Type alias circularly reference error

[luan-xiaokun]

Hi guys, I am using langium 0.5, and I got the error Type alias 'Term' circularly references itself when compiling some parse rules like

Term:
    Literal | BracketTerm;

BracketTerm:
    '(' Term ')';

If the Term rule is replaced with Literal | '(' Term ')' then there is no error. This somehow confuses me, am I missing something?

[msujew]

Hey @luan-xiaokun,

usually you would try to make BracketTerm infer the same type as Term for this. This prevents the circular type reference. See the following example in the arithmetics example:

https://github.com/langium/langium/blob/9f587815f9d14f433d22831ac11d3b9f11696d9f/examples/arithmetics/src/language-server/arithmetics.langium#L31-L34

[luan-xiaokun]

It works after adding infers Term to the BracketTerm rule, thanks for your help!
Actually, I want the Term rule to also include tuple term, so I try to eliminate the left recursion by using

BracketTerm infers Term:
    '(' Term ({infer TupleTerm.members+=current} (',' members+=Term)) ')';

I have a question for this parse rule, when I write current, does it refer to the first Term or the whole already seen part '(' Term?

[msujew]

There's no difference between that, since current only affects the AST, wherease '(' is part of the CST.

[luan-xiaokun]

I see, thanks for your answering!