Hitchhiker's Guide to Software Architecture and Everything Else - by Michael Stal

Monday, February 25, 2008

DSLs revisited

DSLs are everywhere. At least, we can read and hear a lot of good things about Domain Specific Languages. Unfortunately, many experts I meet are not always that sure what a DSL really is.

As a lazy person, I will not bore you with my own definition but just cite what Markus Voelter once said. In a former posting Markus defined a DSL as follows:

A DSL is a concise, precise and processable description of a viewpoint, concern or aspect of a system, given in a notation that suits the people who specify that particular viewpoint, concern or aspect.

Given that definition, UML is a vehicle to define DSLs. Take use case diagrams as an example. However, UML might better suit software engineers.

An ADL (Architecture Description Language) like Wright is a formal language to describe architectural designs.

BPEL or ARIS are DSLs to describe business processes. While BPEL is much more system centric, ARIS can also be understood by business analysts.

Thought questions:

  • Is a programming language such as Java also a DSL?

  • And what about natural language?

DSLs are useful to describe a viewpoint, concern or aspect of a system. But, what means do we have to introduce our own DSLs?

  • Meta data annotations (Java) or attributes (.NET) can serve as the core elements of a DSL. Take the different attributes WCF (Windows Communication Foundation) introduces to help describe contracts.

  • UML provides a meta model for introducing DSLs as well as specific DSLs (the different diagrams to specify viewpoints).

  • XML can serve as a foundation for introducing DSLs. Example: BPEL, WS-*, XAML.

  • Programming languages like Ruby or Smalltalk let you intercept the processing of program statements, thus enabling DSLs. Example: Ruby on Rails.

  • Integrated DSLs like LINQ are embedded into their surrounding language.

  • APIs define a kind of language.

  • The hard way might be designing your own language using plain-vanilla tools such as ANTLR. Note: It is not only the compiler or interpreter but also the run-time system we have to cope with.

According to the Pragmatic Programmers designing a DSL can be very effective.

  • It may help you visualize and communication the viewpoint, aspect, concern the DSL addresses.

  • It may enable you providing a generator that generates significant parts instead of forcing you to hand-craft. This is not restricted to code but you could also generate tests or documents.

Stefan Tilkov provided an excellent example. Suppose, you are developing a Java application. Writing the unit tests in JUnit would be the normal way of operation. But, maybe JRuby would be a much better way to write unit tests. As both run on top of the JVM, this can be achieved easily. In this context, we use JRuby+xUnit APIs as our DSL.

I believe, DSLs help climb over the one-size-fits-all barrier traditional programming languages introduce. Creating a DSL does not need to be as complex as defining a programming language. Using XML Schema or Meta data annotations or UML Profiles are nice examples.


  • I don't count Java as a DSL. It's a general purpose language. You build internal Domain-Specific Languages in General Purpose Languages.

    By Anonymous Anonymous, at 5:57 PM  

  • Hi Michael.
    Very good blog. I posted several comments about this DSL Post in my blog, basically adding the "Domain Oriented " constrain requirement and a discussion of what being a language means.

    For instance, the Macro language for MS Excel seems to me like a good DSL example, where the language has a strong focus on the spreadsheet domain (cells, rows, columns, etc). I also look for other examples and how far can I play with being a language focused in a specific domain. Glad if you have a look.

    William Martinez.

    By Anonymous Anonymous, at 1:20 AM  

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