How to achieve soft keywords?

[ymc9]

Hi,

My Langium schema contains keywords that are contextual. I.e., only has special meaning when used in an expression. For example: X in Y is an InExpression, but I don't want "in" to be an unconditional keyword, so I can use it in other contexts as a regular ID.

Is there a way to achieve this in Langium?

[spoenemann]

The usual solution is to use a "data type rule" for regular IDs:

RegularID returns string: ID | 'in';

That includes the listed keywords where a regular ID is possible. However, that's not considered in syntax highlighting, so in would still be highlighted as keyword regardless where it's used. To avoid that, you can turn off generating the highlighting file and manually remove the keyword.

[msujew]

Note that the in keyword can later still be highlighted as a keyword using the SemanticTokenProvider LSP service. Or in reverse, the in identifier can be highlighted as a variable, even though it is declared as a keyword in the syntactic token provider (textMate).

[ymc9]

Thank you! I found I've already done this in another place, just wasn't sure if it's the right solution 😄.

Yes, the default highlighting behavior is not ideal. I'll look into the solution.

🙏