Prefactoring

3.7. Don't Repeat Yourself

The "Adapt a Prefactoring Attitude" guideline is a specialization of the "Don't Repeat Yourself (DRY) principle of Andrew Hunt and David Thomas (The Pragmatic Programmer: From Journeyman to Master, Addison-Wesley Professional, 1999). The concept is that information should have one authoritative source. If information is needed in multiple ways, a transformation process converts it from the single source into the other formats. By doing so, information needs to be changed only in one place. Dave Thomas says, "The idea is to try to plan ahead to prevent duplication, rather than to waste time removing stuff you've already done."

For example, an XML description of a data table can be transformed into SQL commands to create the table, as well as language-specific classes to access the table. Changes in the organization of the data table need to be made only in one place.

DON'T REPEAT YOURSELF (DRY) Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.