Moose::Cookbook::Recipe1 - The B<Point> example.

NAME

Moose::Cookbook::Recipe1 - The (always classic) Point example.

SYNOPSIS

  package Point;
  use Moose;
        
  has 'x' => (isa => 'Int', is => 'ro');
  has 'y' => (isa => 'Int', is => 'rw');
  
  sub clear {
      my $self = shift;
      $self->{x} = 0;
      $self->y(0);    
  }
  
  package Point3D;
  use Moose;
  
  extends 'Point';
  
  has 'z' => (isa => 'Int');
  
  after 'clear' => sub {
      my $self = shift;
      $self->{z} = 0;
  };

DESCRIPTION

This is the classic Point example. This one in particular I took from the Perl 6 Apocalypse 12 document, but it is similar to the example found in the classic K&R C book as well, and many other places. And now, onto the code:

As with all Perl 5 classes, a Moose class is defined in a package. Moose now handles turning on strict and warnings for you, so all you need to do is say use Moose, and no kittens will die.

By loading Moose, we are enabling the loading of the Moose ``environment'' into our package. This means that we import some functions which serve as Moose ``keywords''. These aren't anything fancy, just plain old exported functions.

Another important thing happens at this stage as well. Moose will automatically set your package's superclass to be the Moose::Object manpage. The reason we do this, is so that we can be sure that your class will inherit from the Moose::Object manpage and get the benefits that provides (see the Moose::Object manpage for details). However, you don't actually have to inherit from the Moose::Object manpage if you don't want to. All Moose features will still be accessible to you.

Now, onto the keywords. The first one we see here is has, which defines an instance attribute in your class:

  has 'x' => (isa => 'Int', is => 'ro');

This will create an attribute named x, which will expect the value stored in the attribute to pass the type constraint Int (1), and the accessor generated for this attribute will be read-only (abbreviated as ro).

The next has line is very similar, with only one difference:

  has 'y' => (isa => 'Int', is => 'rw');

A read/write (abbreviated as rw) accessor will be generated for the y attribute.

At this point the attributes have been defined, and it is time to define our methods. In Moose, as with regular Perl 5 OO, a method is just a subroutine defined within the package. So here we create the clear method.

  sub clear {
      my $self = shift;
      $self->{x} = 0;
      $self->y(0);    
  }

It is pretty standard, the only thing to note is that we are directly accessing the x slot in the instance (2). This is because the value was created with a read-only accessor. This also shows that Moose objects are not anything out of the ordinary, but just regular old blessed HASH references. This means they are very compatible with other Perl 5 (non-Moose) classes as well.

The next part of the code to review is the Point subclass, Point3D. The first item you might notice is that we do not use the standard use base declaration here. Instead we use the Moose keyword extends like so:

  extends 'Point';

This keyword will function very much like use base does in that it will make an attempt to load your class if it has not already been loaded. However, it differs on one important point. The extends keyword will overwrite any previous values in your package's @ISA, where use base will push values onto the package's @ISA. It is my opinion that the behavior of extends is more intuitive in that it is more explicit about defining the superclass relationship.

A small digression here: both Moose and extends support multiple inheritance. You simply pass all the superclasses to extends, like so:

  extends 'Foo', 'Bar', 'Baz';
  
Now, back to our B<Point3D> class. The next thing we do is to create 
a new attribute for B<Point3D> called C<z>.

  has 'z' => (isa => 'Int');

As with Point's x and y attributes, this attribute has a type constraint of Int, but it differs in that it does not ask for any autogenerated accessors. The result being (aside from broken object encapsulation) that z is a private attribute.

Next comes another Moose feature which we call method ``modifiers'' (or method ``advice'' for the AOP inclined). The modifier used here is the after modifier, and looks like this:

  after 'clear' => sub {
      my $self = shift;
      $self->{z} = 0;
  };

This modifier tells Moose to install a clear method for Point3D that will first run the clear method for the superclass (in this case Point::clear), and then run this method after it (passing in the same arguments as the original method).

Now, of course using the after modifier is not the only way to accomplish this. I mean, after all, this is Perl right? You would get the same results with this code:

  sub clear {
      my $self = shift;
      $self->SUPER::clear();
      $self->{z} = 0;
  }

You could also use another Moose method modifier, override here, and get the same results again. Here is how that would look:

  override 'clear' => sub {
      my $self = shift;
      super();
      $self->{z} = 0;
  };
  
The C<override> modifier allows you to use the C<super> keyword 
within it to dispatch to the superclass's method in a very Ruby-ish 
style.

Now, of course, what use is a class if you can't instantiate objects with it? Since Point inherits from the Moose::Object manpage, it will also inherit the default the Moose::Object manpage constructor: new. Here are two examples of how that is used:

  my $point = Point->new(x => 1, y => 2);   
  my $point3d = Point3D->new(x => 1, y => 2, z => 3);

As you can see, new accepts named argument pairs for any of the attributes. It does not require that you pass in the all the attributes, and it will politely ignore any named arguments it does not recognize.

From here on, you can use $point and $point3d just as you would any other Perl 5 object. For a more detailed example of what can be done, you can refer to the t/001_recipe.t test file.

CONCLUSION

I hope this recipe has given you some explanation of how to use Moose to build your Perl 5 classes. The next recipe will build upon the basics shown here with more complex attributes and methods. Please read on :)

FOOTNOTES

(1): Several default type constraints are provided by Moose, of which Int is one. For more information on the builtin type constraints and the type constraint system in general, see the the Moose::Util::TypeConstraints manpage documentation.
(2): Future plans for Moose include allowing for alternate instance structures such as blessed ARRAY refs and such. If you want your Moose classes to be interchangeable, it is advisable to avoid direct instance access, like that shown above.

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

http://www.iinteractive.com

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

Moose::Cookbook::Recipe1 - The B<Point> example.