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Programming Close to the Domain
A Conversation with Andy Hunt and Dave Thomas, Part VI
by Bill Venners
April 7, 2003

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Speaking the Domain Language

Dave Thomas: Since we wrote the book I came across a good example of programming close to the domain. A company makes software to test telephone switches. The software is hundreds of thousands of lines of C++. You configure the software for the switch being tested. It inputs signals and checks for the correct output. Previously, whenever a new switch came along, they had to alter the main program and recompile 800,000 lines of C++ to support the new switch. Now they use the Ruby scripting language. Ruby has a very simple interface to C++ that makes C++ objects look like Ruby objects. They wrapped their entire test suite in Ruby, so they can access it by writing a Ruby program. They now have a library and wrapper code. When a new switch comes along, they write a 50 line Ruby program to drive it. The Ruby program is a domain-specific language that happens to use this boatload of C++ code in the back end. To support a new switch, they now write code at the domain level. And they find that's made them infinitely more productive.

Andy Hunt: A domain language is just another level of leverage. Just as C is a layer of leverage above assembly language, Java is above C++, a domain language is above Java. A domain language is just one more layer in a stack that gets more and more abstract from assembly up to the domain level.

Bill Venners: Ruby is a general language in which I can program anything. To me, a domain language is specific to a particular kind of programming task, such as testing switches. How was Ruby their domain language?

Dave Thomas: Because in Ruby, I can write code that would say things like, "For each line in the input set, set the status to high." I can actually write code in Ruby that looks very domain specific.

Andy Hunt: So they might say: "For each phone line, snap relay on." The phrase we actually use is, "Program in a language close to the domain." You can do that with macros in C. You want code you can read. You want code you could show to a business person and maybe they'd understand roughly what you're talking about. You want to talk in their language, not the computer language.

Dave Thomas: That's a very good test. If you show the code to the customer and they understand it, at least in principle, then you're programming at the right level.

Andy Hunt: There might be some control flow that the business people don't necessarily understand, but in general, you want them to know what you're talking about. You want to be closer to their language than the computer's language.

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