TITLE: When "What" Questions Presuppose "How" AUTHOR: Eugene Wallingford DATE: November 03, 2012 11:17 AM DESC: ----- BODY: John Cook wrote about times in mathematics when maybe you don't need to do what you were asked to do. As one example, he used remainder from division. In many cases, you don't need to do division, because you can find the answer using a different, often simpler, method. We see a variation of John's theme in programming, too. Sometimes, a client will ask for a result in a way that presupposes the method that will be used to produce it. For example, "Use a stack to evaluate these nested expressions." We professors do this to students a lot, because they want the students to learn the particular technique specified. But you see subtle versions of this kind of request more often than you might expect outside the classroom. An important part of learning to design software is learning to tease apart the subtle conflation of interface and implementation in the code we write. Students who learn OO programming after a traditional data structures course usually "get" the idea of data abstraction, yet still approach large problems in ways that let implementations leak out of their abstractions in the form of method names and return values. Kent Beck talked about how this problem afflicts even experienced programmers in his blog entry Naming From the Outside In. Primitive Obsession is another symptom of conflating what we need with how we produce it. For beginners, it's natural to use base types to implement almost any behavior. Hey, the extreme programming principle You Ain't Gonna Need It encourages even us more experienced developers not to create abstractions too soon, until we know we need them and in what form. The convenience offered by hashes, featured so prominently in the scripting languages that many of us use these days, makes it easy to program for a long time without having to code a collection of any sort. But learning to model domain objects as objects -- interfaces that do not presuppose implementation -- is one of the powerful stepping stones on the way to writing supple code, extendible and adaptable in the face of reasonable changes in the spec. -----