Home > DSLs, The How > To mock a…DSL?

To mock a…DSL?

Even with all these language workbenches and DSL frameworks which make creating a DSL a lot easier than it used to be, it’s still not a trivial matter – hence, it takes a non-trivial effort to implement even a rough first draft of a DSL. This also means that it makes sense to get an idea of what your DSL should actually look like before you really start hacking away at the implementation. This is especially true for graphical DSLs which are generally harder to build than textual DSLs, so you can save a lot of wasted efforts by investing in a well-executed mockup phase.

Very rarely you’ll have an actual language specification upfront. The situation that you’re creating a completely new DSL is both much more common and also much more interesting: it is first and foremost an adventure into Domain Land in which you get to learn new people, see interesting things and discover hoards of hidden knowledge, often buried in caches of moldy Word or Excel documents. Often, a language-of-sorts is already around, but without a clearly-defined syntax, let alone a formal definition and without any tooling. It’s our job, as DSL builders, to bring order to this chaos.

Get to know the domain

That’s why the first item on the agenda is getting to know the domain, and the people living it, really well. Us geeks have a tendency to cut corners here: after all, we managed to ace the Compiler and Algorithm Analysis courses, so what could possible be difficult about, say, refrigerator configurations, right? But bear in mind that a DSL is, well, domain-specific so you’d better make sure it fits into the heads of the people which make up your domain and it better fit good, making them more productive even taking into account that they need to learn new tools and probably a new way of working as well.

If the fit is sub-optimal, they’re like to bin your efforts as soon as they’ve come across the first hurdle – which probably even means that you lost your foot in the door regarding model-driven/DSL solutions. (Another way is saying the same thing is that a domain is defined by the group of people which are considered part of it, not the other way around.)

Make mockups

Therefore, the second item on the agenda is coming up with a bunch of mockups. The intent of these mockups is (1) for your domain users to get an idea of their DSL by gauging what actual content would look like expressed in it, and (2) for you to gain feedback from that to validate and guide your DSL design and implementation. It’s important that you use actual content the domain users are really familiar with for these mockups: introducing a DSL tends to be seen as a disruptive innovation even by the most innovation-prone people (and we all know that organizations are rife with that kind…) so your domain users must be able to see where things are going for them.

You don’t achieve that by using something that is too generic/unspecific (e.g., of the ‘Hello world!’ type), too small (which probably means you’re not even beginning to touch corner cases where it really matters what the DSL looks like and how much expressivity it allows) or not broad enough (i.e., overly focusing on a particular aspect the DSL addresses instead of achieving a good spread).

It’s also good to have a few variants for the DSL. There are a lot of, often fairly continuously-valued parameters you can tweak in a DSL:

  1. Is the syntax overly verbose or on-the-verge-of-cryptic minimal?
  2. How is the language factored: one big language or several small ones, each addressing one specific aspect?
  3. How is the prose (i.e., content expressed in the DSL) modularized: one big model, or spread over multiple files?
  4. Is there a way to make content re-usable or to make abstractions?
  5. How is visibility (scoping) and naming (e.g., qualified names with local imports) organized?

Each of these parameters (and also the many more I didn’t list) determine how good the fit with the people in the domain and their way of working is going to be. Also, the eventual language design is going to influence the engineering and the complexity of the implementation directly. The mockup phase is as good a time as any to find out how much leeway you have in the design to lighten the load and optimize implementation efforts and proposing variants is a good way of exploring the space.

What to make mockups with

What do you use for mockups? Anything that allows you to approximate the eventual syntax of the language. For textual DSLs, you can use anything that allows you to mimic the syntax highlighting (i.e., font style and color – I consider highlighting part of the syntax). For graphical DSLs, anything besides Paint could possibly work. In both cases, you’d best pick something that both you and your domain users are already familiar with so that it is easy for everyone involved to contribute to the process by changing stuff and even coming up with their own mockups. Chances are OpenOffice or any of its commercial rivals provide you with more than enough.

Obviously, you’re going to miss out on the tooling aspect: a good editing experience (content assist, navigation of references, outlines, semantic search/compare, etc.) makes up a large part of any modern DSL. Keep in mind though that the DSL prose is going to be read much more often than it is going to be written (i.e., modified or created), and the use of mockups reflects that. Martin Fowler coined the term ‘business-readable DSLs’ because of this and the fact that domain users who are really able to write DSL prose seem to be relatively rare. In any case, you should try whether your domain users that they will be actually be able to create and modify prose, using only the proposed syntax and no tooling.


Having arrived at a good consensus on the DSL’s syntax and design, you can start hammering away at an implementation knowing that the first working draft should not come as a surprise to your domain users. In true Agile fashion, you should present and share working but incomplete versions of the DSL implementation as soon and often as possible and elicit feedback. This also countermands the traditional “MDSD as a magical black box”-thinking which is often present in the uninitiated.

To conclude: making mockups of your DSL before you start implementing is useful and saves you a lot of wasted implementation effort later on.

Categories: DSLs, The How
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