Role::Basic::PhilosophUser Contributed Perl DocumentRole::Basic::Philosophy(3)NAMERole::Basic::Philosophy - Why Role::Basic exists.
RATIONALE
Note: the words "trait" and "role" will be used interchangeably
throughout this documentation.
After years of using roles, your author has found that many people
would be happy to use roles but are not willing/comfortable with using
Moose. This module implements roles and nothing else. It does so in a
(relatively) simple bit of code. However, you should be aware that
there are some differences between Role::Basic and Moose::Role.
Moose is a fantastic technology and your author is quite happy with it.
He urges you to check it out and perhaps even consider Role::Basic a
"stepping-stone" to Moose. However, after an informal poll with many
respondents replying on blogs.perl.org, Twitter, Facebook and private
email unanimously saying they wanted this module for roles and not as a
stepping-stone to Moose, your author took the liberty of deciding to
implement traits in a rather faithful fashion, rather than strictly
adhere to the design of Moose::Role. For areas where we differ,
Role::Basic intends to be more restrictive when syntax is the same.
This allows an easier migration to Moose::Role when the time is right.
Otherwise, Role::Basic will offer a different syntax to avoid
confusion.
TRAITS
As most of you probably know, roles are the Perl implmentation of
traits as described in <http://scg.unibe.ch/research/traits/>. (The
name "role" was chosen because "trait" was already used in Perl 6.) In
particular, we direct you to two papers, both of which are easy to
read:
· <http://scg.unibe.ch/scgbib?_s=tgXJjGrs0380ejY6&_k=Swgdwx_C&query=nathanael+traits+composable+units+ecoop&display=abstract&_n&19>
The seminal "traits paper" which much of the documentation refers
to.
· <http://scg.unibe.ch/scgbib?_s=tgXJjGrs0380ejY6&_k=Swgdwx_C&query=traits+the+formal+model&display=abstract&_n&23>
"Traits: The Formal Model".
While less well-known, this relatively easy to read paper outlines
the mathematical underpinnings of traits and explains several
design decisions taken here.
It is important to refer back to those papers because Role::Basic
attempts to implements traits as described in the research, whereas
Moose::Role attempts to implement something very similar to traits, but
with more of a "Perlish" feel. This is not intended as a criticism of
Moose::Role, but merely an attempt to alert the reader to key
differences.
The Basics
Roles are simply bundles of behavior which classes may use. If you have
two completely unrelated classes, your code may still require each of
them to serialize themselves as JSON even though neither class
naturally has anything to do with JSON (for example, "Person" and
"Order" classes). There are a number of approaches to this problem but
if you're here I'll skip the explanation and assume that you already
understand roles and would like to know why we don't follow the
Moose::Role specification.
As you already probably know, roles allow you to state that your class
"DOES" some behaviour, and allows you to exclude or alias bits and
pieces of the roles you're including. The original specification of
traits made it clear that this was to be done in such a fashion that no
matter how you grouped the traits or in which order you used them, the
outcome behavior would be the same. That's why we have subtle but
forward-compatible differences with Moose::Role.
Commutative
The formal model
(<http://scg.unibe.ch/archive/papers/Scha02cTraitsModel.pdf>) states
that trait composition must be commutative (section 3.4, proposition
1). This means that:
(A + B) = (B + A)
In other words, it should not matter what order you compose the traits
in. It is well known that with both inheritance and mixins, this does
not hold (making refactoring a dicey proposition at times), but when
method modifiers are used with Moose::Role, the same issues arises
(from
<http://blogs.perl.org/users/ovid/2010/12/rolebasic---when-you-only-want-roles.html>):
{
package Some::Role;
use Moose::Role;
requires qw(some_method);
before some_method => sub {
my $self = shift;
$self->some_number( $self->some_number + 2 );
};
}
{
package Another::Role;
use Moose::Role;
requires qw(some_method);
before some_method => sub {
my $self = shift;
$self->some_number( $self->some_number / 2 );
};
}
{
package Some::Class;
use Moose;
my @roles =
int( rand(2) )
? qw(Another::Role Some::Role)
: qw(Some::Role Another::Role);
with @roles;
has some_number => ( is => 'rw', isa => 'Num' );
sub some_method { print shift->some_number, $/ }
}
my $o = Some::Class->new( { some_number => 7 } );
$o->some_method;
If you run this code, it might print 4.5, but it might print 5.5. As
with mixins and multiple inheritance, you have no way of knowing the
exact behaviour which will be exhibited short of running the code. No
introspection will help. This is not an issue with Role::Basic because
we do not allow method modifiers. If you think you need them, please
consider Moose.
Associative
The formal model
(<http://scg.unibe.ch/archive/papers/Scha02cTraitsModel.pdf>) states
that trait composition must be associative (section 3.4, proposition
1). This means that:
(A + B) + C = A + (B + C)
Moose is associative if and only if you do not have multiple methods
with the same name. In Moose, if a role providing method M consumes
one other role which also provides method M, we have a conflict:
package Some::Role;
use Moose::Role;
sub bar { __PACKAGE__ }
package Some::Other::Role;
use Moose::Role;
with 'Some::Role';
sub bar { __PACKAGE__ }
package Some::Class;
use Moose;
with 'Some::Other::Role';
package main;
my $o = Some::Class->new;
print $o->bar;
However, if the role consumes two or more other roles which provide the
same method, we don't have a conflict:
package Some::Role;
use Moose::Role;
sub bar { __PACKAGE__ }
package Some::Other::Role;
use Moose::Role;
sub bar { __PACKAGE__ }
package Another::Role;
use Moose::Role;
with qw(Some::Role Some::Other::Role);
sub bar { __PACKAGE__ }
package Some::Class;
use Moose;
with 'Another::Role';
package main;
my $o = Some::Class->new;
print $o->bar;
This is because, in Moose, when you have two or more roles consumed,
any conflicting methods are excluded and considered to be requirements.
See "Moose::Role composition edge cases" for more explanation:
<http://search.cpan.org/~drolsky/Moose-1.21/lib/Moose/Spec/Role.pod#Composition_Edge_Cases>.
This makes roles easy to use at times, but it means that the following
three groups of roles are not guaranteed to provide the same behavior:
RoleA does RoleB, RoleC
RoleB does RoleA, RoleC
RoleC does RoleA, RoleB
Further, you as a developer have no way of knowing that we have had
methods silently excluded without reading all of the code.
For Role::Basic there are no edge cases. If "RoleA", "RoleB", and
"RoleC" all provide method M, you are guaranteed to get a conflict at
composition time and must specifically address the problem. This
addresses the associative issue because strictly speaking, a trait is
merely a bundle of services provided, not its name. Thus, a trait with
its "foo" method excluded is not the same as itself without the "foo"
method excluded.
Benefits of associative and commutative behaviour
While we recognize that not everyone will be happy with the decisions
we have made, we have several benefits here:
· We adhere to the formal definition of traits
· Ordering and grouping of traits does not alter their behavior
· We're forward-compatible with Moose::Role
CONCLUSION
The primary goal of Role::Basic is to provide traits in a simple and
safe manner. We are huge fans of Moose and Moose::Role and suggest that
everyone check them out. The decision of Moose::Role to deviate from
the "associative" and "commutative" deviations from the original traits
model is, in our experience, less likely to occur with roles than with
mixins and inhertance, so please do not take this as an indictment, but
rather in the spirit of TIMTOWTDI.
perl v5.18.2 2014-05-13 Role::Basic::Philosophy(3)
