Moose: builder requires a value that sometimes is not set (non deterministic) - perl

Question

I'm new to MOOSE and Perl OOP, and I'm struggling understanding the execution order of the code.
I want to create a class reading files, so an attribute of the object should be the filehandle and another the filename to be read.
My problem is that the attribute 'filehandle' has a builder that requires $self->filename, but sometimes at runtime 'filename' is not (yet) available when the builder is called.
Thanks for you help
My ideal object creation:
my $file = FASTQ::Reader->new(
filename => "$Bin/test.fastq",
);
Perl module:
has filename => (
is => 'ro', isa => 'Str', required => 1,
);
has fh => (
is => 'ro', isa => 'FileHandle', builder => '_build_file_handler',
);
sub _build_file_handler {
my ($self) = #_;
say Dumper $self;
open(my $fh, "<", $self->filename) or die ("cant open " . $self->filename . "\n");
return $fh;
}
See: https://gist.github.com/telatin/a81a4097913af55c5b86f9e01a2d89ae

Answer 1

If a value of one attribute depends on another attribute, make it lazy.
#!/usr/bin/perl
use warnings;
use strict;
{ package My::Class;
use Moose;
has filename => (is => 'ro', isa => 'Str', required => 1);
has fh => (is => 'rw', isa => 'FileHandle', lazy => 1, builder => '_build_fh');
# ~~~~~~~~~
sub _build_fh {
my ($self) = #_;
open my $fh, '<', $self->filename or die $!;
return $fh
}
}
my $o = 'My::Class'->new(filename => __FILE__);
print while readline $o->fh;
See Laziness in Moose::Manual::Attributes:
if the default value for this attribute depends on some other attributes, then the attribute must be lazy.