Following pointers from Ned Batchelder's recent excursion into code generation led me to another nice example of the power of dynamic languages. In order to streamline his use of C++, Ned wrote a little tool called cog which enables him to embed, in C++ programs, Python fragments that generate verbose and/or repetitive C++ constructs. He adds:
For more about code generation in general, try:
- Dave Thomas interviewed about code generation. Dave Thomas is one of the Pragmatic Programmers, and I find I agree with him almost universally. He forbids putting the output of code generators under source control, I encourage it. We agree that the output should never be edited.
- The Code Generation page on the c2 wiki. As will happen with a wiki, this fractures off in many directions, with many different viewpoints, both for and against code generation.
In the interview Ned cites, Dave Thomas gives an example of a Ruby feature that I've heard of, but never had occasion to use. In a class definition you can write Ruby code to define a type. That means, as Thomas puts it, that "you can effectively extend the language at runtime from within." Statements like that have a tendency to alienate people. It can sound like the drug-induced fantasy of some idealistic tree-hugging Birkenstock-wearer who isn't living in the real world of Enterprise Software Development. But Thomas backs it up with a great example. In this case, he used Ruby's dynamic extensibility to wrap a database schema in classes that can either persist objects to the database, or create schema documentation, depending on how the methods that dynamically define those classes are defined.
Once upon a time Tom Christiansen gave me a great quote, which he attributes to Andrew Hume: "Programs that write programs are the happiest programs in the world." Templating and code generation are examples of this happy strategy. We've always known that dynamic languages are a great way to create "little languages" for specific tasks. But we don't yet fully appreciate that all programming is a continuous process of language invention. And we don't (yet) evaluate programming-language productivity on those terms. Dave Thomas:
I'm betting that languages such as Java and C++ will in the long term be seen as a curious branch in the evolution of computing. I'm hoping that somewhere out there some bright spark is coming up with a way of letting us write applications expressively and dynamically. Once this happens, the need for these kinds of code generators will diminish.We are linguistic animals endowed with a protean ability to generate language. Naturally we'll want that same generative power in our programming languages.
For example, I rarely (if ever) write a code generator that generates Ruby code: there's just no need, as Ruby is dynamic enough to let be do what I want without leaving the language.
Former URL: http://weblog.infoworld.com/udell/2004/02/11.html#a915