Creating JVM language [PART 6] - Switching to visitor oriented parsing

April 3rd, 2016

Sources

The project can be cloned from github repository.
The revision described in this post is 1fc8131b2752e73776e91084ffeabbfa45fc6307.

Visitor vs Listener

Previously I used listener pattern to implement Enkel parser. There is hovewer another way to do that - Visitor. To enabled it specify -visitor on the commnad line.

I was kind of curious which one would be more suitable for Enkel so I created a small project that exposes the differences. Check out this blog post where you can read full comparison and get sources.

The main benefit is that Visitor returns value where Listener does not:

  • There is less code to write.
  • Less bug prone. No need to store parsing result in the field and rely on getter.
//Listener
class ClassListener extends EnkelBaseListener<ClassDeclaration> {

        private Class parsedClass;

        @Override
        public void enterClassDeclaration(@NotNull EnkelParser.ClassDeclarationContext ctx) {
            String className = ctx.className().getText();
            //do some other stuff
            parsedClass = new Class(className,methods);
        }

        public Class getParsedClass() {
            return parsedClass;
        }
    }

//Visitor
public class ClassVisitor extends EnkelBaseVisitor<ClassDeclaration> {

    @Override
    public ClassDeclaration visitClassDeclaration(@NotNull EnkelParser.ClassDeclarationContext ctx) {
        String name = ctx.className().getText();
        //do some other stuff
        return new ClassDeclaration(name, methods);
    }
}

The decision to switch to Visitor pattern was therefore quite obvious.

Jakub Dziworski

Dev Blog