Use 'type' as an attribute name in interface declaration

[luan-xiaokun]

What I Found

Hi guys, I noticed that type is a keyword in langium grammar language and is disallowed to be used as an attribute name in interface declaration, so the following rule will be rejected by langium.

interface TypeDef {
    type: Type;
    name: string;
}

However, when using the type inference mechanism, the following rule is valid and type is allowed to be an attribute name:

TypeDef:
    'typedef' type=Type 'as' name=ID;

I find that

1. type is actually often used as an attribute name, such as the above example;
2. the explicit type declaration mechanism should be preferred for complicated grammar, as suggested by the document

My Idea

Therefore, I think that supporting 'type' as an attribute name in interface declaration may be helpful to writing complicated grammar.

It seems that type can be used implicitly as an attribute name because of the Assignment parse rule and Action parse rule
https://github.com/langium/langium/blob/9f587815f9d14f433d22831ac11d3b9f11696d9f/packages/langium/src/grammar/langium-grammar.langium#L131-L132
https://github.com/langium/langium/blob/9f587815f9d14f433d22831ac11d3b9f11696d9f/packages/langium/src/grammar/langium-grammar.langium#L83-L84
where the FeatureName includes type and many other keywords
https://github.com/langium/langium/blob/9f587815f9d14f433d22831ac11d3b9f11696d9f/packages/langium/src/grammar/langium-grammar.langium#L198-L199
while the TypeAttribute rule is more restricted, it only allows ID as an attribute name thus excluding type
https://github.com/langium/langium/blob/9f587815f9d14f433d22831ac11d3b9f11696d9f/packages/langium/src/grammar/langium-grammar.langium#L24-L25

I don't know if excluding these keywords in TypeAttribute is an intentional design, such as due to some issues that I have not realized, but as far as I understand, simply replacing ID with FeatureName in the TypeAttribute rule will support the feature I desire, right?

I could be wrong, any comments on this new feature are welcome.

[montymxb]

Hi @luan-xiaokun , this is something we're aware of. I also opened up an issue about this back in June. For now you can preface it with a ^ symbol to allow it, like ^type.

There is a more appropriate data type rule to allow additional property names (like type) internally, which you've found here in as well (FeatureName). It just needs to be added to handle property names in a PR.

[luan-xiaokun]

Thanks for your reply! I didn't find that issue when searching, but I'm glad that there is a workaround.