Cross Referencing Issue

[Danfs64]

This is a minimal sample demonstrating a problem I'm having:

entry App: (elem+=Element)*;

Element: (A|B|C);

A: "A" name=ID;

B: "B" name=ID 'with' d=D;

C: "C" name=ID 'ref' ref=[Ref];

D: name=ID;

Ref: (A|D);

With the grammar declared above, the following code:

A ei
B bee with dee
C cee ref dee

Gives an error saying it could not resolve the reference to Ref named 'dee'
If instead of dee I use ei, it works properly, referencing the A node

I don't know if this is a bug or something I missed while developing my grammar, so instead of opening an Issue, I'm asking here first if anyone knows why that is happening

**Edit: Also, on the langium code example, it also gives a This rule is declared but never referenced on Ref

[luan-xiaokun]

I've had this problem before, your grammar is fine, the solution is to implement your own ScopeComputation (a ScopeProvider can also do this I guess). I think the document explains how langium resolves cross reference very well, you should probably check it out first.

According to my understanding, the problem here is that dee is not visible to the node C (it's not in the scope of C), because dee is in the subtree of B, which is a sibling of C. You need to put it in the scope of C, so the linker can resolve the reference. This includes a guide for scoping in langium, which is a good starting point.

Also, on the langium code example, it also gives a This rule is declared but never referenced on Ref

I think using two different properties for A and D in this case would be helpful. (See Mark's comment below)

[luan-xiaokun]

Yeah, I was wrong about the Ref rule. I will strike it out.

[Danfs64]

Oh well, the Document Lifecycle did mention that children of a non-root node aren't visible outside of that node sub-tree, must've missed it when I read it weeks ago.

That said, after some digging, I did managed to figure out how to extend the ScopeComputation to do what I needed (which in my case was: putting the D nodes on the global scope). Thanks for the help both of you 😄

[joswarmer]

I just read the answer with the type type Ref = A | D;. This was helpful for me to solve a problem I had.
Then I started looking for more explanation of the type in the manual but could not find anything. However, I did find an example where an interface is defined.
I get the strong feeling that typescript types and interfaces are allowed inside langium grammar files, but I cannot find this anywhere.
Am I missing something, or is this something that is missing from the documentation?

[msujew]

We have some explanation about declared types here.

[joswarmer]

Thanks, that is very helpful documentation.