Xtext is an open source Eclipse framework for implementing Domain Specific Languages together with their integration in the Eclipse IDE. Xtext lets you implement languages quickly by covering all aspects of a complete language infrastructure, starting from the parser, code generator, or interpreter, up to a full Eclipse IDE integration (with all the typical IDE features such as editor with syntax highlighting, code completion, error markers, automatic build infrastructure, and so on).
This book will incrementally guide you through the very basics of DSL implementation with Xtext and Xtend, such as grammar definition, validation, and code generation; the book will then cover advanced concepts such as unit testing, type checking, and scoping. Xtext comes with good and smart default implementations for all these aspects. However, every single aspect can be customized by the programmer.
Although Java can be used for customizing the implementation of a DSL, Xtext fosters the use of Xtend, a Java-like programming language completely interoperable with the Java type system which features a more compact and easier to use syntax and advanced features such as type inference and lambda expressions. For this reason, we will use Xtend throughout the book.
Most of the chapters have a tutorial nature and will describe the main concepts of Xtext through uncomplicated examples. The book also uses test driven development extensively.
This book aims at being complementary to the official documentation, trying to give you enough information to start being productive in implementing a DSL with Xtext. This book will try to teach you some methodologies and best practices when using Xtext, filling some bits of information that are not present in the official documentation.
The chapters are meant to be read in order, since they typically refer to concepts that were introduced in the previous chapters.
All the examples shown in the book are available online, see the section Downloading the example code. We strongly suggest that you first try to develop the examples while reading the chapters and then compare their implementations with the ones provided by the author.
After a small introduction to the features that a DSL implementation should cover (including integration in an IDE), the book will introduce Xtend since it will be used in all the examples. The book proceeds by explaining the main concepts of Xtext; for example, validation, code generation, and customizations of runtime and UI aspects. The book will then show how to test a DSL implemented in Xtext with Junit in order to follow a Test Driven Development strategy that will help you to quickly implement cleaner and more maintainable code. The test-driven approach is used in the rest of the book when presenting advanced concepts such as type checking and Scoping. The book also shows how to build and release a DSL so that it can be installed in Eclipse and hints on how to build the DSL headlessly in a continuous integration server. The last chapter briefly introduces Xbase.
Chapter 1, Implementing a DSL, gives a brief introduction to Domain Specific Languages (DSL) and sketches the main tasks for implementing a DSL and its integration in an IDE. The chapter also shows how to install Xtext and gives a first idea of what you can do with Xtext.
Chapter 2, Creating Your First Xtext Language, shows a first example of a DSL implemented with Xtext and gives an introduction to some features of the Xtext grammar language. The chapter describes the typical development workflow of programming with Xtext and provides a small introduction to EMF (Eclipse Modeling Framework), a framework on which Xtext is based.
Chapter 3, The Xtend Programming Language, describes the main features of the Xtend programming language, a Java-like language interoperable with Java. We will use Xtend to implement every aspect of an Xtext DSL.
Chapter 4, Validation, describes validation, in particular, the Xtext mechanism to implement validation, that is, the validator. This chapter is about implementing additional constraint checks that cannot be done at parsing time. It also shows how to implement quickfixes corresponding to the errors generated by the validator.
Chapter 5, Code Generation, shows how to write a code generator for an Xtext DSL using the Xtend programming language. The chapter also shows how a DSL implementation can be exported as a Java standalone command-line compiler.
Chapter 6, Customizations, describes the main mechanism for customizing Xtext components, Google Guice, a Dependency Injection framework. In particular, the chapter shows how to customize both the runtime and the UI aspects of an Xtext DSL.
Chapter 7, Testing, describes how to test a DSL implementation using Junit and the additional utility classes provided by Xtext. The chapter shows the typical techniques for testing both the runtime and the UI aspects of a DSL implemented in Xtext.
Chapter 8, An Expression Language, covers the implementation of a DSL for expressions, including arithmetic, boolean, and string expressions. The chapter shows how to deal with recursive rules and with typical problems when writing Xtext grammars. The implementation will be described incrementally and in a test-driven way. The chapter also shows how to implement a type system for checking that expressions are correct with respect to types and how to implement an interpreter for these expressions.
Chapter 9, Type Checking, covers the implementation of a small object-oriented DSL, which can be seen as a smaller version of Java that we call SmallJava. This chapter shows how to implement some type checking techniques that deal with object-oriented features, such as inheritance and subtyping.
Chapter 10, Scoping, covers the main mechanism behind visibility and cross-reference resolution in Xtext. Since scoping and typing are often strictly connected and inter-dependent especially for object-oriented languages, the chapter is based on the SmallJava DSL introduced in the previous chapter. The chapter describes both local and global scoping and how to customize them.
Chapter 11, Building and Releasing, describes how you can release your DSL implementation by creating an Eclipse p2 repository so that others can easily install it in Eclipse. The chapter is based on the Xtext wizard that creates the infrastructure to build a p2 repository with Buckminster. The wizard will also create all the needed files to build your projects and test them in a headless way so that you can easily run your builds in a continuous integration server.
Chapter 12, Xbase, describes Xbase a reusable expression language interoperable with Java. By using Xbase in your DSL, you will inherit all the Xbase mechanisms for performing type checking according to the Java type system and the automatic Java code generation.
The book assumes that you have a good knowledge of Java; it also assumes that you are familiar with Eclipse and its main features. Existing basic knowledge of a compiler implementation would be useful, though not strictly required, since the book will explain all the stages of the development of a DSL.
This book is for programmers who want to learn about Xtext and how to use it to implement a DSL or a programming language together with the Eclipse IDE tooling.
In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.
Code words in text are shown as follows: "We can include other contexts through the use of the include directive".
A block of code is set as follows:
public static void main(String args[]) {
System.out.println("Hello world");
Where keywords of the languages are typeset in bold and static members are typeset in italics (for example, Java static methods and fields).
Bibliographic references are of the form "Author" "year" when there is a single author, or "First author et al." "year" when there is more than one author. Bibliographic references are used for books, printed articles, or articles published on the Web. The Bibliography can be found at the end of the book.
New terms and important words are shown in bold. Words that you see on the screen, in menus or dialog boxes for example, appear in the text like this: "clicking on the Next button moves you to the next screen". When the user is requested to select submenus, we separate each menu with a pipe, like this: "To create a new project, navigate to File | New | Project...".
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