How to set ro attributes during build? - perl

Question

I'm writing a script to help me get proficient in Moose. I've got the following bit of code:
package Dir;
use Moose;
use Modern::Perl;
use File;
has 'dirs' => (is => 'ro', isa => 'HashRef[Dir]' );
has 'files' => (is => 'ro', isa => 'HashRef[File]');
has 'dir_class' => (is => 'ro', isa => 'ClassName', default => 'Dir');
has 'file_class' => (is => 'ro', isa => 'ClassName', default => 'File');
sub BUILD {
my $self = shift;
my $path = $self->path;
my $name = $self->name;
my (%dirs, %files);
# populate dirs attribute with LaborData::Data::Dir objects
opendir my $dh, $path or die "Can't opendir '$path': $!";
# Get files and dirs and separate them out
my #dirs_and_files = grep { ! m{^\.$|^\.\.$} } readdir $dh;
closedir $dh or die "Can't closedir '$path': $!";
my #dir_names = grep { -d "$path/$_" } grep { !m{^\.} } #dirs_and_files;
my #file_names = grep { -f "$path/$_" } grep { !m{^\.} } #dirs_and_files;
# Create objects
map { $dirs{$_} = $self->dir_class->new ( path => $path . '/' . $_ ) } #dir_names;
map { $files{$_} = $self->file_class->new ( path => $path . '/' . $_ ) } #file_names;
# Set attributes
$self->dirs ( \%dirs );
$self->files ( \%files );
}
The code results in the following error: died: Moose::Exception::CannotAssignValueToReadOnlyAccessor (Cannot assign a value to a read-only accessor at reader Dir::dirs
To get around this error, I could either make the attributes rw or use builder methods for the dirs and files attributes. The former solution is undesirable and the latter solution will require duplication of code (for example, the directory will need to be opened twice) and so is also undesirable.
What is the best solution to this problem?

Answer 1

You can assign a writer to your read-only attribute and use that internally from your BUILD. Name it with an _ to indicate it's internal.
package Foo;
use Moose;
has bar => ( is => 'ro', writer => '_set_bar' );
sub BUILD {
my $self = shift;
$self->_set_bar('foobar');
}
package main;
Foo->new;
This will not throw an exception.
It's essentially the same as making it rw, but now the writer is not the same accessor as the reader. The _ indicates that it's internal, so it's less undesirable than just using rw. Remember that you cannot really protect anything in Perl anyway. If your user wants to get to the internals they will.

Answer 2

I found one possible solution, though it's frowned upon:
# Set attributes
$self->{dirs} = \%dirs;
$self->{files} = \%files;