ANTLR4 Visitor subtree check

antlr, antlr4, visitor-pattern

Solution

The automatically-generated context method `ProcedureCallStatementContext.actualParameters()` will return the `ActualParametersContext` if one was parsed, otherwise it will return `null`.

You might make use of it in a visitor like this:

public T VisitProcedureCallStatement(ProcedureCallStatementContext ctx) {
    if (ctx.actualParameters() != null) {
        // do something here
    }

    ...
}

Problem

I have an Antlr4 question. Given the grammar excerpt below. What is the correct approach to testing for the existence of the optional `actualParameters` subtree within a visitor? I have tried the getChildCount method of the procedureCallStatement context. I've also tested for a null actualParameters parameter on the context. I do not want to visit the actualParameters subtree if it does not exist. Doing so causes an exception. Thank you! Kelvin Johnson ``` program : statement (';' statement)* ';'?; statement : CALLPREFIX('(' actualParameters? ')')? #procedureCallStatement; actualParameters : expressionStatement (';' expressionStatement)* ; expressionStatement : '(' expressionStatement ')' #parensExpression | IDENT'[' expressionStatement ']' #subscript ... ```

Original source