Trying to avoid mutating a readonly attribute

avoid-mutating-readonly-attribute.t

use Test2::V0 -target => 'Client';

subtest 'A class with a [readonly, class_type("URI")] attribute' => sub {
    my $uri    = 'https://example.com';
    my $object = $CLASS->new( service_uri => $uri );

    is(
        $object,
        object { call service_uri => $uri },
        'should return the attribute unchanged',
    );

    $object->get('path');

    is(
        $object,
        object { call service_uri => $uri },
        'should not update the attribute at a distance',
    );
};

done_testing();

Client.pm

package Client;

use Moo;
use Type::Utils qw< class_type >;
use Types::Standard -types;
use URI;

use experimental qw< signatures >;

has service_uri => (
    coerce => 1,
    is     => 'ro',
    isa    => class_type('URI')->plus_constructors(Str, 'new'),
);

sub get ( $self, $path ) { $self->service_uri->clone->path_segments($path) }

1;

In this instance you would have to clone the object otherwise the attribute will get mutated. Other ways around it would be to assign the attribute to a variable and use that.

Interestingly, the Object::Pad solution has the same limitations:

use Object::Pad 0.36;

package Client;
class Client;

use Type::Utils qw< class_type >;
use Types::Standard -types;
use URI;

my $Uri = class_type('URI')->plus_constructors(Str, 'new');

has $service_uri :reader;

BUILD (%args) { $service_uri = $Uri->coerce( $args{service_uri} ) };

method get($path) { $service_uri->clone->path_segments($path) }

1;

You could use the around method modifier to ensure that you are only ever passed a clone on the original object:

package Client;

use Moo;
use Type::Utils qw< class_type >;
use Types::Standard -types;
use URI;

use experimental qw< signatures >;

has service_uri => (
    coerce => 1,
    is     => 'ro',
    isa    => class_type('URI')->plus_constructors(Str, 'new'),
);

around service_uri => sub ( $orig, $self ) {
    $self->$orig()->clone();
};

sub get ( $self, $path ) {
    $self->service_uri->path_segments($path);
}

1;

The same solution exists for Object::Pad:

use Object::Pad 0.36;

package Client;
class Client;

use Class::Method::Modifiers qw< around >;
use Type::Utils qw< class_type >;
use Types::Standard -types;
use URI;

my $Uri = class_type('URI')->plus_constructors(Str, 'new');

has $service_uri :reader;

BUILD (%args) {
    $service_uri = $Uri->coerce( $args{service_uri} );
};

around service_uri => sub ( $orig, $self ) {
    $self->$orig()->clone();
};

method get($path) {
    $self->service_uri->path_segments($path);
}

1;

But the correct way to solve this with Object::Pad (as of v0.51) is:

use Object::Pad 0.51;

package Client;
class Client;

use Type::Utils qw< class_type >;
use Types::Standard -types;
use URI;

my $Uri = class_type('URI')->plus_constructors(Str, 'new');

has $service_uri :param;

ADJUST { $service_uri = $Uri->assert_coerce($service_uri) };

method service_uri () { $service_uri->clone() }

method get ($path) { $self->service_uri->path_segments($path) }