Implementing Domain-Specific Languages with Xtext and Xtend: If you know Eclipse then learning how to implement a DSL using Xtext is a natural progression. And this guide makes it easy to get started through a step-by-step approach accompanied with simple examples.

We will now implement a simple DSL to model entities, which can be seen as simple Java classes; each entity can have a super type entity (you can think of it as a Java superclass) and some attributes (similar to Java fields). This example is a variant of the domain model example that can be found in the Xtext documentation.

Xtext uses the MWE2 DSL to configure the generation of its artifacts; the default generated .mwe2 file already comes with good defaults, thus, for the moment, we will not modify it. However, it is interesting to know that by tweaking this file we can request the Xtext generator to generate support for additional features, as we will see later in this book.

During the MWE2 workflow execution, Xtext will generate artifacts related to the UI editor for your DSL, but most important of all, it will derive an ANTLR specification from the Xtext grammar with all the actions to create the AST while parsing. The classes for the nodes of the AST will be generated using the EMF framework (as explained in the next section).

The generator must be run after every modification to the grammar (the .xtext file). The whole generator infrastructure relies on the Generation Gap Pattern (Vlissides 1996). Indeed, code generators are fine, but when you have to customize the generated code: subsequent...

The Eclipse Modeling Framework (EMF) (Steinberg et al., 2008), http://www.eclipse.org/modeling/emf, provides code generation facilities for building tools and applications based on structured data models. Most of the Eclipse projects that in some way deal with modeling are based on EMF since it simplifies the development of complex software applications with its modeling mechanisms. The model specification ( metamodel) can be described in XMI, XML Schema, UML, Rational Rose, or annotated Java. It is also possible to specify the metamodel programmatically using Xcore, which was implemented in Xtext. Typically, a metamodel is defined in the Ecore format, which is basically an implementation of a subset of UML class diagrams.

Now that we have a working DSL, we can do some improvements and modifications to the grammar.

After every modification to the grammar, as we said in the section The Xtext generator, we must run the MWE2 workflow so that Xtext will generate the new ANTLR parser and the updated EMF classes.

First of all, while experimenting with the editor, you might have noted that while

MyEntity[] myattribute;

is a valid sentence of our DSL, this one (note the spaces between the square brackets)

MyEntity[  ] myattribute;

produces a syntax error.

This is not good, since spaces should not be relevant in a DSL (although there are languages like Python and Haskell where spaces are indeed relevant).

The problem is due to the fact that in the Attribute rule, we specified [], thus, no space is allowed between the square brackets; we can modify the rule as follows:

Attribute: type=[Entity] (array?='[' ']')? name=ID ';';

Since we split the two square brackets into two separate tokens, spaces between...

In this chapter, you learned how to implement a simple DSL with Xtext and you saw that, starting from a grammar definition, Xtext automatically generates many artifacts for the DSL, including IDE tooling.

You also started to learn the EMF API that allows you to programmatically manipulate a model representing a program AST. Being able to programmatically access models is crucial to perform additional checks on a program that has been parsed and also to perform code generation, as we will see in the rest of the book.

In the next chapter, we will introduce the new programming language, Xtend (which is shipped with Xtext, and is implemented in Xtext itself): a Java-like general purpose programming language tightly integrated with Java that allows you to write much simpler and much cleaner programs. We will use Xtend in the rest of the book to implement all the aspects of languages implemented in Xtext.