++ed by:
KJETILK HEYTRAV WRUPPERT DELON JAYALLEN

167 PAUSE users
148 non-PAUSE users.

Author image Dave Rolsky
and 1 contributors

NAME

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

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',
  );

  subtype 'ShirtSize'
      => as 'Str'
      => where { /^(?:s|m|l|xl|xxl)$/i }
      => message { "$_ is not a valid shirt size (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 a 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} = $class->_coerce_birth_date($date );
      $class->_validate_birth_date( $date );

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

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

      my %self = map { $_ => $p{$_} } qw( name shirt_size );
      $self{birth_date} = $date;

      return bless \%self, $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';

      $shirt_size =~ /^(?:s|m|l|xl|xxl)$/
          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 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, which results in much more fragile code.

Did you spot the (intentional) bug?

It's in the _validate_birth_date() method. We should check that that value in $birth_date is actually defined and 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.

There's one bit of code in there worth explaining, which is the handling of the birth date for coercion. In both the constructor and accessor, we first take a copy of the birth date before passing it to the coercion routine. This is to avoid changing the value as it was passed to those methods, which could cause problems for the caller.

Also 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 code was not easy, which just emphasizes the point, that 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.

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

Copyright 2008 by Infinity Interactive, Inc.

http://www.iinteractive.com

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