7 use Scalar::Util 'blessed';
10 our $VERSION = '0.14';
12 # this is our global stash of both
13 # MRO's and method dispatch tables
14 # the structure basically looks like
18 # MRO => [ <class precendence list> ],
20 # orig => <original location of method>,
21 # code => \&<ref to original method>
23 # has_overload_fallback => (1 | 0)
28 # use these for debugging ...
29 sub _dump_MRO_table { %MRO }
31 our $_initialized = 0;
35 # skip if the caller is main::
36 # since that is clearly not relevant
37 return if $class eq 'main';
38 return if $TURN_OFF_C3;
39 # make a note to calculate $class
41 $MRO{$class} = undef unless exists $MRO{$class};
47 # why bother if we don't have anything ...
48 return unless keys %MRO;
51 $MRO{$_} = undef foreach keys %MRO;
53 _calculate_method_dispatch_tables();
54 _apply_method_dispatch_tables();
55 %next::METHOD_CACHE = ();
60 # why bother if we don't have anything ...
61 return unless keys %MRO;
62 _remove_method_dispatch_tables();
63 %next::METHOD_CACHE = ();
67 sub reinitialize { goto &initialize }
69 ## functions for applying C3 to classes
71 sub _calculate_method_dispatch_tables {
73 foreach my $class (keys %MRO) {
74 _calculate_method_dispatch_table($class, \%merge_cache);
78 sub _calculate_method_dispatch_table {
79 my ($class, $merge_cache) = @_;
81 my @MRO = calculateMRO($class, $merge_cache);
82 $MRO{$class} = { MRO => \@MRO };
83 my $has_overload_fallback = 0;
86 # we do @MRO[1 .. $#MRO] here because it
87 # makes no sense to interogate the class
88 # which you are calculating for.
89 foreach my $local (@MRO[1 .. $#MRO]) {
90 # if overload has tagged this module to
91 # have use "fallback", then we want to
93 $has_overload_fallback = ${"${local}::()"}
94 if defined ${"${local}::()"};
95 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
96 # skip if already overriden in local class
97 next unless !defined *{"${class}::$method"}{CODE};
99 orig => "${local}::$method",
100 code => \&{"${local}::$method"}
101 } unless exists $methods{$method};
104 # now stash them in our %MRO table
105 $MRO{$class}->{methods} = \%methods;
106 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
109 sub _apply_method_dispatch_tables {
110 foreach my $class (keys %MRO) {
111 _apply_method_dispatch_table($class);
115 sub _apply_method_dispatch_table {
118 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
119 if $MRO{$class}->{has_overload_fallback};
120 foreach my $method (keys %{$MRO{$class}->{methods}}) {
121 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
125 sub _remove_method_dispatch_tables {
126 foreach my $class (keys %MRO) {
127 _remove_method_dispatch_table($class);
131 sub _remove_method_dispatch_table {
134 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
135 foreach my $method (keys %{$MRO{$class}->{methods}}) {
136 delete ${"${class}::"}{$method}
137 if defined *{"${class}::${method}"}{CODE} &&
138 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
142 ## functions for calculating C3 MRO
145 my ($class, $merge_cache) = @_;
146 return Algorithm::C3::merge($class, sub {
152 package # hide me from PAUSE
158 use Scalar::Util 'blessed';
160 our $VERSION = '0.05';
165 my $indirect = caller() =~ /^(?:next|maybe::next)$/;
166 my $level = $indirect ? 2 : 1;
168 my ($method_caller, $label, @label);
169 while ($method_caller = (caller($level++))[3]) {
170 @label = (split '::', $method_caller);
173 $label eq '(eval)' ||
174 $label eq '__ANON__';
176 my $caller = join '::' => @label;
178 my $class = blessed($self) || $self;
180 my $method = $METHOD_CACHE{"$class|$caller|$label"} ||= do {
182 my @MRO = Class::C3::calculateMRO($class);
185 while ($current = shift @MRO) {
186 last if $caller eq $current;
191 foreach my $class (@MRO) {
192 next if (defined $Class::C3::MRO{$class} &&
193 defined $Class::C3::MRO{$class}{methods}{$label});
194 last if (defined ($found = *{$class . '::' . $label}{CODE}));
200 return $method if $indirect;
202 die "No next::method '$label' found for $self" if !$method;
207 sub can { method($_[0]) }
209 package # hide me from PAUSE
215 our $VERSION = '0.01';
217 sub method { (next::method($_[0]) || return)->(@_) }
227 Class::C3 - A pragma to use the C3 method resolution order algortihm
233 sub hello { 'A::hello' }
243 sub hello { 'C::hello' }
249 # Classic Diamond MI pattern
258 # initializez the C3 module
259 # (formerly called in INIT)
260 Class::C3::initialize();
262 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
264 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
266 D->can('hello')->(); # can() also works correctly
267 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
271 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
272 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
276 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
277 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
278 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
279 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
280 default MRO for Parrot objects as well.
