Yes, I definitely want to have an iterator that I can use to traverse a grammar in Perl6. However, a bit more thought gave me this idea:

class MyFormat is CORE::GrammarVisitor
{
  my $fh is public;

  method date ($node) {
    $fh.printf("%04d-%02d-%02d", $node ^. qw(year month day))
  }
}

$0.traverse(MyFormat.new(fh=>$*STDOUT)); # default ctor?
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This is probably the type of code that would have been instantiated by the hash approach; but is more flexible, and allows for more powerful formating concepts (the visitor can maintain state between the things that it outputs).

But going back to the hash-based approach, I think that it is possible to do something like:

  my $sub = sub {print $value};
  $sub.MY{'$value'} = 42;
  $sub();
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I think this would be kinda scary though.

--Dave

Update: Fixed typo: MY{'$date'} becomes MY{'$value'}