1280 days ago
Domain Specific Languages with Ola Bini and Martin Fowler — QConSF 2009 Impressions

I’m at QConSF all this week, so you’ll get to hear my impressions of every session I go to. Lucky you!

In college, I never took the “compilers” class that most of my other classmates took. For the first five years of my career I felt smug and superior for not having wasted the time, but I’ve spent every year since then regretting the decision.

Domain Specific Languages (or DSLs) are not a new concept (how long has “make” been around?). But they have been catching a lot of renewed attention lately. We are seeing a convergence of enabling technologies that make creating them easier: dynamic languages like ruby and python and easy-to-use parsers like antlr. This ease is important because, by definition, a DSL has to be specific to a domain and therefore you can’t spread the cost of its creation over very many projects.

Nowadays I thoroughly regret not having taken the compilers class, but Bini and Fowler’s day-long session yesterday helped fill some of the gap. The session was very technical and (frankly) very dry. They talked a great deal about parsers, parse trees and symantic models. But the density was warranted because you can’t really understand the full power of a DSL until you know those concepts. Fowler and Bini gave us just enough background not to hang ourselves with our shiny new ropes.

One point they hammered again and again was that you need to keep your syntax, your semantic model, and your execution separate. For example, if you were to write a new kind of Spring configuration file format in JSON, then JSON would be your syntax, the Spring BeanDefinition interface would be your semantic model, and the Spring GenericApplicationContext might be your executing code. Many implementations of DSLs might be tempted to leap directly from parsing the input to calling code on the fly but according to the presenters that usually leads to heartache as your DSL becomes more complex.

They also went into detail about the difference between an External DSL (something you have to write a parser for, like Ant) and an Internal DSL (basically helper functions on top of an existing language, like Rake).