Reuse: A goal or a by-product?

I found this gem in the comp.object mailing list:

"Re-use is a happy side-effect of good designs. The primary goal is managing dependencies between modules, by making them more pluggable"

I like this statement because it captures an important insight that is too often overlooked due to us following common conceptions without questioning them. Reuse is not a goal. Reuse is a trait of source code that usually goes hand in hand with good programs. This does not mean that a high degree of reuse is a must for achieving high quality code.

If your program's complexity is well controlled then you're in a good position regardless of the amount of duplicated code therein. Conversely, having no duplications does not mean that you successfully tamed complexity. One reason for that is that highly-reused modules are also highly-coupled: even a tiny a modification in a (highly reused) module is likely to break a good fraction of its many clients. This is sheer mathematics: the more clients you have, the greater the chances that you break something.

Degradation of locality is also a risk: when you reuse (for instance by means of inheritance), the various pieces making up your class are actually defined outside the scope the class. This will make it difficult to understand what the class is doing because it's actual definition is scattered across several locations.

These ideas are not new. They were already expressed by Riachard Gabriel in his book "Patterns of Software".

Gabriel coined the term "compression" to describe reuse within a program. In the very first chapter, "Reuse vs. Compression", he says:

"Compression is a little dangerous because it requires the programmer to understand a fair amount about the context from which compressed code will take its meaning.

...The primary feature for easy maintenance is locality: Locality is that characteristic of source code that enables a programmer to understand that source by looking at only a small portion of it. Compressed code doesn’t have this property, unless you are using a very fancy programming environment."