In computer programming, the interpreter pattern is a design pattern that specifies how to evaluate sentences in a language. The basic idea is to have a class for each symbol (terminal or nonterminal) in a specialized computer language. The syntax tree of a sentence in the language is an instance of the composite pattern and is used to evaluate (interpret) the sentence.

More about this pattern.

Along with the visitor pattern, this is still a very useful pattern, but in a very small but important context. Parsing and executing code. We see those quite often. In particular, JavaScript is probably the most common place where we see interpreters.

That said, unless you are actually dealing with executing code, there is very little reason for you to want to apply this pattern. In fact, I have seem people go for that several times for purposes that I really can’t explain.

Interpreter is for code execution. It has some interesting differences from compiling the code. For one, it is a lot easier to write, and for the most part, performance is great. This is especially true because the hard parts (the text parsing) are usually done up front and then you are just executing the AST.

From my perspective, we use Jint, a JS interpreter in RavenDB because compiling to IL and running that was complex. Any bugs there was complex to figure out, and most important from our point of view, we really needed to be able to place limits on what you could do. The number of steps that can be taken, the recursion depth, and so on. Doing so with compiled code requires you to have kernel level access, or doing things like Thread Abort.

So Interpreters are good, but watch out when you use it, if it ain’t code that you are going to run, why are you writing this in the first place?