Defining Primitives within xtext Grammar

Question

I would like to create an incredibly simple DSL using xtext, with the following features:

• It will have two primitive types: Number and String
• Users can define their own classes, which are composed of field declarations
• A field declaration associates a name with a type, where a type can be a class or a primitive

The following is my attempt at a DSL, the class definition and references work fine, but I cannot figure out how to have primitive types. The 'String' and 'Number' literals are not working:

Model:
    (classes+=Class)*
    (fields+=Field)*;

FieldType: Class | 'String' | 'Number';

Field:
    type=[FieldType] name=ID ";";

Class:
    "class" name=ID
    "{"
        (fields+=Field)*
    "}";

Here is an example I expect to be valid against the above DSL:

Class SomeClass {

}

// This works!
SomeClass reference;

// This does not, doesn't recognise the "String" literal
String string;

Note that further down the line I will be supporting assignment. Thus my DSL will need to incorporate the concept of number / string literals, such that it supports Number someNumber = 123;

Answer 1

Having slept on it, I think the right answer is to change my approach. In the above definition we have

Field:
    type=[FieldType] name=ID ";";

This defines a rule called "Field" which is made up of two parts; the "type" and the "name". It is the type part that is presenting the issue. The square brackets denote we are expecting an instance of FieldType, which is:

FieldType: Class | 'String' | 'Number';

Now, it's clear to see that you can have an instance of class, but semantically there's no way to have an instance of the 'String' or 'Number' Literals.

I believe this to be why my DSL above won't let me declare the primitives. The String/Number "types" simply aren't elements that you can have an instance of.

---

Thinking further, there are some very important distinctions between the definitions of fields that are primitive vs. those which are instances of a class. For example, you can only invoke methods on instances of a class (in my case I am treating String as a true primitive, so no methods).

Thus, it's probably important to have two different type of declarations, one for PrimitiveField and one for ObjectField. A field can be either of these:

Model:
    (classes+=Class)*
    (fields+=Field)*;

PrimitiveType: 'String' | 'Number' | 'Boolean';

Field:
    PrimitiveField | ObjectField
;

PrimitiveField:
    type=PrimitiveType name=ID ";"
;
ObjectField:
    type=[Class] name=ID ";";

Class:
    "class" name=ID
    "{"
    (fields+=Field)*
    (methods+=Method)*
    "}";