Refactoring the /Can't return (?:array|hash) to scalar context/ croak

[perl5.git] / lib / mro.pm

#      mro.pm
#
#      Copyright (c) 2007 Brandon L Black
#
#      You may distribute under the terms of either the GNU General Public
#      License or the Artistic License, as specified in the README file.
#
package mro;
use strict;
use warnings;

# mro.pm versions < 1.00 reserved for MRO::Compat
#  for partial back-compat to 5.[68].x
our $VERSION = '1.00';

sub import {
    mro::set_mro(scalar(caller), $_[1]) if $_[1];
}

package # hide me from PAUSE
    next;

sub can { mro::_nextcan($_[0], 0) }

sub method {
    my $method = mro::_nextcan($_[0], 1);
    goto &$method;
}

package # hide me from PAUSE
    maybe::next;

sub method {
    my $method = mro::_nextcan($_[0], 0);
    goto &$method if defined $method;
    return;
}

1;

__END__

=head1 NAME

mro - Method Resolution Order

=head1 SYNOPSIS

  use mro; # enables next::method and friends globally

  use mro 'dfs'; # enable DFS MRO for this class (Perl default)
  use mro 'c3'; # enable C3 MRO for this class

=head1 DESCRIPTION

The "mro" namespace provides several utilities for dealing
with method resolution order and method caching in general.

These interfaces are only available in Perl 5.9.5 and higher.
See L<MRO::Compat> on CPAN for a mostly forwards compatible
implementation for older Perls.

=head1 OVERVIEW

It's possible to change the MRO of a given class either by using C<use
mro> as shown in the synopsis, or by using the L</mro::set_mro> function
below.  The functions in the mro namespace do not require loading the
C<mro> module, as they are actually provided by the core perl interpreter.

The special methods C<next::method>, C<next::can>, and
C<maybe::next::method> are not available until this C<mro> module
has been loaded via C<use> or C<require>.

=head1 The C3 MRO

In addition to the traditional Perl default MRO (depth first
search, called C<DFS> here), Perl now offers the C3 MRO as
well.  Perl's support for C3 is based on the work done in
Stevan Little's module L<Class::C3>, and most of the C3-related
documentation here is ripped directly from there.

=head2 What is C3?

C3 is the name of an algorithm which aims to provide a sane method
resolution order under multiple inheritance. It was first introduced in
the language Dylan (see links in the L</"SEE ALSO"> section), and then
later adopted as the preferred MRO (Method Resolution Order) for the
new-style classes in Python 2.3. Most recently it has been adopted as the
"canonical" MRO for Perl 6 classes, and the default MRO for Parrot objects
as well.

=head2 How does C3 work

C3 works by always preserving local precendence ordering. This essentially
means that no class will appear before any of its subclasses. Take, for
instance, the classic diamond inheritance pattern:

     <A>
    /   \
  <B>   <C>
    \   /
     <D>

The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A>
appears before B<C>, even though B<C> is the subclass of B<A>. The C3 MRO
algorithm however, produces the following order: (D, B, C, A), which does
not have this issue.

This example is fairly trivial; for more complex cases and a deeper
explanation, see the links in the L</"SEE ALSO"> section.

=head1 Functions

=head2 mro::get_linear_isa($classname[, $type])

Returns an arrayref which is the linearized MRO of the given class.
Uses whichever MRO is currently in effect for that class by default,
or the given MRO (either C<c3> or C<dfs> if specified as C<$type>).

The linearized MRO of a class is an ordered array of all of the
classes one would search when resolving a method on that class,
starting with the class itself.

If the requested class doesn't yet exist, this function will still
succeed, and return C<[ $classname ]>

Note that C<UNIVERSAL> (and any members of C<UNIVERSAL>'s MRO) are not
part of the MRO of a class, even though all classes implicitly inherit
methods from C<UNIVERSAL> and its parents.

=head2 mro::set_mro($classname, $type)

Sets the MRO of the given class to the C<$type> argument (either
C<c3> or C<dfs>).

=head2 mro::get_mro($classname)

Returns the MRO of the given class (either C<c3> or C<dfs>).

=head2 mro::get_isarev($classname)

Gets the C<mro_isarev> for this class, returned as an
arrayref of class names.  These are every class that "isa"
the given class name, even if the isa relationship is
indirect.  This is used internally by the MRO code to
keep track of method/MRO cache invalidations.

Currently, this list only grows, it never shrinks.  This
was a performance consideration (properly tracking and
deleting isarev entries when someone removes an entry
from an C<@ISA> is costly, and it doesn't happen often
anyways).  The fact that a class which no longer truly
"isa" this class at runtime remains on the list should be
considered a quirky implementation detail which is subject
to future change.  It shouldn't be an issue as long as
you're looking at this list for the same reasons the
core code does: as a performance optimization
over having to search every class in existence.

As with C<mro::get_mro> above, C<UNIVERSAL> is special.
C<UNIVERSAL> (and parents') isarev lists do not include
every class in existence, even though all classes are
effectively descendants for method inheritance purposes.

