How to get all types and each rules required fields

[Lemour-sudo]

I have parser rules that expect multiple fields to be included: Assert: 'assert' '(' value1=STRING possibleValues+=STRING ')';.
For each parser rule I want to get its metadata that will tell me the type and the required fields. In this example, I would expect something like:

{
    name: 'Assert',
    parameters: {
        value1: string;
        value2: string[];
    }
}

I did come across the mandatory property returned as part of a type's metadata with the getTypeMetaData method in the generated "ast.ts" file. However, the mandatory property is just a list of one required field:

{
    name: 'Assert',
    mandatory: [
        { name: 'value2', type: 'array' }
    ]
}

First, value1 is not included. Second, from the above metadata, it is not clear what type value2 is an array of.

Is it possible in Langium to get all a parser rule's required fields like in the above example?

[msujew]

Hey @Lemour-sudo,

in the current next release (see also #1165), all properties are now being exposed by the API. Note that the getTypeMetaData in v2 is mainly an internal API: It tells the parser which fields on an AST node can be filled with reasonable defaults. This only includes arrays (defaulting to []) and booleans (defaulting to false). Filling other fields would lead to unexpected side effects.

Note that you can always get the fields of an AST node by using Object.keys(node) and filtering out the fields starting with $. This assumes that the node has been parsed correctly without parser errors.

[Lemour-sudo]

Thanks for the reply.

I see that AST is an interface in "ast.ts", how can I treat it as an object instead in order to go through its keys as you suggested?

[msujew]

Interfaces are a compile time construct of TypeScript. They are deleted when generating the JavaScript code, since JS doesn't have a notion of an interface. There's also no corresponding object that gets generated instead.

[Lemour-sudo]

I was actually referring to the node object you mentioned in the first reply. How can I find this node in Langium?

[msujew]

I meant an instance of an AstNode interface object (i.e. any of your AST nodes). Assuming there was no parser error, they should contain all non-nullable elements.