Chapter 8. An Expression Language

In this chapter, we will implement a DSL for expressions, including arithmetic, boolean, and string expressions. We will do that incrementally and in a test-driven way. Since expressions are by their own nature recursive, writing a grammar for this DSL requires some additional efforts, and this allows us to discover additional features of Xtext grammars.

You will also learn how to implement a type system for a DSL to check that expressions are correct with respect to types, for example, you cannot add an integer and a boolean. We will implement the type system so that it fits the Xtext framework and integrates correctly with the corresponding IDE tooling.

Finally, we will implement an interpreter for these expressions. We will use this interpreter to write a simple code generator that creates a text file with the evaluation of all the expressions of the input file, and also to show the evaluation of an expression in the editor.

This chapter will...