AttrX::InitArg

Moose like init_arg for Perl 6

use AttrX::InitArg;

class SecretEnvoy {
    has $!message is init-arg is required;
    has $.steed is init-arg(False) = 'Shadowfax';
    has $.rider is init-arg('messenger') = 'Gandalf';

    method get-message($password){
        $password eq 'opensesame' ?? $!message !! Nil;
    }

}

my $msg = SecretEnvoy.new(
    message => 'TOP SECRET',
    messenger => 'BatMan',
    steed => 'Bat-Mobile' # this one won't work
);

say so $msg.can('message'); #-> False
say $msg.get-message('opensesame'); #-> TOP SECRET
say $msg.steed #-> Shadowfax;
say $msg.rider #-> Batman;

Description

Perl 6 is presently quite opinionated about the concepts of public and private attributes. It doesn't allow for:

1. Attributes that can be set by .bless but do not have accessors
2. Attributes that cannot be set by .bless but do have accessors

Perl 6 OO is flexible enough to create attributes that behave in the above way, it just involves a lot of boilerplate. This module takes care of that in the same way as Perl 5's Moose. Moose has an attribute trait called init_arg which this module attempts to emulate.

There are three ways of using init arg.

With no argument

class Foo {
    has $!attr is init-arg;
    method works { say $!attr }
}
Foo.new( attr => 'win' ).works #-> win

This is intended to be used with $! attributes. The attribute will be set by .bless as if it were a $. attribute but won't have accessors set up for it as usual.

With a string argument

class Foo {
    has $.attr is init-arg('other-name');
}
say Foo.new(other-name => "bar").attr #-> bar
say Foo.new(attr => "bar").attr #-> (Any)

Sets the attribute's argument name in .bless for both $! and $. attributes. name. $. attributes will no longer be set with their usual name in .bless, but it won't die if you try.

With a False argument

class Foo {
    has $.attr is init-arg(False) = "foo";
}

say Foo.new(attr => "bar").attr #-> foo

For use with $. attributes only. Does not allow the attribute to be set by .bless. It will ignore you if you try. Usually used when you want an attribute with accessors but always want the attribute to start as its default value.

Usage Notes

roles

doesn't work in roles yet. Sorry! It works inside roles too!

.gist and .perl

This module modfiies .gist and .perl (in a fairly ugly way) so that .perl's EVALablilty is preserved. However, .gist is modified so that it never prints out the values of $! attributes.

Is this even a good idea?

It depends. If you are using the no-argument form of init-arg with $! attributes, make sure you have considered just using $.. Is public read-only not good enough? Does it have to have no accessors?

BUILDALL

This trait creates a custom BUILDALL on the role/class containing the trait. If you write your own one this won't work atm. This Should be fine now. You can write your own BUILD or BUILDALL and it won't conflict.