Code completion inside function call expression

[AignerGames]

Hello, I have the following syntax for a function call expression using langium 4:

interface FunctionCallExpression extends Expression
{
FunctionReference: @FunctionDeclarationStatement
Arguments: Expression[];
}

CallExpressionRule returns FunctionCallExpression:
FunctionReference=[FunctionDeclarationStatement:ID] '(' (Arguments+=ExpressionRule (',' Arguments+=ExpressionRule)*)? ')';

My problem is, that the default code completion doesn't work after the first argument.

SomeFunction( <- Code completion here will show variables

SomeFunction(someVariable, <- Code completion here will NOT show variables, instead it will show the error "Expecting token of type ')' but found ,"
But if I finish the expression manually, without code completion, like SomeFunction(someVariable, someOtherVariable); then the error will go away and everything will be parsed correctly.

How can I fix this, so that the variable code completion is also working after the comma?

[msujew]

Hey @AignerGames,

can you provide a fully reproducible example? It's hard to tell what is wrong without that. Note that we have a list of issues tagged with completion that might be the culprit of this. As you can imagine, completely generic code completion is a rather difficult problem :/

[AignerGames]

Hello, sorry I can't provide a full example right now. It will take a while.

But I asked Copilot and it thinks that the issue happens, because I also have a Grouping Expression, which looks like this:

GroupExpressionRule returns GroupingExpression:
'(' Expression=ExpressionRule ')';

Copilot assumes that the incomplete function call like "SomeFunction(A," could be mistaken for a grouping in this stage and this could cause the code completion to fail after the comma.

Since this is only a "hobby language" I implemented a maybe overkill solution: I added a custom completion handler and check if the base class returns any results, if not I assume that I'm inside a incomplete expression and just return all variables and functions as fallback.