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1 | # mro.pm |
2 | # |
3 | # Copyright (c) 2007 Brandon L Black |
4 | # |
5 | # You may distribute under the terms of either the GNU General Public |
6 | # License or the Artistic License, as specified in the README file. |
7 | # |
8 | package mro; |
9 | use strict; |
10 | use warnings; |
11 | |
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12 | # mro.pm versions < 1.00 reserved for MRO::Compat |
13 | # for partial back-compat to 5.[68].x |
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14 | our $VERSION = '1.00'; |
15 | |
16 | sub import { |
17 | mro::set_mro(scalar(caller), $_[1]) if $_[1]; |
18 | } |
19 | |
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20 | package # hide me from PAUSE |
21 | next; |
22 | |
23 | sub can { mro::_nextcan($_[0], 0) } |
24 | |
25 | sub method { |
26 | my $method = mro::_nextcan($_[0], 1); |
27 | goto &$method; |
28 | } |
29 | |
30 | package # hide me from PAUSE |
31 | maybe::next; |
32 | |
33 | sub method { |
34 | my $method = mro::_nextcan($_[0], 0); |
35 | goto &$method if defined $method; |
36 | return; |
37 | } |
38 | |
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39 | 1; |
40 | |
41 | __END__ |
42 | |
43 | =head1 NAME |
44 | |
45 | mro - Method Resolution Order |
46 | |
47 | =head1 SYNOPSIS |
48 | |
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49 | use mro; # enables next::method and friends globally |
50 | |
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51 | use mro 'dfs'; # enable DFS MRO for this class (Perl default) |
52 | use mro 'c3'; # enable C3 MRO for this class |
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53 | |
54 | =head1 DESCRIPTION |
55 | |
56 | The "mro" namespace provides several utilities for dealing |
57 | with method resolution order and method caching in general. |
58 | |
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59 | These interfaces are only available in Perl 5.9.5 and higher. |
60 | See L<MRO::Compat> on CPAN for a mostly forwards compatible |
61 | implementation for older Perls. |
62 | |
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63 | =head1 OVERVIEW |
64 | |
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65 | It's possible to change the MRO of a given class either by using C<use |
66 | mro> as shown in the synopsis, or by using the L</mro::set_mro> function |
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67 | below. The functions in the mro namespace do not require loading the |
68 | C<mro> module, as they are actually provided by the core perl interpreter. |
69 | |
70 | The special methods C<next::method>, C<next::can>, and |
71 | C<maybe::next::method> are not available until this C<mro> module |
72 | has been loaded via C<use> or C<require>. |
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73 | |
74 | =head1 The C3 MRO |
75 | |
76 | In addition to the traditional Perl default MRO (depth first |
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77 | search, called C<DFS> here), Perl now offers the C3 MRO as |
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78 | well. Perl's support for C3 is based on the work done in |
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79 | Stevan Little's module L<Class::C3>, and most of the C3-related |
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80 | documentation here is ripped directly from there. |
81 | |
82 | =head2 What is C3? |
83 | |
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84 | C3 is the name of an algorithm which aims to provide a sane method |
85 | resolution order under multiple inheritance. It was first introduced in |
86 | the language Dylan (see links in the L</"SEE ALSO"> section), and then |
87 | later adopted as the preferred MRO (Method Resolution Order) for the |
88 | new-style classes in Python 2.3. Most recently it has been adopted as the |
89 | "canonical" MRO for Perl 6 classes, and the default MRO for Parrot objects |
90 | as well. |
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91 | |
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92 | =head2 How does C3 work |
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93 | |
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94 | C3 works by always preserving local precendence ordering. This essentially |
95 | means that no class will appear before any of its subclasses. Take, for |
96 | instance, the classic diamond inheritance pattern: |
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97 | |
98 | <A> |
99 | / \ |
100 | <B> <C> |
101 | \ / |
102 | <D> |
103 | |
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104 | The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> |
105 | appears before B<C>, even though B<C> is the subclass of B<A>. The C3 MRO |
106 | algorithm however, produces the following order: (D, B, C, A), which does |
107 | not have this issue. |
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108 | |
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109 | This example is fairly trivial; for more complex cases and a deeper |
110 | explanation, see the links in the L</"SEE ALSO"> section. |
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111 | |
112 | =head1 Functions |
113 | |
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114 | =head2 mro::get_linear_isa($classname[, $type]) |
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115 | |
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116 | Returns an arrayref which is the linearized MRO of the given class. |
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117 | Uses whichever MRO is currently in effect for that class by default, |
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118 | or the given MRO (either C<c3> or C<dfs> if specified as C<$type>). |
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119 | |
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120 | The linearized MRO of a class is an ordered array of all of the |
121 | classes one would search when resolving a method on that class, |
122 | starting with the class itself. |
123 | |
124 | If the requested class doesn't yet exist, this function will still |
125 | succeed, and return C<[ $classname ]> |
126 | |
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127 | Note that C<UNIVERSAL> (and any members of C<UNIVERSAL>'s MRO) are not |
128 | part of the MRO of a class, even though all classes implicitly inherit |
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129 | methods from C<UNIVERSAL> and its parents. |
130 | |
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131 | =head2 mro::set_mro($classname, $type) |
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132 | |
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133 | Sets the MRO of the given class to the C<$type> argument (either |
