The grammar for the Expressions DSL

The DSL that we want to implement in this chapter should allow us to write lines containing either a variable consisting of an identifier and an initialization expression (the angle brackets denote non-terminal symbols):

<ID> = <Expression>

or an expression by itself:

<Expression>

If we write something like this

ExpressionsModel:
  variables += Variable*
  expressions += Expression*
; 

We will not be able to write a program where variables and expressions can be defined in any order; we could only write variables first and then expressions.

To achieve the desired flexibility, we introduce an abstract class for both variable declarations and expressions; then, our model will consist of a (possibly empty) sequence of such abstract elements.

For the moment, we consider a very simple kind of expression: integer constants. These are the first rules (we skip the initial declaration parts of the grammar):

ExpressionsModel:
  elements += AbstractElement...