NAME

       Moose::Manual::Unsweetened - Moose idioms in plain old Perl 5 without the sugar

VERSION

       version 2.1005

DESCRIPTION

       If you're trying to figure out just what the heck Moose does, and how it saves you time,
       you might find it helpful to see what Moose is really doing for you. This document shows
       you the translation from Moose sugar back to plain old Perl 5.

CLASSES AND ATTRIBUTES

       First, we define two very small classes the Moose way.

         package Person;

         use DateTime;
         use DateTime::Format::Natural;
         use Moose;
         use Moose::Util::TypeConstraints;

         has name => (
             is       => 'rw',
             isa      => 'Str',
             required => 1,
         );

         # Moose doesn't know about non-Moose-based classes.
         class_type 'DateTime';

         my $en_parser = DateTime::Format::Natural->new(
             lang      => 'en',
             time_zone => 'UTC',
         );

         coerce 'DateTime'
             => from 'Str'
             => via { $en_parser->parse_datetime($_) };

         has birth_date => (
             is      => 'rw',
             isa     => 'DateTime',
             coerce  => 1,
             handles => { birth_year => 'year' },
         );

         enum 'ShirtSize' => qw( s m l xl xxl );

         has shirt_size => (
             is      => 'rw',
             isa     => 'ShirtSize',
             default => 'l',
         );

       This is a fairly simple class with three attributes. We also define an enum type to
       validate t-shirt sizes because we don't want to end up with something like "blue" for the
       shirt size!

         package User;

         use Email::Valid;
         use Moose;
         use Moose::Util::TypeConstraints;

         extends 'Person';

         subtype 'Email'
             => as 'Str'
             => where { Email::Valid->address($_) }
             => message { "$_ is not a valid email address" };

         has email_address => (
             is       => 'rw',
             isa      => 'Email',
             required => 1,
         );

       This class subclasses Person to add a single attribute, email address.

       Now we will show what these classes would look like in plain old Perl 5. For the sake of
       argument, we won't use any base classes or any helpers like "Class::Accessor".

         package Person;

         use strict;
         use warnings;

         use Carp qw( confess );
         use DateTime;
         use DateTime::Format::Natural;

         sub new {
             my $class = shift;
             my %p = ref $_[0] ? %{ $_[0] } : @_;

             exists $p{name}
                 or confess 'name is a required attribute';
             $class->_validate_name( $p{name} );

             exists $p{birth_date}
                 or confess 'birth_date is a required attribute';

             $p{birth_date} = $class->_coerce_birth_date( $p{birth_date} );
             $class->_validate_birth_date( $p{birth_date} );

             $p{shirt_size} = 'l'
                 unless exists $p{shirt_size}:

             $class->_validate_shirt_size( $p{shirt_size} );

             return bless \%p, $class;
         }

         sub _validate_name {
             shift;
             my $name = shift;

             local $Carp::CarpLevel = $Carp::CarpLevel + 1;

             defined $name
                 or confess 'name must be a string';
         }

         {
             my $en_parser = DateTime::Format::Natural->new(
                 lang      => 'en',
                 time_zone => 'UTC',
             );

             sub _coerce_birth_date {
                 shift;
                 my $date = shift;

                 return $date unless defined $date && ! ref $date;

                 my $dt = $en_parser->parse_datetime($date);

                 return $dt ? $dt : undef;
             }
         }

         sub _validate_birth_date {
             shift;
             my $birth_date = shift;

             local $Carp::CarpLevel = $Carp::CarpLevel + 1;

             $birth_date->isa('DateTime')
                 or confess 'birth_date must be a DateTime object';
         }

         sub _validate_shirt_size {
             shift;
             my $shirt_size = shift;

             local $Carp::CarpLevel = $Carp::CarpLevel + 1;

             defined $shirt_size
                 or confess 'shirt_size cannot be undef';

             my %sizes = map { $_ => 1 } qw( s m l xl xxl );

