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Man Pages


Manual Reference Pages  -  ROLE::BASIC (3)

.ds Aq ’

NAME

Role::Basic - Just roles. Nothing else.

CONTENTS

VERSION

Version 0.13

SYNOPSIS

In a role:



    package Does::Serialize::AsYAML;
    use Role::Basic;
    use YAML::Syck;
    requires as_hash;

    sub serialize {
        my $self = shift;
        return Dump( $self->as_hash );
    }

    1;



In your class:



    package My::Class;
    use Role::Basic with;

    with qw(
        Does::Serialize::AsYAML
    );

    sub as_hash { ... } # because the role requires it



BETA CODE

This code appears to be stable and currently passes over 300 tests. We’ve not (yet) heard of any bugs. There are no functional changes with this release. It’s merely here to let early-adopters know it’s safe to give it a spin.

DESCRIPTION

For an extended discussion, see <http://blogs.perl.org/users/ovid/2010/12/rolebasic---when-you-only-want-roles.html>.

Sometimes you want roles. You’re not sure about Moose, Mouse, Moo and what was that damned Squirrel thing anyway? Then there’s Class::Trait, but it has a funky syntax and the maintainer’s deprecated it in favor of Moose::Role and you really don’t care that it handles overloading, instance application or has a workaround for the SUPER:: bug. You think a meta-object protocol sounds nifty, but you don’t understand it. Maybe you’re not sure you want the syntactic sugar for object declaration. Maybe you’ve convinced your colleagues that roles are a good idea but they’re leery of dragging in Moose (your author has had this happen more than once and heard of others making the same complaint). Sometimes you just want good old-fashioned roles which let you separate class responsibility from code reuse.

Whatever your reasons, this is the module you’re looking for. It only provides roles and its major design goals are safety and simplicity. It also aims to be a subset of Moose::Role behavior so that when/if you’re ready to upgrade, there will be minimal pain.

DECLARING A ROLE

To declare the current package as a role, simply add the following line to the package:



    use Role::Basic;



You can then use with to consume other roles and requires to list the methods this role requires. Note that the only methods the role will provide are methods declared directly in the role or consumed from other roles. Thus:



    package My::Role;
    use Role::Basic;
    use List::Util sum; # this will not be provided by the role
    with Some::Other::Role; # any methods from this role will be provided

    sub some_method {...} # this will be provided by the role



    Allowed methods in roles

<B>WarningB>: this functionality is experimental and is subject to change with no warning.

As mentioned, methods imported into a role are not provided by that role. However, this can make it very hard when you want to provide simple getters/setters. To get around this limitation, a role (and only roles, not classes) may specify one class which they ’allow’ to provide additional methods:



    package My::Role;
    use Role::Basic allow => Class::BuildMethods;
    use Class::BuildMethods qw/foo bar/;

    # your role will now provide foo and bar methods
    # rest of role definition here



Please note that if you do this, the code which provides these ’extra’ methods should not provide them in a way which is incompatible with your objects. For example, many getter/setters generation classes assume you’re using a blessed hashref. Most objects are, but the role should not make such an assumption about the class which consumes it. In the above example, we use Class::BuildMethods. It’s agnostic about your object implementation, but it’s slow.

See <http://blogs.perl.org/users/ovid/2011/01/happy-new-yearroles.html> and search for ’glue’ to understand why this is important.

CONSUMING ROLES

To declare the current package as a class that will use roles, simply add the following line to the package:



    use Role::Basic with;



Just as with Moose, you can have -alias, -excludes, and -version.

Unlike Moose, we also provide a -rename target. It combines -alias and -excludes. This code:



    package My::Class;
    use Role::Basic with;

    with My::Role => {
        -rename => { foo => baz, bar => gorch },
    };



Is identical to this code:



    package My::Class;
    use Role::Basic with;

    with My::Role => {
        -alias    => { foo => baz, bar => gorch },
        -excludes => [qw/foo bar/],
    };



EXPORT

Both roles and classes will receive the following methods:
o with

with accepts a list and may only be called <B>onceB> per role or class. This is because calling it multiple times removes composition safety. Just as with Moose::Role, any class may also have -alias or -excludes.



    package My::Class;
    use Role::Basic with;

    with Does::Serialize::AsYAML => { -alias => { serialize => as_yaml } };



And later:



    print $object->as_yaml;



o DOES

Returns true if the class or role consumes a role of the given name:



 if ( $class->DOES(Does::Serialize::AsYAML) ) {
    ...
 }



Every role DOES itself.

Further, if you’re a role, you can also specify methods you require:
o requires



    package Some::Role;
    use Role::Basic;

    # roles can consume other roles
    with Another::Role;

    requires qw(
        first_method
        second_method
        another_method
    );



In the example above, if Another::Role has methods it requires, they will be added to the requirements of Some::Role.

