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Mark Devine
September 9, 2019 00:59

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Would you be able to expand on how to implement the trait that wraps into what is effectively the ‘get_value’ accessor of an attribute?  I’m not finding enough in the documentation to connect the dots.  I’ve looked at XML::Class, which does some pretty heavy trait’ing, but my intuition has expired.

my role XML-attribute {
    has $attr;
    method get_value () {
        say 'in XML-attribute method get_value with ' ~ $.name;
    method set_value ($) {
        say 'in XML-attribute method set_value with ' ~ $.name;
        $attr = $;

multi sub trait_mod:<is> (Attribute $a, :$xmlattr) {
    say "⟨is xmlattr⟩ has been called with ⟨$xmlattr⟩ on {$a.WHICH}";
    $a does XML-attribute;

class System {
    has Str $.SerialNumber is xmlattr;


I’ve twisted myself into a pretzel.  I couldn’t even find where a role gets ‘$’, but I saw it in JSTOWE’s code so I mimicked.

I need a little more broad-based help on what goes where and when.



From: Vadim Belman <>
Sent: Sunday, September 8, 2019 14:57
To: Mark Devine <>
Cc: perl6-users <>
Subject: Re: Wrap an attribute’s ^get_value method in TWEAK

Hi Mark,

get_value won't give you what you expect. It's use is narrowly limited to trait 'is handles' and method .perl. Besides, are you sure all of your attributes will be considered as belonging to XML? I mean, won't there be need for utility attributes serving exclusively internal needs of a class? If so, then the best solution is to create a trait for an attribute and have it like:

has $.SerialNumber is xmlattr;

The trait would then install an accessor which would do what you need.

Best regards,
Vadim Belman

On Sep 8, 2019, at 12:41 PM, Mark Devine <<>> wrote:

Perl6 Community,

How do I properly wrap an attribute’s ^get_value method in TWEAK?  If a condition exists, I’d like to wrap all (:local) attributes so that they can do some extra work.

The module that I’m working on has classes/attributes for hundreds of fields in dozens of different, big XML files -- there’s a lot.  And XML is easily extended, so I may very well be adding hundreds more attributes in the future.  I would very much prefer to preserve the automatic accessor behavior of ‘has $.attr;’ versus writing a billion set/get accessors myself – truly prohibitive.  Wrapping seems the elegant/extensible approach.

class System {
    has $.SerialNumber;
    submethod TWEAK {
        for self.^attributes(:local) -> $attr {
            self.$attr.^get_value.wrap: { say 'in ' ~ $ ~ "'s get_value"; nextsame; };

# OUTPUT: No such method 'CALL-ME' for invocant of type 'Attribute'

I know the above .wrap is probably misguided, but it sort of demonstrates what I’m trying to do.

I’m not finding the proper approach.  Any hints?  Any tutorials?  Any existing examples in the ecosystem that I could mimic?



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