Moose how to change the attribute value only when it is $undef? - perl

Question

Now have:
has 'id' => (
is => 'rw',
isa => 'Str',
default => sub { "id" . int(rand(1000))+1 }
);
Works OK, the:
PKG->new(id => 'some'); #the id is "some"
PKG->new() #the id is #id<random_number>
In the next scenario:
my $value = undef;
PKG->new(id => $value);
(of course) got an error:
Attribute (id) does not pass the type constraint because: Validation failed for 'Str' with value undef at /Users/me/perl5/perlbrew/perls/perl-5.16.3/lib/site_perl/5.16.3/darwin-thread-multi-2level/Moose/Exception.pm line 37
The question is:
How to achieve changing the value after it is set to undef (and only when it is $undef)? So,
has 'id' => (
is => 'rw',
isa => 'Str|Undef', #added undef to acceptable Type
default => sub { "id" . int(rand(1000))+1 }
);
Now, it accepting the $undef, but I don't want $undef but want "id" . int(rand(1000))+1. How to change the attribute value after it is set?
The after is called only for the accessors not for constructors. Maybe some weird coercion from Undef to Str - but only for this one attribute?
Ps: using the PKG->new( id => $value // int(rand(10000)) ) is not an acceptable solution. The module should accept the $undef and should silently change it to the random number.

Answer 1

Type::Tiny has as one of its aims to make it easy to add coercions to individual attributes really easy. Here's an example:
use strict;
use warnings;
{
package Local::Test;
use Moose;
use Types::Standard qw( Str Undef );
my $_id_default = sub { "id" . int(rand(1000)+1) };
has id => (
is => 'rw',
isa => Str->plus_coercions(Undef, $_id_default),
default => $_id_default,
coerce => 1,
);
__PACKAGE__->meta->make_immutable;
}
print Local::Test->new(id => 'xyz123')->dump;
print Local::Test->new(id => undef)->dump;
print Local::Test->new->dump;
You could also look at MooseX::UndefTolerant which makes undef values passed to the constructor act as if they were entirely omitted. This won't cover passing undef to accessors though; just constructors.

Answer 2

Here is an alternative, using Moose' BUILD method, which is called after an object is created.
#!/usr/bin/perl
package Test;
use Moose;
has 'id' => (
is => 'rw',
isa => 'Str|Undef',
);
sub BUILD {
my $self = shift;
unless($self->id){
$self->id("id" . (int(rand(1000))+1));
}
}
1;
package Main;
my $test = Test->new(id => undef);
print $test->id; ###Prints random number if id=> undef
More info on BUILD here:
http://metacpan.org/pod/Moose::Manual::Construction#BUILD

Answer 3

#choroba in a comment mentioned about the triggers. Based on this, found a next solution. The trigger is called twice in the case id=>undef, but otherwise it works.
use Modern::Perl;
package My;
use namespace::sweep;
use Moose;
my $_id_default = sub { "id" . int(rand(100_000_000_000)+1) };
my $_check_id = sub { $_[0]->id(&$_id_default) unless $_[1] };
has id => (
is => 'rw',
isa => 'Str|Undef',
default => $_id_default,
trigger => $_check_id,
);
__PACKAGE__->meta->make_immutable;
package main;
say My->new->id;
say My->new(id=>'aaa')->id;
say My->new(id=>undef)->id;