ParseTreeVisitor or ParseTreeListener

Question

I am trying to write an AST interpreter / REPL. ANTLRv4 provides two very similar interfaces (ParseTreeVisitor and ParseTreeListener) to walk the parse tree. I cannot seem to find any major differences between them, and the documentation is rather sparse. Is one interface preferable to the other?

Answer 1

The interfaces are used for different purposes. The primary differences are as follows:

ParseTreeListener

• Provides separate enter/exit methods for before/after the children of a parse tree node are examined.
• All methods return void. Any values collected for "return" by the listener must be held in fields or elsewhere.
• Control of which tree nodes are examined is external (via ParseTreeWalker or a derived class).

ParseTreeVisitor

• Provides one method which is responsible for all analysis/behavior for each parse tree node.
• Each method returns generic type parameter T, which may be Void if the visitor methods do not return a value.
• Control of which tree nodes are examined is internal (via visitChildren and/or calls to visit for specific children).