Thursday, 18 December 2008

Domain Specific Languages (III)

Understanding the Domain Specific Language

The generated graphical DSL includes the following features:
  • Domain Model: a DSL is defined by its domain model. The domain model includes the domain classes and domain relationships that form the basis of the DSL. The domain model is not the same as a model. The domain model is the design-time representation of the DSL, while the model is the run-time instantiation of the DSL. Domain classes are used to create the various elements in the domain, and domain relationships are the links between the elements. The domain model can be seen in the Classes and Relationships swimlane of the image below.

    Every domain relationship has two roles: a source role and a target role. The Name property of a role is the name that is used to navigate from a domain relationship to one of the domain classes that it connects. The PropertyName property of a role is used to navigate from an element at one end of the relationship to the element or elements at the opposite end of the relationship. The DisplayName property of a role is used in the DSL Explorer to show relationships between elements. By default, the value of the DisplayName property is the same as that of the Name property.

    Multiplicities specify how many elements can have the same role in a domain relationship. In most cases, the number of relationships that are connected to a given element is the same as the number of elements that are connected through those relationships. In the diagram, the zero-to-many (0..*) multiplicity setting on the Elements role specifies that any instance of the ExampleModel domain class can have as many ExampleModelHasElements relationship links as you want to give it. A multiplicity of one (1..1) on a role specifies that each instance of the domain class can have only one link in the relationship.

  • Graphical Notation: a DSL has a set of elements that can be easily defined and extended to represent domain-specific constructs. A graphical notation consists of shapes, which represent the elements, and connectors, which represent the relationships between elements, on a graphical diagram surface. The graphical notation can be seen in the Diagram Elements swimlane in the image above.
  • Artifact Generators: one of the main purposes of a DSL is to generate an artefact, for example, source code. A change in the model typically results in a change in the artifact. Here, artifacts are generated from T4 files (file extension .tt). In the Dsl project, look at the T4 files in the GeneratedCode folder, and the corresponding C# files they generate.

  • Serialization: a DSL must be persisted in a format that can be edited, saved and reloaded. DSL Tools use an XML format.

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