Xtext uses the MWE2 (Modeling Workflow Engine 2) DSL to configure the generation of its artifacts. The 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...

The EMF (Eclipse Modeling Framework) (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 mechanisms. The model specification (metamodel) can be described in XMI, XML Schema, Unified Modeling Language (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 similar to 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 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

MyEntity[] myattribute;

is a valid statement of our DSL, while the one below (note the spaces between the square brackets):

MyEntity[  ] myattribute;

produces a syntax error.

This is not good, since we do not want spaces to be relevant (although there are languages such as 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...

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 programming language Xtend, which is shipped with Xtext and is implemented in Xtext itself. Xtend is a Java-like general purpose programming language, completely inter-operable 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 Xtext languages.