NAME

Unknown::Values::Instance::Object - Internal null value object for the "Unknown::Values" distribution

VERSION

version 0.102

SYNOPSIS

    package Employee {
        use parent 'Person';
        use Unknown::Values ':OBJECT';

        sub new {
            my ( $class, $id ) = @_;
            my $self = $self->next::method($id) // return unknown;
            ...
            return $self
        }
        
        # ...
    }

    ...

The following assumes that $employee is unknown:

    my $employee = Employee->new($id);

    # you can call any method on $employee
    if ( $employee->salary > $threshold ) {
        ... will never get here if $employee is unknown
    }

    say $employee->name; # fatal
    if ( is_unknown $employee ) {
        ... works as expected
    }

    if ( $employee->isa('Employee') ) {
        ... isa always returns unknown
    }

DESCRIPTION

use Unknown::Values ':OBJECT' implements a variation of the NULL object pattern.

In addition to having all of the behavior of Unknown::Values, you can call any method on the object and it will return the unknown object instances.

Subclassing Unknown Objects

Sometimes you want to provide a default value for a method. You can subclass unknown objects to allow this:

    package Unknown::Person {
        use parent 'Unknown::Values::Instance::Object';

        sub name { return '<unknown>' }
    }

    package Person {
        sub new {
            my ( $class, $name, $age ) = @_;

            if ( not defined $name ) {
                return Unknown::Person->new;
            }
            return bless {
                name => $name,
                age  => $age,
            } => $class;
        }

        sub name { $_[0]->{name} }
        sub age  { $_[0]->{age} }
    }

AUTHOR

Curtis "Ovid" Poe <ovid@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2021 by Curtis "Ovid" Poe.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.