antlr4: how to know which alternative is chosen given a context

antlr, antlr4

Solution

No, you can either use the method you described (checking if an item is non-null), or you can label the outer alternatives of the rule using the `#` operator.

type
  : IDENTIFIER     # someType
  | typeDescriptor # someOtherType
  ;

When you label the outer alternatives, it will produce `ParserRuleContext` classes for each of the labels. In the example above, you'll either get a `SomeTypeContext` or a `SomeOtherTypeContext`, which applies equally to the generated listener and visitor interfaces.

Problem

Assume there is a rule about 'type'. It is either a predefined type (referred by IDENTIFIER) or a typeDescriptor. ``` type : IDENTIFIER | typeDescriptor ; ``` In my program, I have got an instance of typeContext 'ctx'. How do I know if the path IDENTIFIER is chosen, or typeDescriptor is chosen. I recognise one way which is to test `ctx.IDENTIFIER() == null` and `ctx.typeDescriptor() == null`. But it seems not working very well when there are a lot more alternatives. Is there a way to return an index to indicate which rule is chosen? Thanks.

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