Comment Driven Design

Jef Raskin suggests comments are more important that code (via a link from Raganwald). Like test first design, you should try out comment first design:

A competent programmer who has learned the documentation-first style will sometimes think of a solution in terms of code, write that first, and then document, or will apply a mixed strategy—especially when no convoluted algorithm design is involved. This should not be discouraged so long as the programmer generally adheres to (and sincerely supports) the documentation-first approach.

— Jef Raskin

I actually remember working this way long ago in my perl days and finding it a reasonable way to get started. It’s perl so generally you’re not writing a large application all at once so outlining some of the design with comments and then beginning to build functions worked for me as an approach and as a bonus I got a decent level of documentation for my perl scripts.

Javadoc was also a great find as I migrated into Java. Here you could have generated HTML comments all inline with the code. This used to be pitched as a major advantage over Java other older languages. It’s been a long time since I saw Javadoc talked about as a major feature of the language anymore.

Gradually I left comments behind except when documenting a tricky design decision or a hack that was left in the code for now. I use it still fairly extensively when attempting to add a bug fix or enhancement to some legacy code base so at least future maintenance programmers can have an idea of what I was fixing.

With TDD driving my designs I found more readable code more compelling than any sort comments. I like Ken Pugh’s concept of ‘extreme readability’ from Prefactoring. The idea is the code is so much like the business user’s language that they could review the code and pick up the meaning. At least that’s the goal. Jef Raskin argues that while this idea from XP is a step in the right direction:

When programmers speak of “self-documenting code,” they mean that you should use techniques such as clear and understandable variable names. Instead of n or count, it is better to use a readable, self-explanatory name such as numberOfApricotsPickedToDate. This is a minimalist’s documentation. Nonetheless, it helps—the use of explanatory names, whether of variables, modules, objects, or programs, should be encouraged.

Comments still need to come first because they can’t explain the why of a design decision. His example here is a comment about why an algorithm was chosen:

:Comment: A binary search turned out to be slower than the Boyer-Moore algorithm for the data sets of interest, thus we have used the more complex, but faster method even though this problem does not at first seem amenable to a string search technique. :End Comment

I wonder if this isn’t a straw man type argument. Obviously comments on why a certain algorithm was chosen would be helpful and a good idea for any developer to include. Otherwise they either get lost or end up a design document people are unlikely to read.

It’s a very short article so he doesn’t spend much time with the issues around comments as documentation such as when they get out of sync with the code. There’s no talk of tests as executable documentation.

If one were actually to take a comment driven design approach I can’t see how you’d be able to write useful comments about design decisions. You have to actually write tests and code to make any reasonable comment about why a design was chosen. And then how are you doing document first? Does your comment look like:

// A binary search is used here for better performance.
public String findShortestNode() {

Then doing a few cycles of writing tests and refactoring the code you update it to:

/**
 * A binary search turned out to be slower than
 * the Boyer-Moore algorithm for the data 
 * sets of interest, thus we have used the more
 * complex, but faster method even though 
 * this problem does not at first seem amenable
 * to a string search technique.
 */
public String findShortestNode() {

I think I’ll just keep adding the rare comment to explain why when I need it rather than starting out comment first.

3 comments to Comment Driven Design

  • Thanks. I enjoyed reading this post and know that it will change my code style for the better.

    Cheers.

  • Glad to hear you got some value out of it. I just really prefer well named methods and variables over trying to contain a lot of information in comments.