Implementing existing DSLs with Xtext – a case study, part 1

Unlike the previous installment, which held no technical details whatsoever, this one’s going to get a tad greasier, but not much.

Obtaining the language specs

First item on the agenda is obtaining the specifications of the language, if they exist. If they don’t exist, you want to get hold of as many people as you can which might have the specification in their heads. In both cases, you also want to get hold of ‘proza’, i.e. actual content written using the language. It’s important to verify whether that proza is complete and valid, and if not: what error messages are expected. Also, pre-existing tooling can and should be used to validate your own efforts by comparing output from both on the same input.

In the case of CSS it’s relatively easy: all specifications can be found on the W3C web site. However, here it’s already clear that it’s not going to be a walk in the park: the specification is divided into numerous modules which have different status (recommendation vs. official) and varying applicability (through profiles/media: HTML, printing, hearing assistance, etc.).

For Less it’s slightly more difficult: the web site provides a lot of examples but it doesn’t constitute a spec. In fact, careful inspection of the less.js parser sources and unit tests shows that the spec is a bit wider than the examples with respect to selectors of mixins.

This showcases an important non-technical aspect of endeavors such as these:

Customer expectation management

Since the DSL already exists, it’s quite probable that it will have domain users which have been using this DSL and have certain expectations towards this newer implementation. Also, management will have its reasons to go greenlight something that already exists in some shape or form for which they already paid.

It’s important to get social with these customers, gauge their expectations and to set goals for your implementation accordingly. The golden rule here is to provide something that’s good enough to make it worth changing from the existing tooling to the newer tooling. By providing this value as early as possible, you will have an army of testers to battle-harden your implementation and iron out any bugs or incompatibilities.

The silver rule is not to overdo it: pick the initial scope responsibly, get a green light on the scope from users and management, deliver the value and harvest feedback.

In the case of an open-source endeavor like Less/CSS, this is quite a bit trickier: there are plenty of users (certainly of CSS) but probably none share a manager with you so you have to rely on social media to get people to know your stuff, try it out and provide you with feedback. But even here you have to manage  expectations. Note that you can also get feedback some by measuring download numbers and page traffic on the GitHub repository.

For Less/CSS, I’ve set the following initial scope after careful consideration:

Be able to completely parse all CSS and Less unit tests in the GitHub repo, giving errors only where the content really is invalid.

For CSS, this means that I’m ‘vaguely’ going to support CSS3. Obviously, this leaves quite a number of aspects out to dry. To name just a few:

• Compliance to a documented sub set of the CSS specs.
• Knowledge about actual CSS properties.
• Validation of Less expressions (type safety) – this requires implementing a evaluation engine.
• More sensible highlighting than the defaults provide.
• Formatting.
• Generation of CSS from Less.

However, I feel that a basic compliance to the CSS and Less syntax with the niceties that Xtext provides out-of-the-box based on the grammar and a minimal implementation of scoping, reflects the minimum that would make the Less editor useful in the sense that it improves the development experience over eliciting a response from the less.js parser.

Of course, a good CSS editor already exists in the Eclipse WTP project, so I only intend to support CSS to the point that it makes the Less editor more useful. The CSS editor is therefore currently nothing more than a runtime dependency of the Less editor. In fact, it’s more of a development convenience than anything else, at the moment. I might roll the CSS language into the Less language somewhere along the line – probably when I start generating the CSS-intrinsic parts of the grammar.

Future ‘releases’ (i.e., the occasional new download) will gradually provide more functionality, depending on the amount of time I allocate for this project which will also depend on the amount of feedback I get. So, if you like it, be sure to drop me a line saying that and also what you would like to see in the plug-ins.

Finally, a word on:

Harvesting pre-existing implementations

It’s rare that pre-existing tooling can actually be used or even harvested to create an Xtext version, however, for numerous reasons. For example, the parsing tech used might be sufficiently different from Xtext’s LL(*) attribute grammar language to make a pre-existing grammar useless: e.g., it may be LALR(k), it may not be an attribute grammar (meaning that parsing and subsequent model creation are separate phases) or it may use things like semantic predicates and symbol tables to resolve parsing issues. Even an ANTLR grammar is usually quite insufficient to transform into an Xtext grammar.

So, whenever someone (probably a manager) suggests this approach, just point him/her to this paragraph 😉 On the other hand, having the source code of a pre-existing implementation really is very useful, especially as supplement or even substitute of a real language spec.

Please leave a comment if you are finding this useful. In case you’re not able to duplicate my efforts for your own DSL(s) or lack the time to do so yourself: you can hire me as a consultant.