TITLE: Thoughts on How to Design AUTHOR: Eugene Wallingford DATE: February 19, 2010 4:33 PM DESC: ----- BODY: Smalltalk Best Practice Patterns is one of my favorite programming books. In it, Kent Beck records some of the design and implementation patterns that he has observed in Smalltalk systems over the years. What makes the book most valuable, though, is that most of its patterns apply beyond Smalltalk, to other object-oriented programming languages and even to non-OO languages. That's because it is really about how we think about and design our programs. Kent's latest design endeavor is what he calls the Responsive Design Project, and he reports some of this thinking so far in a recent blog entry. The entry includes a number of short patterns of design. These are not patterns that show up in designs, but patterns of thinking that help give rise to designs. Being hip deep in teaching functional design style to students whose experience is imperative programming, many of Kent's lessons hit home for me.
Inside or Outside. Change the interface or the implementation but not both at the same time.This is a classic that bears repeating. It's tempting to start making big changes to even a small piece of code, but whenever we conflate changes to interface and implementation, we risk creating more complexity than our small brains can manage.
Isolate Changes. Before making a change, isolate the area to be changed from the rest of the system so you can change an entire element at a time. For example, before changing a part of a procedure, extract the area to be changed into its own procedure. Make the change, then inline the changed sub-procedure if appropriate.This one is beyond the ken of most intermediate-level students, so it doesn't show up in my courses often. When we confine a change to a small box, we control the complexity of the change and the range of its effect. This technique can even be used to control software evolution at a higher level.
Exploit Symmetries. Divide similar elements into identical parts and different parts.Many beginning programmers find it counter-intuitive that the best way to eliminate duplicated code is to increase the level of duplication, maximizing the repetition to the point that it can be factored out in the cleanest and sharpest way. I have come to know this pattern well but sense that its value runs much deeper than the uses to which I've put it thus far. Then there is the seemingly contradictory pair
Cultivate Confidence. Master your tools. Your feeling of mastery will improve your cognition.and
Cultivate Humility. Try tools or techniques you aren't comfortable with. Being aware of your limitations will improve your effectiveness.Of course, the practices themselves aren't contradictory at all, though the notion that one can be confident and humble at the same time might seem to be. But even that holds no contradiction, because it's all about the edge between mastery and learning. I often talk about these patterns in my programming classes, if only hope that a student who is already starting to sense the tension between hubris and humility will know that it's okay to walk the line. Finally, my nominee for best new pattern name:
Both. Faced with design alternatives without a clear winner, do it every way. Just coding each alternative for an hour is more productive than arguing for days about which is better in theory, and a lot more satisfying.You may have heard the adage, "Listen to your code", which is often attributed to Kent or to Ward Cunningham. This pattern goes one step beyond. Create the code that can tell you what you need to hear. Time spent talking about what code might do is often much less productive than simply writing the code and finding out directly. Early in his essay, Kent expresses the lesson that summarizes much of what follows as
Our illusion of control over software design is a dangerous conceit best abandoned.He says this lesson "disturbs and excites me". I guess I'm not too disturbed by this notion, because feeling out of control when working in a new domain or style or language has become routine for me. I often feel as if I'm stumbling around in the dark while a program grows into just what it needs to be, and then I see it. Then I feel like I'm in control. I knew where I was going all the time. In my role as a teacher, this pattern holds great danger. It is easy after the fact to walk into a classroom and expound at length about a design or a program as if I understood it all along. Students sometimes think that they should feel in control all the time, too, and when they don't they become discouraged or scared. But the controlled re-telling of the story is a sham; I was no more in control while writing my program than they will be when they write theirs. What excites me most about this pattern is that it lifts a burden from our backs that we usually didn't know we were carrying. Once we get it, we are able to move on to the real business of writing and learning, confident in the knowledge that we'll feel out of control much of the time. -----