In Moose, how do I require one of multiple attributes? - perl

Question

I would like to be able to declare one of a set of mutually dependent attributes required.
Let's assume a simple example of Number 'nr_two' being 'nr_one' + 1, and 'nr_one' being 'nr_two' -1, with one of either having to be passed in upon initialization.
So far, I have seen this problem solved for example through BUILDARGS checks and a lazy builder on each:
has 'nr_one' => (
is => 'ro',
isa => 'Num',
lazy => 1,
builder => '_build_nr_one',
);
sub _build_nr_one { shift->nr_two - 1; }
has 'nr_two' => (
is => 'ro',
isa => 'Num',
lazy => 1,
builder => '_build_nr_two',
);
sub _build_nr_two { shift->nr_one + 1; }
around 'BUILDARGS' => sub {
my $orig = shift;
my $self = shift;
my $args = is_hashref($_[0])? $_[0] : { #_ };
die "Either nr_one or nr_two is required!" unless defined $args{nr_one} || defined $args{nr_two};
return $self->$orig($args);
};
Or, avoiding the around:
has 'nr_one' => (
is => 'ro',
isa => 'Num',
predicate => 'has_nr_one',
lazy => 1,
builder => '_build_nr_one',
);
sub _build_nr_one { shift->nr_two - 1; }
has 'nr_two' => (
is => 'ro',
isa => 'Num',
predicate => 'has_nr_two',
lazy => 1,
builder => '_build_nr_two',
);
sub _build_nr_two { shift->nr_one + 1; }
sub BUILD {
my $self = shift;
die "Either nr_one or nr_two is required!" unless $self->has_nr_one || $self->has_nr_two;
}
However, I am looking for something that can be declared on the attributes,
for example a grouping of some sort that can then be introspected and, for example, triggered in BUILD.
Ideally, I'd like to ship this into a generic role or Meta class to make it available
with some sort of nicer syntax, to avoid having to check for BUILD(ARGS) checks
or rely on the pod to declare things accurately.
Is there cpan module that could help with this, or a pattern someone is aware of to achieve this?
Any hints / partial solutions are appreciated, if not :)
An example of what I would imagine would look something like this:
has 'nr_one' => (
is => 'ro',
isa => 'Num',
lazy => 1,
builder => '_build_nr_one',
required_grouping => 'NumberGroup',
);
sub _build_nr_one { shift->nr_two - 1; }
has 'nr_two' => (
is => 'ro',
isa => 'Num',
lazy => 1,
builder => '_build_nr_two',
required_grouping => 'NumberGroup',
);
sub _build_nr_two { shift->nr_one + 1; }
# when initialized without any attributes, error thrown:
# "One of 'nr_one', 'nr_two' is required"
# or, probably easier: "NumberGroup required!"

Answer 1

I did not find a way to make a custom MooseX::Type or attribute trait automatically add a method modifier to BUILDARGS() that would validate the attributes. But it is simple to do that with a Moose::Role like this:
#! /usr/bin/env perl
package NumberGroup;
use Moose::Role;
around 'BUILDARGS' => sub {
my $orig = shift;
my $self = shift;
my $args = (ref $_[0]) eq "HASH" ? $_[0] : { #_ };
die "Either nr_one or nr_two is required!" unless defined $args->{nr_one} || defined $args->{nr_two};
return $self->$orig($args);
};
package Main;
use Moose;
with 'NumberGroup';
has 'nr_one' => (
is => 'ro',
isa => 'Num',
);
has 'nr_two' => (
is => 'ro',
isa => 'Num',
);
package main;
use strict;
use warnings;
Main->new();
Output:
Either nr_one or nr_two is required! at ./p.pl line 8.