How to use ValueConverter service for converting e.g. ID and STRING values

[hallvard]

I'm traversing the AST I receive from the parser, and can traverse using the generated isXYZ predicates and types. However, when I read a STRING feature, it includes the quotes, and although I can strip them off myself, I guess the correct way to do it is to use a ValueConverter. However, it has a convert methods that takes the value and a CstNode and I haven't found out how to obtain the the latter. My first guess was that this was related to my grammar access, but although I can get the RuleCall corresponding to the STRING feature, I don't know how to turn that into a CstNode. I notice the AstNode as a $cst field and that inside there is a child corresponding to my STRING feature, but I cannot find API for getting it.

What is the correct approach?

[msujew]

Hi @hallvard, I guess we f'd something up. The default value converter will strip the quotes if you rename the STRING terminal to string (even though it will show a warning). The reason for that is this code:

https://github.com/langium/langium/blob/fbdef5d08b059e182a86db60dc06ef46e44d35e5/packages/langium/src/parser/value-converter.ts#L32-L36

On a more general note: You don't have to retrieve the CstNode yourself, you can just override the protected runConverter method, and bind your custom ValueConverter in your services declaration.

[spoenemann]

Just use boolean as the return type. The default value converter will take care of that:

https://github.com/langium/langium/blob/fbdef5d08b059e182a86db60dc06ef46e44d35e5/packages/langium/src/parser/value-converter.ts#L37-L41

[msujew]

Actually, the default value converter does not support transforming yes/no to true/false. @hallvard you are right, you would have to define your own value converter for that like this:

export class QuizValueConverter extends DefaultValueConverter {
    protected runConverter(rule: GrammarAST.AbstractRule, input: string, cstNode: CstNode): ValueType {
        if (rule.name === 'YesNo') {
            return input === 'yes';
        } else {
            return super.runConverter(rule, input, cstNode);
        }
    }
}

Then, bind this new value converter in your services like this:

export const QuizModule: Module<QuizServices, PartialLangiumServices & QuizAddedServices> = {
    // The rest of the module comes here
    parser: {
        ValueConverter: () => new QuizValueConverter()
    },

[drhagen]

@msujew Would you mind updating this solution? Rule does not appear to be defined anymore, and VSCode is giving me red squiggles on parser.

[msujew]

I've updated the Rule type reference. What kind of error do you get on parser? That part at least hasn't changed in the meantime.

[drhagen]

Never mind, parser is fine now. Thanks for updating the example.