In My Toolkit: Midje

When I switched to Clojure as my primary programming language, the top item on my shopping list was a testing library. Clojure comes with clojure.test, but after 5 years of using RSpec, I wanted something more in its style. I stopped searching when I found Midje. It was exactly what I was looking for. It even had something I didn’t realize I’d been yearning for: metaconstants–more on those later.


Below are some of the highlights of Midje that are intended to give you a taste of its style and capabilities. For the details and additional features, please see its user guide and annotated example.


The fact macro is the basic Midje building block. With it you declare the expected result of an expression. It is striking in its simplicity:

(fact (+ 1 1) => 2)

The expression before the => is the expression being tested and the one after it is the expected result. In this case the result is a specific value, but a rich set of checkers allow for more generality.

Optional document strings allow you to elaborate on your tests:

(fact "basic addition"
  (+ 1 1) => 2)

Multiple related facts may be combined into a single fact–or facts, if you prefer–expression:

(facts "about basic addition"
  "one and one are two"
  (+ 1 1) => 2
  "two and two are four"
  (+ 2 2) => 4)


RSpec has mocks and stubs. Midje has prerequisites. They are specified with the provided clause within a fact:

(fact (sum-of-squares 2 3) => 13
    (square 2) => 4
    (square 3) => 9))

Prerequisites both provide return values for a (possibly not-yet-implemented) function and ensure that it is being called as expected.


Metaconstants are my favorite feature of Midje. They are an elegant solution to what would require extensive use of mock objects in RSpec. For example:

  (sum-scores [..game1.. ..game2..]) => 10
    (score ..game1..) => 3
    (score ..game2..) => 7))

This states that sum-scores will be passing ..game1.. and ..game2.. through to the score function and then summing the results to get its result. The beauty is that the implementation of sum-scores does not care what sort of values ..game1.. and ..game2.. are and, thanks to metaconstants, neither does the test.

Midje Mode for Emacs

If you are an Emacs user, it is worth installing Midje mode. It gives you the ability to send individual facts to the browser and see the results as comments added to the buffer above the fact. This is a big time saver compared to running the tests through, e.g., the Leiningen Midje plugin and makes the red/green/refactor cycle very quick.

As an added bonus, it also updates the Clojure mode indentation rules so that facts format as shown in the above examples.

Getting Midje

Ready to give Midje a try? The source code is available on GitHub and the library is available on Clojars.

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