No list of patterns, no matter how exhaustive, can cover every situation that comes up while programming. Eventually (or even frequently) you'll come upon a situation where none of the cookie cutters fits. This need for general approaches to unique problems is one reason to study the theory of programming. Another is the sense of mastery that comes of knowing both what to do and why. Conversations about programming are also more interesting when they cover both theory and practice.
Each pattern carries with it a little bit of theory. There are larger and more pervasive forces at work in programming than are covered in individual patterns, however. This section describes these cross-cutting concerns. They are divided here into two types: values and principles. The values are the universal overarching themes of programming. When I am working well, I hold dear the importance of communicating with other people, removing excess complexity from my code, and keeping my options open. These values -- communication, simplicity, and flexibility -- color every decision I make while programming.
The principles described here aren't as far-reaching or pervasive as the values, but each one is expressed by many of the patterns. The principles bridge between the values, which are universal but often difficult to apply directly, and the patterns, which are clear to apply but specific. I have found it valuable to make the principles explicit for those situations where no pattern applies, or when two mutually exclusive patterns apply equally. Faced with ambiguity, understanding the principles allows me to "make something up" that is consistent with the rest of my practice and likely to turn out well. These three elements -- values, principles, and patterns -- form a balanced expression of a style of development. The patterns describe what to do. The values provide motivation. The principles help translate motive into action. The values, principles, and patterns here are drawn from my own practice, reflection, and conversation with other programmers. We all draw from the experience of previous generations of programmers. The result is a style of development, not the style of development. Different values and different principles will lead to different styles. One of the advantages of laying out a programming style as values, principles, and practices is that it is easier to have productive conflict about programming this way. If you want to do something one way and I another, we can identify the level of our disagreement and avoid wasting time. If we disagree about principles, arguing about where curly braces belong won't solve the underlying discord.
Values
Three values that are consistent with excellence in programming are communication, simplicity, and flexibility. While these three sometimes conflict, more often they complement each other. The best programs offer many options for future extension, contain no extraneous elements, and are easy to read and understand.
