Sum types having members with no attributes

[drhagen]

What is the appropriate way to model sum types where one or more of the members has no payload. For example, the naive grammar for the Option type would be:

grammar HelloWorld

entry Option:
    Some | None;

Some:
    'Some' '(' value = ID ')';

None:
    'None' '(' ')';

hidden terminal WS: /\s+/;
terminal ID: /[a-z]+/;

But this does not work because None has no attributes, which triggers an error: Data type rules cannot infer a type.

How do I tell Langium that None should be inferred to be an interface like:

export interface None extends AstNode {
    readonly $type: 'None';
}

[msujew]

Hey @drhagen,

you can just invoke an action to force Langium to instantiate an object (and therefore create a corresponding type in the ast.ts):

None:
    {infer None} 'None' '(' ')';