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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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.

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)
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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

Thank you, I quite agree. This is inherited code and I wanted to understand the limitations before changing it.