Antlr4 Listeners and Visitors - which to implement?

user_2403657 0
Viewed 21284

I'm reading "The Definitive Antlr 4 Reference" and get the idea in relation to how Listeners and Visitors work. The book explains particularly well how Listeners relate to SAX parsers and makes it obvious when methods are going to be called during the implementation of each. I can see also that listeners are quite good for transforming input to output but I would appreciate a short explanation/example as to when to use a Listener and when to use a Visitor (or should they both be used in certain cases?).

My particular intention is to create an interpreter (Cucumber-style / TinyBasic Interpreter with some custom calls) that will check for syntax errors and stop executing on an error from a custom function without recovering - would love to see a complete implementation of such a thing in antlr - if anyone happens to know of one.

Thanks in advance for any advice.

antlr4
4 Answers

There's another important difference between these two patterns: a visitor uses the call stack to manage tree traversals, whereas the listener uses an explicit stack allocated on the heap, managed by a walker. This means that large inputs to a visitor could blow out the stack, while a listener would have no trouble.

If your inputs could be potentially unbounded, or you might call your visitor very deep in a call tree, you should use a Listener, rather than a Visitor, or at least validate that the parse tree is not too deep. Some companies' coding practices discourage or even outright forbid non-tail recursion for this reason.

user_21379960

From the book, page 120.

Visitors work very well if we need application-specific return values because we get to use the built-in Java return value mechanism. If we prefer not having to explicitly invoke visitor methods to visit children, we can switch to the listener mechanism. Unfortunately, that means giving up the cleanliness of using Java method return values.

This is why I use visitors.

user_16244540
Related