Comments on: Visitor pattern with closures

By: Alex Miller - Dynamic visitor builder with closures

Alex Miller - Dynamic visitor builder with closures — Sun, 16 Mar 2008 08:05:27 +0000

[...] « Visitor pattern with closures | Home | [...]

By: Howard Lovatt

Howard Lovatt — Fri, 14 Mar 2008 21:16:48 +0000

In my own pet project, Pattern Enforcing Compiler (PEC):

http://pec.dev.java.net/nonav/compile/index.html

You can use multiple dispatch instead of visitor:

http://pec.dev.java.net/nonav/compile/javadoc/pec/compile/multipledispatch/package-summary.html#WalkingAnExpressionTree

By: Mark Mahieu

Mark Mahieu — Fri, 14 Mar 2008 14:13:36 +0000

I’ll be curious to see what you come up with as a ‘Visitor Builder’ – I was planning to write a short blog entry on that subject myself. The ability to modify local variables and (in BGGA) use non-local returns would make some of my existing uses of the Visitor pattern much simpler…

By: Alex

Alex — Fri, 14 Mar 2008 13:11:45 +0000

@Neal – gotcha, I’ll give that a shot and you can tell me how I did. :)

@Peter – yeah, I know. If you take a look at my older visitor article (linked above), you’ll see a full treatment of the navigation issue.

By: Peter Veentjer

Peter Veentjer — Fri, 14 Mar 2008 13:01:11 +0000

@Alex.
One of the things I do when writing visitors is to make the accept method really stupid:
all implementation just have visitor.visit(this). The traversal should be task of the visitor. If I need reusable traversals, I create visitor guides:

class DepthFirstNodeVisitorGuide implements NodeVisitor{
private final NodeVisitor guest;

public DepthFirstNodeVisitorGuide(NodeVisitor guest){
this.guest = guest;
}

public void visit(ConcreteNode node){
node.accept(guest);
}

public void visit(ConcreteNode node){
for(Node child: node.getChildren()){
node.accept(this);
}

node.accept(guest);
}
}

If you want to apply it, you can say something like this:

node.accept(new DepthFirstNodeVisitorGuide(new PrintNodeVisitor());

@back on subject.
The last example you gave (with the ‘if(v instanceof ConcreteNode)’) typically is something you would not see with a visitor. The question is: it is possible to lift on a different dynamic dispatching mechanism other than the polymorphic dispatch, or to use that polymorphic dispatch for closures.

It is a shame that Java has so many features but not something ‘simple’ as multi-dispatch.