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Langium version: 1.0.0 Package name: langium

Chevrotain version; 10.4.2

Steps To Reproduce

1. Create Hello World Example
2. Replace Grammar with the following grammar

grammar HelloWorld

entry Model: GroupContent;

Person: 'person' name=ID;

fragment SimpleGroupContent:
    (persons+=Person | greetings+=Greeting )*;
fragment GroupContent:
    (persons+=Person | greetings+=Greeting | groups+=Group)*;

Group: 'group' name=ID ('(' 'info' '=' info=STRING ')')? (
        ('{'
            GroupContent
        '}')|
        (  // ';' // <.. this line seems to fix the effect
            SimpleGroupContent
        )
    );

Greeting:
    'Hello' person=[Person:ID] '!';

hidden terminal WS: /\s+/;
terminal ID: /[_a-zA-Z][\w_]*/;
terminal INT returns number: /[0-9]+/;
terminal STRING: /"[^"]*"|'[^']*'/;

hidden terminal ML_COMMENT: /\/\*[\s\S]*?\*\//;
hidden terminal SL_COMMENT: /\/\/[^\n\r]*/;

3. Compile and run the IDE, enter the following model

person Pi
Hello Pi!
group G1 ( info="Hello") {
    person Tim1
    Hello Tim1!
}
group G2 ( info="Hello") { // resulting error indicated here: "Expecting end of file but found `{`."
    p_ERROR_erson Pi  // <--- Error entered here, expecting something like: "Expecting token of type '}' but found `p_ERROR_erson`."
    Hello Pi!
}

The current behavior

• The IDE indicates the error at "{"
• Completion does not work properly

The expected behavior

• Correct error message in the line of the error
• Better completion performance

The strange thing is, that if a ';' is added to the grammar (sketched above), the problem gets fixed. Also I did not observe this with my Langium 0.0.5 version of hello-world.

Maybe the following Chevrotain issue is related https://github.com/Chevrotain/chevrotain/issues/1868

This is likely related to https://github.com/langium/langium/pull/755. Do you mind adding the chevrotainParserConfig.maxLookahead property to the langium config? Similar to here.

Thank you, @msujew ! This helped... Adding the property - like you suggested - solved the issue.

{
    "projectName": "HelloWorld",
    "languages": [{
        "id": "hello-world",
        "grammar": "src/language-server/hello-world.langium",
        "fileExtensions": [".hello"],
        "chevrotainParserConfig": {
            "maxLookahead": 3
        },
        "textMate": {
            "out": "syntaxes/hello-world.tmLanguage.json"
        }       
    }],
    "out": "src/language-server/generated"
}

It remains unclear to me, if this is a bug or not. Or if my grammar was some kind of "sub-optimal"? If you prefer I can copy the issue to the discussion section... (other people might have similar issues.) What do you think?

It's a bug, though I thought I fixed all of them in https://github.com/langium/chevrotain-allstar. I'll look into it once my vacation ends.