Re: Exporting constants from a Moo role

One of the features of Moo (and Moose) is to define attributes as readonly with a default value. So it seems only natural that the authors of Moose and Moo would expect such values to be exported as readonly accessors instead of explicitly using constants.

Update: For example:-

package MyRole;
use Moo::Role;
use strictures 2;

has 'INDEX_FOO'=> (
    is      => 'ro',
    isa     => 'Int',
    default => 42
);
[download]

One world, one people

Comment on Re: Exporting constants from a Moo role Download Code

Replies are listed 'Best First'.
Re^2: Exporting constants from a Moo role by tobyink (Canon) on Aug 02, 2018 at 16:00 UTC
You'd probably also want to prevent people from overriding the constant using: `my $obj = MyClass->new(INDEX_FOO => 43);` [download] To do this, use: `has 'INDEX_FOO'=> ( is => 'ro', default => 42 init_arg => undef, );` [download] But by that point, you have to ask, why not just do this? `sub INDEX_FOO () { 42 }` [download] toby döt ink	[reply] [d/l] [select]
Re^3: Exporting constants from a Moo role by 1nickt (Canon) on Aug 02, 2018 at 21:40 UTC
Thanks Toby! `init_arg => undef` is a new one on me. Why do you need to declare a default value (or what is the point) when you use that parameter to the attribute? I quite often use simple subs for constants if only because the syntax to declare them is simpler and shorter. I guess the author of this code (which is used to access the elements of a work queue job) prioritized simpler and shorter syntax at the calling end, preferring `$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]
Re^2: Exporting constants from a Moo role by 1nickt (Canon) on Aug 02, 2018 at 21:26 UTC
Thank you, I quite agree. This is inherited code and I wanted to understand the limitations before changing it. The way forward always starts with a minimal test.	[reply]