I misjudged the travel requirements this week, so this newsletter goes out a tad late. Sorry!

Purely functional digital circuit simulator (SICP 3.3)

Lots of people enjoyed the previous article of this series and are looking forward for the next installment – this one – where we make the simulator properly pure.

The approach we'll take is that of a generator function that consumes (and produces) wire signal changes in one lazy sequence, then re-emits those events in another lazy sequence meant for external observers. (This is similar to a scan in FP terminology, but not quite, because the processing function appends to the sequence consumed.)

This is not the ideal implementation for a real digital circuit simulator, but for a toy program with the requirements laid out by Abelson and Sussman, it is very practical.

Full article (4–11 minute read): Purely functional digital circuit simulator (SICP 3.3)

Flashcard of the week

The collection of science in Making Software: What Really Works and Why We Believe It (edited by Oram and Wilson) is useful. Here's one of the things I learned from it.

How much slower does software development go when we require all code to be reviewed by another person, considering first-order effects only?

We can argue that code review requirements make development go faster in the end, because it raises the quality bar and quality is speed. But setting up a randomised trial powerful enough to test that hypothesis would be very expensive. However, we can test (and have tested!) the first-order effect, which is that development goes slower because each change requires more work. The question is how much slower.

It makes development twice as slow.

Involving an external reviewer, compared to reviewing one's own work only, makes each change take twice as long. Much of this time is not time spent reviewing, but (a) time the author spends waiting for input from the reviewer and vice versa, as well as (b) time the author spends rewriting the change due to (hopefully good) feedback from the reviewer.

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