Better Living through Thoughtful Technology

## Doc-driven Development Using lein-midje-doc

• Apr 11 / 2014
Clojure

## Doc-driven Development Using lein-midje-doc

This is one of a series of posts on my experiences developing Clortex in Clojure, a new dialect of LISP which runs on the Java Virtual Machine. Clortex is a re-implementation of Numenta’s NuPIC, based on Jeff Hawkins’ theories of computational neuroscience. You can read my in-progress book by clicking on the links to the right. Clortex will become public this month.

One of the great things about Clojure is the vigour and quality of work being done to create the very best tools and libraries for developing in Clojure and harnessing its expressive power. Chris Zheng‘s lein-midje-doc is an excellent example. As its name suggests, it’s uses the comprehensive Midje testing library, but in a literate programming style which produces documentation or tutorials.

## Doc-driven Development

Before we get to DDD, let’s review its antecedent, TDD.

### Test-driven Development

Test-driven Development (TDD) has become practically a tradition, arising from the Agile development movement. In TDD, you develop your code based on creating small tests first (these specify what your code will have to do); the tests fail at first because you haven’t yet written code to make them pass. You then write some code which makes a test pass, and proceed until all tests pass. Keep writing new tests, failing and coding to pass, until the tests encompass the full spec for your new feature or functionality.

For example, to write some code which finds the sum of two numbers, you might first do the following:
 

(fact "adding two numbers returns their sum" ; "fact" is a Midje term for a property under test
(my-sum 7 5) => 12 ; "=>" says the form on the left should return or match the form on the right
)

 

This will fail, firstly because there is no function my-sum. To fix this, write the following:
 

(defn my-sum [a b]
12)
)

 

Note that this is the correct way to pass the test (it’s minimal). Midje will go green (all tests pass). Now we need to force the code to generalise:
 

(fact "adding two numbers returns their sum"
(my-sum 7 5) => 12
(my-sum 14 10) => 24
)

 

Which makes us actually write the code in place of 12:
 

(defn my-sum [a b]
(+ a b)
)

 

The great advantage of TDD is that you don’t ever write code unless it is to pass a test you’ve created. As Rich Hickey says: “More code means… more bugs!” so you should strive to write the minimum code which solves your problem, as specified in your tests. The disadvantage of TDD is that it shifts the work into designing a series of tests which (you hope) defines your problem well. This is better than designing by coding, but another level seems to be required. Enter literate programming.

### Literate Programming

This style of development was invented by the famous Donald Knuth back in the 70’s. Knuth’s argument is that software is a form of communication. The first is obvious: a human (the developer) is communicating instructions to the machine in the form of source code. The second is less obvious but perhaps more important: you are communicating your requirements, intentions and design decisions to other humans (including your future self). Knuth designed and built a system for literate programming, and this forms the basis for all similar systems today.

This post is an example of literate programming (although how ‘literate’ it is is left to the reader to decide), in that I am forming a narrative to explain a concrete software idea, using text, and interspersing it with code examples which could be executed by a system processing the document.

### Doc-driven Development

DDD is essentially a combination of TDD and Literate Programming. Essentially, you write a document about some part of your software, a narrative describing what it should do and examples of how it should work. The examples are framed as “given this, the following should be expected to happen”, which you write as facts (a type of test which is easier for humans to read). The DDD system runs the examples and checks the return values to see if they match the expectations in your document.

This big advantage of this is that your documentation is a much higher-level product than a list of unit tests, in that your text provides the reader (including your future self returning to the code) with much more than a close inspection of test code would yield. In addition, your sample code and docs are guaranteed to stay in synch with your code, because they actually run your code every time it changes.

## lein-midje-doc

lein-midje-doc was developed by Chris Zheng as a plugin for the Clojure Leiningen project and build tool. It leverages Midje to convert documents written in the Literate Programming style into suites of tests which can verify the code described.

It’s simple to set up. You have to add dependencies to your project.clj file, and then you add an entry for each document you wish to include (instructions are in the README.md, full docs on Chris’ site). then you use two shells to run things. In one, you run lein midje-doc, which repeatedly creates the readable documents as HTML files from your source files, and in the other you run midje.repl‘s (autotest) function to actually spin through your tests.

Here’s Chris demonstrating lein-midje-doc: