Is UML a domain specific language (DSL)?

Question

Is it appropriate to think of UML as a DSL?

I know that everybody thinks of UML as a way to draw pictures, but underlying the "view" of UML diagrams there is a model...and the "L" in UML stands for Language.

Another question - is SysML a DSL?

Answer 1

I am a MDA enthusiast so I think I can provide you a very detailed answer to your question. What is the UML: The Object Management Group (OMG), a consortium of companies aimed at providing standard languages and technologies, defined a meta-meta modeling language called "The Meta Object Facility" or MOF (http://www.omg.org/mof). A meta-model is a model describing a model or, in other terms, describing the vocabulary (the elements you can use in a model), the syntax (ho they relate each other) and their semantics (what does each entity mean and how its meaning changes in a given context, etc.). A meta-model plays the same role played by Context-Free grammars with respect to the languages they produce. You can thus think at a meta-meta model as a language you can use to define meta-model. This is what the OMG actually did with the UML. The UML language has a meta-model described by means of the MOF in two documents: The UML Infrastructure and the UML Supersturcture (http://www.omg.org/spec/UML).

The UML meta-model has been defined with the aim of been generic enough to cope with the modeling of different systems belonging to different domains. When you define a new UML model you create an instance of the UML meta-model. You could do that for many reasons: to analyse some characteristics of the system, to share some aspects of the system with other stakeholders and so on. However, one of the most important aspects of the OMG vision are model transformations. You can think at a transformation as a set of rules telling an interpreter how to explore a model and produce something else. You can basically transform a model into two different kind of thins, other models (Model2Model, M2M transformations, defined by means of the QVT language) or text such as code or documentation (Model2Text, M2T, transformations defined by means of the MOFM2T Transformation language). So it is VERY IMPORTANT to understand that a UML model is not its diagram. A diagram is just a pictorical representation of the model contents, useful for umans, but not machine readable. You can't apply transformations to a diagram.

The Eclipse Modeling Framework (EMF) is a very powerful (and FREE!) framework implementing all the technologies I have mentioned. A subset of the MOF is implemented in the Eclipse ECORE language. By means of the ECORE the UML meta-model is defined so grafical UML editors (i.e. Papyrus, TopCased, etc.) actually creates XMI representation of the graphically defined UML models conform to the ECORE representation of the UML meta-model. Such representation can be provided as input to transformations engine. The two transformation languages, and related engines, are also available in the EMF with the QVTo plugin and ACCELEO (implementing the MOFM2T transformation language).

As mentioned UML is intentionally generic. However it also provide lightweight extension mechanisms to extends original language vocabulary with domain specific constructs. This can be done by means of stereotypes. A stereotype is a sort of label (actually with meta-attributes) you can attach on model elements to create new entities in the language. You can for instance say in your models some of the classes could be requirements or something else. There are of course some rules, for instance when you stereotype a meta-class you can not violate its original semantics, just reduce it.

SySML is a profile of the UML http://www.omgsysml.org/. A SysML Block is just a UML class stereotyped as Block, a SysML Requirement is just another UML class stereotyped as Block and so on.

Profiling a meta-model like the UML is a quite easy way of creating a sort of DSL (as with stereotypes you add to a more general language some constructs which belong to your domain) which is compatible with the UML (i.e. you can use SysML and UML together). There is another way of creating a DSL which is defining its meta-model by means of the MOF (ECORE). In this case you create a brand new language which is conceptually at the same level of the UML itself.

Many people say UML is just about diagrams because in many cases they do not know what they are talking about. The topic is far more complex, interesting and promising.

Answer 2

UML is a DSL.

A "domain specific language" lets one specify a problem or a solution in a narrow area of application; banking, telephony circuit design, .... One way to distinguish a DSL is that it cannot do general purpose computation (although there are some DSLs that can). Java, C#, Python and COBOL fail this test. (Some would say COBOL is domain-specific for "business" but its only serious concession to that is a decimal data type, and C# has that too.) ColdFusion fails this test; nonstandard syntax does not a DSL make, but IIRC ColdFusion has some support for generating HTML. Fortran fails this test, but its array(-section) sublanguage is only good for arrays and not general purpose computation. Verilog is very domain specific: it is designed to let you write down digital circuits.

UML focuses on specifying different aspects of how software is structured. [You'll note it can't do general purpose computation; one hallmark]. (It actually has 9 or more different aspects it addresses: classes, statecharts, deployment, ... I'll stick to the class aspect for this discussion). The class diagram aspect lets one describe how data is organized, and operations on that data. You can argue this about software so it can't be "domain-specific". What, building software isn't a problem domain?

SYSML is focused on expressing how systems are joined, so it fits this category too.

A more useful question to ask IMHO is, "If I think of UML as a DSL, what do I gain?" Here I don't think you get a lot. The concept of DSL is useful when you arguing for one you don't have (designed or possess) with the point being better expressiveness for a common problem, and might be useful for arguing "you don't want implement your system entirely in it because it isn't Turing capable". It is also useful if you want to explain that your language is going to have a lot of funny notations, precisely because they serve special purposes. People already know this about UML, so... nothing learned.

While I'm a big fan of DSLs, I'm also a big fan of GPLs (general purpose languages). I think in big systems you should necessarily find a "lot" of both: the DSLs to express what they can succinctly (cuts engineering and maintenance costs), and the GPLs to provide arbitrary computation and glue between the system parts. For me what counts in a language is:

• what's the class of problem it claims to address and how well does it do it?
• what's the syntax (and is it relatively standard for the problem domain)?
• what are the precise semantics (this is where you learn the most)?
• how good is the tool support?
• how well does the DSL integrate into other parts of a big system?
• how big and supportive is the community?

UML has (after 15 years) arrived at pretty good answers to these questions.

Homegrown DSLs often don't do so well, partly due to poor design, but often due to the fact that tool support is difficult to get. My company provides machinery to give DSL builders excellent support to improve this situation.

Answer 3

UML is NOT a DSL because UML can be used to model any vertical domain (insurance software, embedded systems,...)

UML is a (horizontal) DSL because UML is a specialized language to model software systems.

So UML is and is not a DSL depending on how you look at it. You could apply the same reasoning to many other languages like html or SQL. They are general because they can be used to represent/manipulate any kind of data but they are specific because they are focused on one task

Answer 4

I think the answer to your first question depends on how to define "General" in the term "General Purpose Language". Wikipedia says it is not a DSL:

The opposite is:

• a general-purpose programming language, such as C, Java or Python,

• or a general-purpose modeling language such as the Unified Modeling Language (UML).

Answer 5

UML is a general modelling language that is not specific to any domain whilst the S in DSL stands for Specific. UML is used for modelling systems that can also be represented by multi-purpose programming languages. DSL on the other hand are constrained programming/scripting languages which are specific to a particular domain.

Answer 6

Short answer - NO - to both questions.

Think of UML as a tool that lets you describe software architectures, software interactions and so on ... describe them in a general way, language agnostic.

DSLs are specialised syntaxes meant to make it easier to describe some specific set of problems