=head2 mro::is_universal($classname)

Returns a boolean status indicating whether or not
the given classname is either C<UNIVERSAL> itself,
or one of C<UNIVERSAL>'s parents by C<@ISA> inheritance.

Any class for which this function returns true is
"universal" in the sense that all classes potentially
inherit methods from it.

For similar reasons to C<isarev> above, this flag is
permanent.  Once it is set, it does not go away, even
if the class in question really isn't universal anymore.

=head2 mro::invalidate_all_method_caches()

Increments C<PL_sub_generation>, which invalidates method
caching in all packages.

=head2 mro::method_changed_in($classname)

Invalidates the method cache of any classes dependent on the
given class.  This is not normally necessary.  The only
known case where pure perl code can confuse the method
cache is when you manually install a new constant
subroutine by using a readonly scalar value, like the
internals of L<constant> do.  If you find another case,
please report it so we can either fix it or document
the exception here.

=head2 mro::get_pkg_gen($classname)

Returns an integer which is incremented every time a
real local method in the package C<$classname> changes,
or the local C<@ISA> of C<$classname> is modified.

This is intended for authors of modules which do lots
of class introspection, as it allows them to very quickly
check if anything important about the local properties
of a given class have changed since the last time they
looked.  It does not increment on method/C<@ISA>
changes in superclasses.

It's still up to you to seek out the actual changes,
and there might not actually be any.  Perhaps all
of the changes since you last checked cancelled each
other out and left the package in the state it was in
before.

This integer normally starts off at a value of C<1>
when a package stash is instantiated.  Calling it
on packages whose stashes do not exist at all will
return C<0>.  If a package stash is completely
deleted (not a normal occurence, but it can happen
if someone does something like C<undef %PkgName::>),
the number will be reset to either C<0> or C<1>,
depending on how completely package was wiped out.

=head2 next::method

This is somewhat like C<SUPER>, but it uses the C3 method
resolution order to get better consistency in multiple
inheritance situations.  Note that while inheritance in
general follows whichever MRO is in effect for the
given class, C<next::method> only uses the C3 MRO.

One generally uses it like so:

  sub some_method {
    my $self = shift;
    my $superclass_answer = $self->next::method(@_);
    return $superclass_answer + 1;
  }

Note that you don't (re-)specify the method name.
It forces you to always use the same method name
as the method you started in.

It can be called on an object or a class, of course.

The way it resolves which actual method to call is:

=over 4

=item 1

First, it determines the linearized C3 MRO of
the object or class it is being called on.

=item 2

Then, it determines the class and method name
of the context it was invoked from.

=item 3

Finally, it searches down the C3 MRO list until
it reaches the contextually enclosing class, then
searches further down the MRO list for the next
method with the same name as the contextually
enclosing method.

=back

Failure to find a next method will result in an
exception being thrown (see below for alternatives).

This is substantially different than the behavior
of C<SUPER> under complex multiple inheritance.
(This becomes obvious when one realizes that the
common superclasses in the C3 linearizations of
a given class and one of its parents will not
always be ordered the same for both.)

B<Caveat>: Calling C<next::method> from methods defined outside the class:

There is an edge case when using C<next::method> from within a subroutine
which was created in a different module than the one it is called from. It
sounds complicated, but it really isn't. Here is an example which will not
work correctly:

  *Foo::foo = sub { (shift)->next::method(@_) };

The problem exists because the anonymous subroutine being assigned to the
C<*Foo::foo> glob will show up in the call stack as being called
C<__ANON__> and not C<foo> as you might expect. Since C<next::method> uses
C<caller> to find the name of the method it was called in, it will fail in
this case. 

But fear not, there's a simple solution. The module C<Sub::Name> will
reach into the perl internals and assign a name to an anonymous subroutine
for you. Simply do this:

  use Sub::Name 'subname';
  *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };

and things will Just Work.

=head2 next::can

This is similar to C<next::method>, but just returns either a code
reference or C<undef> to indicate that no further methods of this name
exist.

=head2 maybe::next::method

In simple cases, it is equivalent to:

   $self->next::method(@_) if $self->next_can;

But there are some cases where only this solution
works (like C<goto &maybe::next::method>);

=head1 SEE ALSO

=head2 The original Dylan paper

=over 4

=item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>

=back

=head2 The prototype Perl 6 Object Model uses C3

=over 4

=item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>

=back

=head2 Parrot now uses C3

=over 4

=item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>

=item L<http://use.perl.org/~autrijus/journal/25768>

=back

=head2 Python 2.3 MRO related links

=over 4

=item L<http://www.python.org/2.3/mro.html>

=item L<http://www.python.org/2.2.2/descrintro.html#mro>

=back

=head2 C3 for TinyCLOS

=over 4

=item L<http://www.call-with-current-continuation.org/eggs/c3.html>

=back 

=head2 Class::C3

=over 4

=item L<Class::C3>

=back

=head1 AUTHOR

Brandon L. Black, E<lt>blblack@gmail.comE<gt>

Based on Stevan Little's L<Class::C3>

=cut