antlr4

Antlr4 Listeners and Visitors - which to implement?


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.


Solution

  • 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.