The Point of WebDSL

Jay asks in the comments:

I don’t mean to be mean, because I really like your blog and I read it all the time… but could you just explain to me the point of WebDSL? I honestly don’t mean it in any kind of negative way, I’m just wondering why you’re dedicating your valuable time to building something like that when RoR, php, et al. already exist.

Just from a cursory look, it seems like the syntax is halfway between Visual Basic (yikes!) and C. Why do you prefer that type of syntax to the Lisp-style syntax of Clojure/Compojure?

Good questions!

WebDSL was started about 3 years ago by Eelco Visser as an exercise the the design and implementation of domain-specific languages. His focus up to then had been on parsing and meta-programming, but it was time to focus on a new domain: the web. When he started Eelco had never built a web application. He investigated a number of Java frameworks as a basis and eventually decided to use JBoss Seam as a target.

The goal of WebDSL is to get rid of the boilerplate code you would have to write when building a Java application and raise the level abstraction. The vision was to have simple, domain-specific sub-languages that allow a programmer to specify a certain aspect of the application and the WebDSL compiler would generate all the implementation code for that aspect. Initially there were three sub-languages: a data modeling language, a user interface language and a simple action language to specify logic. As others joined the project (including myself), we added more sub-languages and more features: access control, workflow, data validation, ajax support and more recently search. Work is also done in the area of data evolution (i.e. migrating databases as you change your data model).

Although WebDSL is mainly a research project, we are increasingly working to make it useable by anybody with some programming experience. We currently have a few websites in production built using WebDSL (researchr, tweetview, and and the manual is growing.

The idea of building abstractions for the web itself is hardly novel. As Jay mentions, there are many web frameworks that already do this: Rails, Django and so on. There are a few things that we do differently in WebDSL, compared to existing frameworks:

  • We create our own custom syntax. Whereas Rails and Django are struggling to express everything using Ruby and Python, respectively, we designed our own clean syntax. Whether you like this syntax is a matter of taste. Personally I like it, although, indeed, it inconsistent here and there.
  • WebDSL is a statically typed and checked language. I wrote a number of posts about this issue and its advantages.
  • WebDSL compiles to low-level Java code, which has good performance characteristics. The code we generate does not rely on run-time meta-programming and reflection features of the language which are typically rather slow.
  • WebDSL is platform independent. We generate Java code now, but it can be ported relatively easily to .NET, Python or PHP. We have prototypes of this utilizing the PIL language that I developed.
  • Within the next few months WebDSL will have excellent IDE support for Eclipse, built using Spoofax/IMP. My colleagues are working on this. It will feature syntax highlighting, as-you-type error reporting, code completion and eventually refactoring support.

A drawback that WebDSL has today is that it’s not trivial to install, but with the IDE plug-in and Java-version that should become a lot easier soon.

So, why am I putting so much effort into this? As you may be aware I’m doing a Ph.D. in the area of domain-specific languages, so we investigate how to best build them. WebDSL is a case study for us. Soon I intend to work on another DSL, in the domain of mobile applications (yes, a DSL to build iPhone and Android applications, people!). It’s interesting from a research perspective to see how to best do this.

In addition I regularly experiment with alternative ways of creating DSLs, like in Clojure and Scala. I’d like to see how far you can push these languages to build the DSLs you like. Clojure allows you to define your own custom syntax, in some sense, as long as you adhere to the rule of the parenthesis. Static error checking is much more problematic. Clojure is also rather tied to one platform, sure, there’s also ClojureCLR, but to write programs that work on both CLR and JVM is, well, challenging. IDE support for a Clojure DSL is also non-trivial.

On the other hand, the flexibility of a DSL like WebDSL also has its downsides. Basically you can design the language any way you like, both its syntax and semantics, you don’t get much for free. Whereas an internal DSL built on Clojure or Scala gets a lot for free: some error reporting, support for namespaces (something we still don’t have in WebDSL), a type system (in Scala’s case), an escape to a powerful language (Clojure or Scala) and a rich set of libraries you can use. In WebDSL we have to design all of this from scratch.

So in the end both approaches have their advantages and disadvantages. I intend to continue to explore them both.