How to define a `separator` tag in play-1.x without modifing play's source code - tags

Question

I want to define a tag separator tag, which inside a list tag, can add separator between items.
The sample code is:
List<String> users = new ArrayList<String>();
users.add("Jeff");
users.add("Mike");
#{list users, as: 'user'}
#{separator ' + ' /}
<span>${user}</span>
#{/list}
If I don't use the separator tag, the code will be:
#{list users, as: 'user'}
${user_isFirst ? '' : ' + '}
<span>${user}</span>
#{/list}
The generated html code will be:
<span>Jeff</span> + <span>Mike</span>
I tried defined a fastTag:
public static void _separator(Map<?, ?> args, Closure body, PrintWriter out, GroovyTemplate.ExecutableTemplate template, int fromLine) {
Object value = args.get("arg");
// TODO how to get the value of `as` defined in parent `list` tag?
out.print(value);
}
But the problem is I can't get the value of as defined in list tag (which is user) in this case.

Answer 1

You can create a custom list tag in groovy like this
#{list items:_arg, as:'tmp'}
%{
attrs = [:]
attrs.put(_as, tmp)
}%
#{ifnot tmp_isFirst}${_sep}#{/ifnot}
#{doBody vars:attrs /}
#{/list}
and use it like this
#{myList users, as:'user', sep:','}
${user}
#{/myList}

Answer 2

You should trace into your FastTag implementation. I think you'll see all the variables in scope inside the args map. This is from memory - so, sorry if not.
That said, I think it might be simpler if you copy the Java code for #{list} and add a new parameter, like
#{list users, as: 'user', separator: '+' }
and handle the logic in there. It seems a bit cleaner too from a design point of view - if it is a separator, how come you can put it anywhere you like in the code (and why not put it in twice!).
A final option is to look at Groovy or Java collection operators. http://groovy.codehaus.org/groovy-jdk/java/util/Collection.html