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

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