Antlr4 Listeners and Visitors - which to implement?

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.

If you plan to directly use the parser output for interpretation, the visitor is a good choice. You have full control of the traversal, so in conditionals only one branch is visited, loops can be visited n times and so on.

If you translate the input to a lower level, e.g. virtual machine instructions, both patterns may be useful.

You might take a look at "Language Implementation Patterns", which covers the basic interpreter implementations.

I mostly use the visitor pattern, as it's more flexible.

Here is quote from the book that I think is relevant:

The biggest difference between the listener and visitor mechanisms is that listener methods are called by the ANTLR-provided walker object, whereas visitor methods must walk their children with explicit visit calls. Forgetting to invoke visit() on a node’s children means those subtrees don’t get visited.

In visitor pattern you have the ability to direct tree walking while in listener you are only reacting to the tree walker.