Monday, February 23, 2009

Three Lessons from a Thirty-Year Bug

Reader Michael Bolton writes:

I'm reading General Principles of Systems Design, and enjoying it. I'm confused by something, and I think it's because of an error in the text.

On page 106, there's a matrix that is intended to describe the bathtubs illustrated in Figure 5.1 and diagrammed in figure 5.2. My interpretation is that the last row of the matrix should read

0 0 1 1 0

The text suggests

0 1 1 1 0

I interpret this as meaning that bathtub 1 could supply water directly to K, the sink; but neither Figure 5.1 nor 5.2 suggest that. Am I misunderstanding, or is there an error?

My response

It's an error in the text, previously unreported.

Seems as though it's been sitting there for 30 years and tens of thousands of readers.

Moral Number One:

Several morals, but to me, the most important one is about testability. Figures 5.1 and 5.2 are in the previous chapter, and difficult to look at while you're looking at the matrix. This makes testing quite difficult. I should have repeated one of the figures so it appeared on the same page as the matrix.

Moral Number Two:

In writing, as in software development, there's no such thing as perfection. (For more on this subject, see my book, Perfect Software, and other illusions about testing.) Just because nobody's found a bug for 29 years doesn't mean one won't turn up in year 30. If you start believing in perfection, you may be in for a nasty shock.

Moral Number Three

: It would have been easy to blame my readers for being careless, inattentive, or just plain dumb not to have detected and reported this bug (which actually appears twice). If I did that, however, I would have shielded myself from learning the first moral, which puts the responsibility squarely on me. If you don't take responsibility for your mistakes, learning doesn't happen.

Slow Learning is Better than No Learning

So, that's three lessons in thirty years. I'm a pretty slow learner, but at least three is better than zero. So, I'm going to be proud of myself for learning at all.

1 comment:

Pradeep Soundararajan said...

Great lessons, not just for the author of the book but for the readers as well.