [msg] :as all]
                   2      (count all) ; check there were just two calls
                   42     x
                   "good" msg)
          (side-effects [[my-pred true] println] (my-code 42))))

Expecting Side Effects

Further Reading