Error while parsing with an optional reference

[antoinelilly]

Hello,
I am a beginner with Langium and I am trying to build a simple language with Classes, Attributes and an Import functionality. Eventually, I would like to be able to scope for attributes using a Python-like structure : Import.Class.Attribute where the import alias is optional. Here is the grammar I wrote:

grammar EasyClass

entry Model:
    (classes+=Class | imports=Import)*;

Import:
    'import' name=ID;

Class:
    'class' name=ID ('{' (attributes+=Attribute)* '}')?;

Attribute: 
    name=ID ('from' class=ClassCall "." attr=[Attribute:ID])?;

ClassCall:
   (import=[Import:ID] ".")? refclass=[Class:ID]);

It works well when I run the extension on the following code:

import MyImport

class Firstclass {
    zeroAttr
    firstAttr from myImport.SecondClass.zeroAttr
}

class SecondClass {
    secondAttr
}

but if I remove 'myImport.' in my firstAttr declaration, I get the following error message: Could not resolve reference to Import named 'SecondClass'. It looks like Langium is not looking for a Class ID at all, but in my understanding my Attribute grammar should allow Langium to search for either an Import or a Class ID.

But what I find most confusing is that the error disappears once I change the "." by "->" in either my Attribute or ClassCall grammar. Then I am able to call myImport before SecondClass or not. myImport.SecondClass->zeroAttr or SecondClass->zeroAttr both work then .

I also found that I can make it work with "." appearing twice by removing the ClassCall parser rule and rewrite my attributes like this:

Attribute: 
    name=ID ('from' (import=[Import:ID] ".")? refclass=[Class:ID] "." attr=[Attribute:ID])?;

but I don't understand what changed for Langium once I do that.

Can somebody explain to me why the error occurs if I try to do the same thing with "." in both my Attribute and my ClassCall grammar? I'm sure there is something I misunderstand.

Thank you for your help.

[msujew]

Hey @antoinelilly, thanks for trying out Langium :)

When trying to parse the (import=[Import:ID] ".")? refclass=[Class:ID]) part of your grammar, the underlying parser of Langium doesn't know about the semantic meaning of those references. All it sees are: (ID '.')? ID. When trying to decide whether it should parse the (ID '.')? part, the parser will need to look at the next few tokens to see whether that's a valid option. It sees [ID, '.'] in the stack and decides to parse the optional part, even though there isn't enough left afterwards to parse the rest of the parser rule successfully.

What you've just created is called a parser ambiguity since the part after the optional element looks exactly like the optional element itself - the parser cannot differentiate between those options.

In order to support such a grammar, you will need to refactor it into multiple steps of member references. See our guide here. We have a very similar issue in our Langium implementation of SQL. For example, how to decide whether the name A.B is a reference to a table inside of a schema, or an attribute inside of a globally defined table. We solve this using custom scoping as well.

[antoinelilly]

OK I think I understand the problem now and how to fix it for next times.

Thank you so much for this quick answer!