My Secret Life as a Spaghetti Coder
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This one refers to the 40+ minute presentation by Obie Fernandez on Agile DSL Development in Ruby. (Is InfoQ not one of the greatest resources ever?)

You should really view the video for better details, but I'll give a little rundown of the talk here. Obie starts out talking about how you should design the language first with the domain expert, constantly refining it until it is good - and only then should you worry about implementing it (this is about the same procedure you'd follow if you were building an expert system as well). That's where most of the Agility comes into play.

Later, he moves on to describe four different types of design for your DSL. This was something I hadn't really thought about before, therefore it was the most interesting for me. Here are the four types:
  • Instantiation: The DSL consists simply of methods on an object. This is not much different from normal programming. He says it is "DSLish," perhaps the way Blaine Buxton looks at it.

  • Class Macros: DSL as methods on some ancestor class, and subclasses can then use those methods to tweak the behavior of themselves and their subclasses. This follows a declarative style of programming, and is the type of DSL followed by Ruby on Rails (and cfrails) if I understand him correctly.

  • Top-level methods: Your application defines the DSL as a set of top-level methods, and then invokes load with the path to your DSL script. When those methods are called in the configuration file, they modify some central (typically global) data, which your application uses to determine how it should execute. This is like scripting, and (again) if I understood correctly, this is the style my (almost working correctly) Partial Order Planner uses.

  • Sandboxing (aka Contexts): Similar to the Instantiation style, but with more magic. Your DSL is defined as methods of some object, but that object is really just a "sandbox." Interaction with the object's methods modifies some state in the sandbox, which is then queried by the application. This is useful for processing user-maintained scripts kept in a database, and you can vary the behavior by changing the execution context.
Note: These are "almost" quotes from the slides/talk (with a couple of comments by myself), but I didn't want to keep rewinding and such, so they aren't exact. Therefore, I'm giving him credit for the good stuff, but I stake a claim on any mistakes made.

The talk uses Ruby as the implementation language for the example internal DSLs, but I think the four types mentioned above can be generalized. At the least, they are possible in ColdFusion - but with Java, I'm not so sure. In particular, I think you'd need mixin ability for the Top-level methods style of DSL.

Next, Obie catalogs some of the features of Ruby you see used quite often in crafting DSLs: Symbols (because they have less noise than strings), Blocks (for delayed evaluation of code), Modules (for cleaner separation of code), Splats (for parameter arrays), the different types of eval (for dynamic evaluation), and define_method and alias_method.

Finally, he winds up the presentation with a bit about BNL. I liked this part because I found Jay Fields' blog and his series of articles about BNL.

As always, comments, questions, and thoughts are appreciated. Flames - not so much.

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Thanks for the info, it was an interesting presentation.

Posted by Mike on Feb 02, 2010 at 07:23 PM UTC - 5 hrs

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