DESIGN GOALS AND LIMITATIONS

There are two overriding design goals for Role::Basic: <B>simplicityB> and <B>safetyB>. We make it a bit harder to shoot yourself in the foot and we aim to keep the code as simple as possible. Feature requests are welcomed, but will not be acted upon if they violate either of these two design goals.

Thus, if you need something which Role::Basic does not support, you’re strongly encouraged to consider Moose or Mouse.

The following list details the outcomes of this module’s goals.
o Basic role support

This includes composing into your class, composing roles from other roles, roles declaring requirements and conflict resolution.

o Moose-like syntax

To ease migration difficulties, we use a Moose-like syntax. If you wish to upgrade to Moose later, or you find that others on your project are already familiar with Moose, this should make Role::Basic easier to learn.

o No handling of SUPER:: bug

A well-known bug in OO Perl is that a SUPER:: method is invoked against the class its declared in, not against the class of the invocant. Handling this properly generally involves eval’ing a method directly into the correct package:



    eval <<"END_METHOD";
    package $some_package;

    sub some_method { ... }
    END_METHOD



Or using a different method resolution order (MRO) such as with Class::C3 or friends. We alert you to this limitation but make no attempt to address it. We consider this a feature because roles should not know or care how they are composed and probably should not know if a superclass exists. This helps to keep this module simple, a primary design goal.

o Composition Safety

In addition to the normal conflict resolution, only one with statement is allowed:



    package Foo;
    use Role::Basic;
    with Some::Role;
    with Another::Role; # boom!



This is because when you have more than one with statement, the latter will ignore conflicts with the first. We could work around this, but this would be significantly different from the behavior of Moose.

o Override Safety

By default, we aim to behave like Moose::Role. This means that if a class consuming a role has a method with the same name the role provides, the class silently wins. This has been a somewhat contentious issue in the Moose community and the silent behaviour has won. However, there are those who prefer that they don’t have their methods silently ignored. We provide two optional environment variables to handle this:



    $ENV{PERL_ROLE_OVERRIDE_WARN}
    $ENV{PERL_ROLE_OVERRIDE_DIE}



If you prefer, you can set one of those to true and a class overridding a role’s method will warn or die, as appropriate. As you might expect, you can handle this with normal role behaviour or exclusion or aliasing.



    package My::Class;
    use Role::Basic with;
    with My::Role => { -excludes => conflicting_method };



From your author’s email exchanges with the authors of the original traits paper (referenced here with permission), the class silently wins behaviour was not intended. About this, Dr. Andrew P. Black wrote the following:



    Yes, it is really important that a programmer can see clearly when a trait
    method is being overridden -- just as it is important that it is clear
    when an inherited method is being overridden.

    In Smalltalk, where a program is viewed as a graph of objects, the obvious
    solution to this problem is to provide an adequate tool to show the
    programmer interesting properties of the program.  The original traits
    browser did this for Smalltalk; the reason that we implemented it is that
    traits were really NOT a good idea (that is,they were not very usable or
    maintainable) without it.  Since then, the same sort of "virtual
    protocols" have been built into the browser for other properties, like
    "overridden methods".



Note that those are provided as environment variables and not as syntax in the code itself to help keep the code closer to the Moose syntax.

o No instance application

Role::Basic does not support applying roles to object instances. This may change in the future.

o No method modifiers

These have been especially problematic. Consider a before modifier which multiplies a value by 2 and another before modifier which divides a value by 3. The order in which those modifiers are applied becomes extremely important. and role-consumption is no longer entirely declarative, but becomes partially procedural. This causes enough problems that on Sep 14, 2010 on the Moose mailing list, Stevan Little wrote:



    I totally agree [with the described application order problems], and if I
    had to do it over again, I would not have allowed method modifiers in
    roles. They ruin the unordered-ness of roles and bring about edge cases
    like this that are not so easily solved.



Thus, Role::Basic does not and will not support method modifiers. If you need them, consider Moose.

AUTHOR

Curtis ’Ovid’ Poe, <ovid at cpan.org>

BUGS

Please report any bugs or feature requests to bug-role-basic at rt.cpan.org, or through the web interface at <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Role-Basic>. I will be notified, and then you’ll automatically be notified of progress on your bug as I make changes.

SUPPORT

You can find documentation for this module with the perldoc command.



    perldoc Role::Basic



You can also look for information at:
o RT: CPAN’s request tracker

<http://rt.cpan.org/NoAuth/Bugs.html?Dist=Role-Basic>

o AnnoCPAN: Annotated CPAN documentation

<http://annocpan.org/dist/Role-Basic>

o CPAN Ratings

<http://cpanratings.perl.org/d/Role-Basic>

o Search CPAN

<http://search.cpan.org/dist/Role-Basic/>

SEE ALSO

o Role::Tiny
o Moose::Role
o Mouse::Role

LICENSE AND COPYRIGHT

Copyright 2010 Curtis ’Ovid’ Poe.

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.

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