Re^2: Exporting constants from a Moo role

You'd probably also want to prevent people from overriding the constant using:

my $obj = MyClass->new(INDEX_FOO => 43);
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To do this, use:

has 'INDEX_FOO'=> (
    is       => 'ro',
    default  => 42
    init_arg => undef,
);
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But by that point, you have to ask, why not just do this?

sub INDEX_FOO () { 42 }
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toby döt ink

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Re^3: Exporting constants from a Moo role by 1nickt (Canon) on Aug 02, 2018 at 21:40 UTC
Thanks Toby! `init_arg => undef` is a new one on me. Why do you need to declare a default value (or what is the point) when you use that parameter to the attribute? I quite often use simple subs for constants if only because the syntax to declare them is simpler and shorter. I guess the author of this code (which is used to access the elements of a work queue job) prioritized simpler and shorter syntax at the calling end, preferring `$self->job->[INDEX_FOO]` to `$self->job->[ $self->INDEX_FOO ]` ... I think I like the solution you showed in your other reply. Thanks again! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^4: Exporting constants from a Moo role by tobyink (Canon) on Aug 03, 2018 at 11:49 UTC
`init_arg` is used to tell Moo what key will get passed to the constructor for that attribute. `use v5.16; package Person { use Moo; has "nick-name" => ( is => "ro", init_arg => "nick", reader => "nick_name", ); }; my $robert = Person->new(nick => "Bob"); # init_arg say $robert->nick_name; # reader say $robert->{"nick-name"}; # attribute name (direct +hashref access)` [download] Setting `init_arg` to undef means that the attribute cannot be passed to the constructor at all. (And if the attribute is also read-only, so there's no writer method, you should probably be setting a default for the attribute too!) (And this is not Moo-specific. Moose and Mouse have the exact same features.) toby döt ink	[reply] [d/l] [select]