Can I create a visitor class without having an instance of the parser ?

[dhowe]

Is there a way to create a visitor class without having an instance of the parser ?

[msujew]

Hey @dhowe,

you can always create a class that implements the ICstVisitor interface yourself, without a parser instance. The method provided by the parser will just allow validation of the visitor based on the rules defined in your parser. Without that, your visitor might run into undefined behavior.

[bd82]

Hello @dhowe

Following up on @msujew's answer:
You can use the existing visitor implementation as a reference:

• https://github.com/Chevrotain/chevrotain/blob/master/packages/chevrotain/src/parse/cst/cst_visitor.ts

BTW I believe these visitor APIs could be improved, see:
#1039

[dhowe]

Thanks @msujew and @bd82

Do I understand correctly that the visit() method is basically the same for all (or most) visitors, while validateVisitor() is specific to each, thus requiring the dynamic superclass?

If so I think it would be useful to have a sort of default Visitor to subclass that provides a simple implementation of the visit() method (In Java this is sometimes called an 'adapter' pattern, though I'm not sure its correct), and maybe a no-op version of validateVisitor(). Does this make sense, and if so, would it be useful to others beside me?

[bd82]

Do I understand correctly that the visit() method is basically the same for all (or most) visitors, while validateVisitor() is specific to each, thus requiring the dynamic superclass?

I am rusty in regards to this code, but that seems to be the case.

It seems to me that this validateVisitor could be extracted as a utility function which end users could decide to use or not.

• e.g: only call it once during their unit-tests

And this would significantly simplify things.

[dhowe]

Seems its as simple as this (unless I'm missing something):

class BaseVisitor {
  visit(cstNode, param) {
    if (Array.isArray(cstNode)) {
      cstNode = cstNode[0];
    }
    if (typeof cstNode === 'undefined') {
      return undefined;
    }
    return this[cstNode.name](cstNode.children, param);
  }
  validateVisitor() {
    // no-op
  }
}

[bd82]

Seems correct.
The CST structure is quite simple.

Note the beyond the validations the built-in CST visitor also implements a default handler for a un-implemented methods.

• https://github.com/Chevrotain/chevrotain/blob/master/packages/chevrotain/src/parse/cst/cst_visitor.ts#L13-L29

But this is not mandatory either...