What is Domain Specific Language or DSL ?

What is Domain Specific Language or DSL?

In his book, Martin Fowler defines the DSLs as follows.

 “A computer programming language of limited expressiveness focused on a particular domain.” (Fowler, 2010)

“The basic idea of a domain specific language (DSL) is a computer language that’s targeted to a particular kind of problem, rather than a general purpose language that’s aimed at any kind of software problem.” (Fowler, 2010)

Summing up the above two quotes, a DSL can be define as follows

“A computer programming language of limited expressiveness that are specialized and focused to specify a particular aspect of a software system at a much higher level of abstraction than the general purpose programming language. It is capable of capturing the abstractions related to that particular aspect of the software system under development.”

In the literature, this particular aspect of the software system is also referred by the terms “viewpoint” or the “domain”. So in this context of DSLs, the viewpoint or the domain can be defined as follows

Domain or Viewpoint is the related set of concerns that represent the particular aspect of a whole software system under development that is important from the software development perspective. We can define the two types of domain or viewpoint for a software system, the horizontal and vertical domain (Maier, et al., 200X) (MDA Guide)

Horizontal Domain describes the technical aspects related to the software system like the architectural view, persistency view, user interface view, user interface look and feel view and user interface interaction view. (?)

Vertical Domain describes the functional aspects related to the software system like the insurance policy domain, banking domain, air traffic control domain, air flight reservation domain, medical domain, mobile software domain are the examples of few. (?)

DSLs do not live in isolation but they are integrated. As a DSL fulfils only a narrow purpose and specify a particular aspect of a software system. But the real world software has multiple aspects associated with it and all such aspects are overlapping and tightly integrated to each other. So a DSL in isolation is worthless. In order to specify all aspects of the software system we must have multiple integrated set of DSLs. (Fowler, 2010)

DSLs are not needed to be turing-complete languages like the GPLs. As the scope of these languages is much narrowed to specialized domain of a software system, therefore their capabilities are also limited as compared to the GPLs like Java and C#. A DSL is not needed to be a turing complete language like the GPLs having all kind of controls structures like conditional structures, looping structures. Also these languages are not necessarily to have the capability to create new abstraction styles. The narrowed capabilities are enough to fully specify the particular aspect of the software system. As these languages are more tilted toward the specialized domain, therefore they are expected to have more user-friendly syntax as compared to the GPLs. Due to this feature the programs or models specified in such languages are more expressive, more focused, more fluent and more readable as compared to the GPLs programs or models. (Fowler, 2010)

Examples of the DSLs include CSS, XML configuration files , regular expressions, make, rake, ant, SQL, HQL, many bits of Rails, expectations in JMock, graphviz's dot language, FIT, strut's configuration file.  (Fowler, 2010)