--- name: "Moose::Cookbook::Recipe1" section: "3pm" platform: "linux" package: "libmoose-perl" version: "0.54-1" release: "Debian 5.0 (Lenny)" platforms: ["linux"] url: "https://new.carta.tech/Moose::Cookbook::Recipe1" --- # Moose::Cookbook::Recipe1(3pm) The (always classic) 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 Moose::Object. The reason we do this, is so that we can be sure that your class will inherit from Moose::Object and get the benefits that provides (such as a constructor; see Moose::Object for details). However, you don't actually *have* to inherit from Moose::Object 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 **Point3D** class. The next thing we do is to create a new attribute for **Point3D** called `"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 `"override"` modifier allows you to use the `"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 Moose::Object, it will also inherit the default Moose::Object 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/000\_recipes/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 Moose::Util::TypeConstraints documentation. - (2) Moose supports using instance structures other than blessed hash references (such as in a glob reference -- see MooseX::GlobRef::Object). If you want your Moose classes to be interchangeable, it is advisable to avoid direct instance access, like that shown above. Moose does let you get and set attributes directly without exposing the instance structure, but that's an advanced topic (intrepid readers should refer to the "Moose::Meta::Attribute documentation"). ## SEE ALSO - Method Modifiers The concept of method modifiers is directly ripped off from CLOS. A great explanation of them can be found by following this link. \ ## AUTHOR Stevan Little \ ## COPYRIGHT AND LICENSE Copyright 2006-2008 by Infinity Interactive, Inc. \ This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.