             $sizes{$shirt_size}
                 or confess "$shirt_size is not a valid shirt size (s, m, l, xl, xxl)";
         }

         sub name {
             my $self = shift;

             if (@_) {
                 $self->_validate_name( $_[0] );
                 $self->{name} = $_[0];
             }

             return $self->{name};
         }

         sub birth_date {
             my $self = shift;

             if (@_) {
                 my $date = $self->_coerce_birth_date( $_[0] );
                 $self->_validate_birth_date( $date );

                 $self->{birth_date} = $date;
             }

             return $self->{birth_date};
         }

         sub birth_year {
             my $self = shift;

             return $self->birth_date->year;
         }

         sub shirt_size {
             my $self = shift;

             if (@_) {
                 $self->_validate_shirt_size( $_[0] );
                 $self->{shirt_size} = $_[0];
             }

             return $self->{shirt_size};
         }

       Wow, that was a mouthful! One thing to note is just how much space the data validation
       code consumes. As a result, it's pretty common for Perl 5 programmers to just not bother.
       Unfortunately, not validating arguments leads to surprises down the line ("why is
       birth_date an email address?").

       Also, did you spot the (intentional) bug?

       It's in the "_validate_birth_date()" method. We should check that the value in $birth_date
       is actually defined and an object before we go and call "isa()" on it! Leaving out those
       checks means our data validation code could actually cause our program to die. Oops.

       Note that if we add a superclass to Person we'll have to change the constructor to account
       for that.

       (As an aside, getting all the little details of what Moose does for you just right in this
       example was really not easy, which emphasizes the point of the example. Moose saves you a
       lot of work!)

       Now let's see User:

         package User;

         use strict;
         use warnings;

         use Carp qw( confess );
         use Email::Valid;
         use Scalar::Util qw( blessed );

         use base 'Person';

         sub new {
             my $class = shift;
             my %p = ref $_[0] ? %{ $_[0] } : @_;

             exists $p{email_address}
                 or confess 'email_address is a required attribute';
             $class->_validate_email_address( $p{email_address} );

             my $self = $class->SUPER::new(%p);

             $self->{email_address} = $p{email_address};

             return $self;
         }

         sub _validate_email_address {
             shift;
             my $email_address = shift;

             local $Carp::CarpLevel = $Carp::CarpLevel + 1;

             defined $email_address
                 or confess 'email_address must be a string';

             Email::Valid->address($email_address)
                 or confess "$email_address is not a valid email address";
         }

         sub email_address {
             my $self = shift;

             if (@_) {
                 $self->_validate_email_address( $_[0] );
                 $self->{email_address} = $_[0];
             }

             return $self->{email_address};
         }

       That one was shorter, but it only has one attribute.

       Between the two classes, we have a whole lot of code that doesn't do much. We could
       probably simplify this by defining some sort of "attribute and validation" hash, like
       this:

         package Person;

         my %Attr = (
             name => {
                 required => 1,
                 validate => sub { defined $_ },
             },
             birth_date => {
                 required => 1,
                 validate => sub { blessed $_ && $_->isa('DateTime') },
             },
             shirt_size => {
                 required => 1,
                 validate => sub { defined $_ && $_ =~ /^(?:s|m|l|xl|xxl)$/i },
             }
         );

       Then we could define a base class that would accept such a definition, and do the right
       thing. Keep that sort of thing up and we're well on our way to writing a half-assed
       version of Moose!

       Of course, there are CPAN modules that do some of what Moose does, like "Class::Accessor",
       "Class::Meta", and so on. But none of them put together all of Moose's features along with
       a layer of declarative sugar, nor are these other modules designed for extensibility in
       the same way as Moose. With Moose, it's easy to write a MooseX module to replace or extend
       a piece of built-in functionality.

       Moose is a complete OO package in and of itself, and is part of a rich ecosystem of
       extensions. It also has an enthusiastic community of users, and is being actively
       maintained and developed.

AUTHOR

       Moose is maintained by the Moose Cabal, along with the help of many contributors. See
       "CABAL" in Moose and "CONTRIBUTORS" in Moose for details.

COPYRIGHT AND LICENSE

       This software is copyright (c) 2013 by Infinity Interactive, Inc..

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