The Interpreter Pattern

Languages are written in other languages (at least at first). PHP itself, for example, is written in C. By the same token, odd as it may sound, we can define and run our own languages using PHP. Of course, any language we might create will be slow and somewhat limited. Nonetheless, minilanguages can be very useful, as we will see in this chapter.

The Problem

When we create web (or command line) interfaces in PHP, we give the user access to functionality. The trade-off in interface design is between power and ease of use. As a rule, the more power you give your user, the more cluttered and confusing your interface becomes. Good interface design can help a lot here, of course, but if 90 percent of users are using the same 30 percent of your features, the costs of piling on the functionality may outweigh the benefits. You may wish to consider simplifying your system for most users. But what of the power users, that 10 percent who use your system's advanced features? Perhaps you can accommodate them in a different way. By offering such users a domain language (often called a DSL—Domain Specific Language), you might actually extend the power of your application.