QCon November 6th: DSL Tutorial

For those not following my more stream-of-consciousness reporting on QCon via Tumblr, I thought I'd summarize on a daily basis here.

On November 6th, I attended the Domain Specific Languages tutorial put on by Martin Fowler and Kent Beck. They started the presentation by letting us know they were only prepared for a half-day tutorial and had only recently realized they were scheduled for a full day. They then proceeded to get through the first 'hour' of material by lunch, and took the afternoon for the rest, so all in all it seems like the content was well-suited to the timeframe.

They did a good job of describing the kinds of domain specific languages (they used the terms internal and external, rather than the 'embedded' term that's been used elsewhere), showed some good examples of DSLs in Java and Ruby as well as completely custom DSLs, the tiny languages that the Pragmatic Programmers talk about. They also spent some time describing the boundaries of DSLs: What's a DSL vs. an API; a DSL is usually a thin veneer over a framework; the hardest part in writing a DSL is writing the framework beneath it.

The last portion focused on those external DSLs, using Antlr as an example on how you might build and use a parser for a domain-specific language. This was the most interesting to me, because although I've read a little on the subject, including some of Martin Fowler's exploration of Antlr already, it's the area that I knew the least about (having used jMock's Java DSL for years, and having done a few months of Ruby in anger).

There were a few interesting moments:

• Charlie Poole indicated that some domain specific languages make it harder for non-english speakers to use the API, because they lack the common use of the idioms of the language. For instance, is "2.days.from.tomorrow" easier than Calendar.getInstance().add(Calendar.DAY,2).getTime() for a german speaker? (I dunno, I'm not German).
• Neal Ford, describing the muddy boundary between an API and a DSL, said, "DSLs are like pornography: Hard to define, but I know it when I see it."
• Martin Fowler and Neal Ford have slightly different takes on open classes, the ability to add methods and behavior to classes you don't control, easy to do in Ruby. Martin argued that it was reasonable within the context of an application you control, but riskier in a library that will be integrated with someone else's application (and other libraries). Neal Ford argued that experienced Rubyists find this conflict arises very rarely, and that the benefits outweigh the risks.
  

In the end, I don't know that I actually learned that much, but that's to be expected when you already know a fair bit about the area. I expect that when they release a book on the subject, it'll be a good one, with the kind of detail you can't stuff into a day-long presentation, and particularly valuable to those of you who are interested in, but have little experience with, DSLs.

I ended the day by having some Mexican food from Mijita Cocina Mexicana, possibly the food highlight of my trip (so far).