282 =head2 How does C3 work.
284 C3 works by always preserving local precendence ordering. This essentially means that no class will
285 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
293 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
294 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
295 (D, B, C, A), which does not have this same issue.
297 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
298 the L<SEE ALSO> section.
300 =head2 How does this module work?
302 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
303 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
304 gathers information from the symbol tables of each of those classes, and builds a set of method
305 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
306 then adds the method aliases into the local classes symbol table.
308 The end result is actually classes with pre-cached method dispatch. However, this caching does not
309 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
310 your classes to be effectively closed. See the L<CAVEATS> section for more details.
312 =head1 OPTIONAL LOWERCASE PRAGMA
314 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
315 the regular install since lowercase module names are considered I<"bad"> by some people. However I
316 think that code looks much nicer like this:
326 But hey, it's your choice, thats why it is optional.
332 =item B<calculateMRO ($class)>
334 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
338 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
339 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
340 use C3. Here is a quick code example:
344 # ... Foo methods here
349 # ... Bar methods here
353 Class::C3::initialize(); # now it is safe to use Foo and Bar
355 This function used to be called automatically for you in the INIT phase of the perl compiler, but
356 that lead to warnings if this module was required at runtime. After discussion with my user base
357 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
358 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
359 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
360 your own INIT method which calls this function.
364 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
365 clear the MRO cache first.
367 =item B<uninitialize>
369 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
370 style dispatch order (depth-first, left-to-right).
372 =item B<reinitialize>
374 This is an alias for L</initialize> above.
378 =head1 METHOD REDISPATCHING
380 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
381 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
382 method along the C3 linearization. This is best show with an examples.
384 # a classic diamond MI pattern ...
398 sub foo { 'B::foo => ' . (shift)->next::method() }
403 sub foo { 'C::foo => ' . (shift)->next::method() }
408 sub foo { 'D::foo => ' . (shift)->next::method() }
410 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
412 A few things to note. First, we do not require you to add on the method name to the C<next::method>
413 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
414 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
416 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
417 not automatically use the current C<@_>.
419 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
420 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
422 $self->next::method(@_) if $self->next::can;
424 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
425 The previous example could be simply written as:
427 $self->maybe::next::method(@_);
429 There are some caveats about using C<next::method>, see below for those.
433 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
434 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
435 whatever your needs might be.
437 But there are still caveats, so here goes ...
441 =item Use of C<SUPER::>.
443 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
444 However, it's use in conjuntion with this module is very much not recommended, and in fact very
445 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
446 more details on it's usage above.
448 =item Changing C<@ISA>.
450 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
451 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
452 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
453 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
454 in F<t/20_reinitialize.t> for more information.
456 =item Adding/deleting methods from class symbol tables.
458 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
459 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
460 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
461 changes you make to take effect.
463 =item Calling C<next::method> from methods defined outside the class
465 There is an edge case when using C<next::method> from within a subroutine which was created in a different
466 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
467 will not work correctly:
469 *Foo::foo = sub { (shift)->next::method(@_) };
471 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
472 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
473 uses C<caller> to find the name of the method it was called in, it will fail in this case.
475 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
476 assign a name to an anonymous subroutine for you. Simply do this:
478 use Sub::Name 'subname';
479 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
481 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
482 manage to find a workaround for it, so until someone gives me a working patch this will be a known
483 limitation of this module.
489 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
492 ---------------------------- ------ ------ ------ ------ ------ ------ ------
493 File stmt bran cond sub pod time total
494 ---------------------------- ------ ------ ------ ------ ------ ------ ------
495 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
496 ---------------------------- ------ ------ ------ ------ ------ ------ ------
497 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
498 ---------------------------- ------ ------ ------ ------ ------ ------ ------
502 =head2 The original Dylan paper
506 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
510 =head2 The prototype Perl 6 Object Model uses C3
514 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
518 =head2 Parrot now uses C3
522 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
524 =item L<http://use.perl.org/~autrijus/journal/25768>
528 =head2 Python 2.3 MRO related links
532 =item L<http://www.python.org/2.3/mro.html>
534 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
538 =head2 C3 for TinyCLOS
542 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
546 =head1 ACKNOWLEGEMENTS
550 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
551 and finding many bugs and providing fixes.
553 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
554 calls inside C<eval> and anon-subs.
556 =item Thanks to Robert Norris for adding support for C<next::can> and
557 C<maybe::next::method>.
563 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
565 =head1 COPYRIGHT AND LICENSE
567 Copyright 2005, 2006 by Infinity Interactive, Inc.
569 L<http://www.iinteractive.com>
571 This library is free software; you can redistribute it and/or modify
572 it under the same terms as Perl itself.