I gave Miles a set of Logo programming problems:

• sv 3 draws a square divided vertically into three columns

• sh 4 draws a square divided horizontally into four rows

• svn 3 4 draws a square with three columns and four rows

(These are going to build towards some work with fractions, but he won’t know that unless he reads my web site. Hi, Miles!)

The first thing he did was place a slider and a button on the screen. The slider ranges from 1 to 10, and the button calls sv with the value of the slider. He used these to test the program while he wrote sv, to quickly try it on different arguments without typing. When he added sh he added another button, and so on for svn.

This looked to me like some sort of hybrid between test-driven development (with a unit testing framework or FIT), and using the command line. It’s more parameterizable than unit tests, but easier to fit into a development cycle than the command line.

What was really interesting, though, was that Extract Method wasn’t a new concept. The initial implementation of sh was copy-pasted from sv, and svn was copy-pasted from both of them. I started on my DRY lecture — “see how this part of sh is the same as this part of sv” — and he jumped the gun. “Oh, cool, you can use aliases?!” he exclaimed, before I even modified any code.

The analogy is between files in a directory and callees in a method. If directory could transclude its contents — list one of its children’s contents as its own — then the analogy would be exact.