Unix programmers divide all programming into three parts -- policy, mechanism, and glue. Mechanism is code that tells how to do things, policy is code that tells what to do -- and glue is the stuff that binds policy and mechanism together. The punch line: glue is evil and must be destroyed, or at least minimized. Your typical huge honkin' C++ application with classes stacked twelve deep is an un-maintainable mess because the top two layers are policy, the bottom two are mechanism, and the middle eight are glue. And the trouble with glue is that it's opaque -- it impedes your ability to see clear down through the system from the top, or clear up from the bottom. You can't debug what you can't see through, because you can't form an adequate mental model of its behavior. This is very closely matched by the separation of mechanism, policy and glue. "Mechanism" is roughly equivalent to the data "model", "policy" is roughly equiva...