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134 | C<c3> or C<dfs>). |
135 | |
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136 | =head2 mro::get_mro($classname) |
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137 | |
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138 | Returns the MRO of the given class (either C<c3> or C<dfs>). |
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139 | |
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140 | =head2 mro::get_isarev($classname) |
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141 | |
142 | Gets the C<mro_isarev> for this class, returned as an |
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143 | arrayref of class names. These are every class that "isa" |
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144 | the given class name, even if the isa relationship is |
145 | indirect. This is used internally by the MRO code to |
146 | keep track of method/MRO cache invalidations. |
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147 | |
148 | Currently, this list only grows, it never shrinks. This |
149 | was a performance consideration (properly tracking and |
150 | deleting isarev entries when someone removes an entry |
151 | from an C<@ISA> is costly, and it doesn't happen often |
152 | anyways). The fact that a class which no longer truly |
153 | "isa" this class at runtime remains on the list should be |
154 | considered a quirky implementation detail which is subject |
155 | to future change. It shouldn't be an issue as long as |
156 | you're looking at this list for the same reasons the |
157 | core code does: as a performance optimization |
158 | over having to search every class in existence. |
159 | |
160 | As with C<mro::get_mro> above, C<UNIVERSAL> is special. |
161 | C<UNIVERSAL> (and parents') isarev lists do not include |
162 | every class in existence, even though all classes are |
163 | effectively descendants for method inheritance purposes. |
164 | |
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165 | =head2 mro::is_universal($classname) |
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166 | |
167 | Returns a boolean status indicating whether or not |
168 | the given classname is either C<UNIVERSAL> itself, |
169 | or one of C<UNIVERSAL>'s parents by C<@ISA> inheritance. |
170 | |
171 | Any class for which this function returns true is |
172 | "universal" in the sense that all classes potentially |
173 | inherit methods from it. |
174 | |
175 | For similar reasons to C<isarev> above, this flag is |
176 | permanent. Once it is set, it does not go away, even |
177 | if the class in question really isn't universal anymore. |
178 | |
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179 | =head2 mro::invalidate_all_method_caches() |
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180 | |
181 | Increments C<PL_sub_generation>, which invalidates method |
182 | caching in all packages. |
183 | |
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184 | =head2 mro::method_changed_in($classname) |
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185 | |
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186 | Invalidates the method cache of any classes dependent on the |
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187 | given class. This is not normally necessary. The only |
188 | known case where pure perl code can confuse the method |
189 | cache is when you manually install a new constant |
190 | subroutine by using a readonly scalar value, like the |
191 | internals of L<constant> do. If you find another case, |
192 | please report it so we can either fix it or document |
193 | the exception here. |
194 | |
195 | =head2 mro::get_pkg_gen($classname) |
196 | |
197 | Returns an integer which is incremented every time a |
198 | real local method in the package C<$classname> changes, |
199 | or the local C<@ISA> of C<$classname> is modified. |
200 | |
201 | This is intended for authors of modules which do lots |
202 | of class introspection, as it allows them to very quickly |
203 | check if anything important about the local properties |
204 | of a given class have changed since the last time they |
205 | looked. It does not increment on method/C<@ISA> |
206 | changes in superclasses. |
207 | |
208 | It's still up to you to seek out the actual changes, |
209 | and there might not actually be any. Perhaps all |
210 | of the changes since you last checked cancelled each |
211 | other out and left the package in the state it was in |
212 | before. |
213 | |
214 | This integer normally starts off at a value of C<1> |
215 | when a package stash is instantiated. Calling it |
216 | on packages whose stashes do not exist at all will |
217 | return C<0>. If a package stash is completely |
218 | deleted (not a normal occurence, but it can happen |
219 | if someone does something like C<undef %PkgName::>), |
220 | the number will be reset to either C<0> or C<1>, |
221 | depending on how completely package was wiped out. |
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222 | |
223 | =head2 next::method |
224 | |
225 | This is somewhat like C<SUPER>, but it uses the C3 method |
226 | resolution order to get better consistency in multiple |
227 | inheritance situations. Note that while inheritance in |
228 | general follows whichever MRO is in effect for the |
229 | given class, C<next::method> only uses the C3 MRO. |
230 | |
231 | One generally uses it like so: |
232 | |
233 | sub some_method { |
234 | my $self = shift; |
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235 | my $superclass_answer = $self->next::method(@_); |
236 | return $superclass_answer + 1; |
237 | } |
238 | |
239 | Note that you don't (re-)specify the method name. |
240 | It forces you to always use the same method name |
241 | as the method you started in. |
242 | |
243 | It can be called on an object or a class, of course. |
244 | |
245 | The way it resolves which actual method to call is: |
246 | |
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247 | =over 4 |
248 | |
249 | =item 1 |
250 | |
251 | First, it determines the linearized C3 MRO of |
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252 | the object or class it is being called on. |
253 | |
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254 | =item 2 |
255 | |
256 | Then, it determines the class and method name |
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257 | of the context it was invoked from. |
258 | |
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259 | =item 3 |
260 | |
261 | Finally, it searches down the C3 MRO list until |
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262 | it reaches the contextually enclosing class, then |
263 | searches further down the MRO list for the next |
264 | method with the same name as the contextually |
265 | enclosing method. |
266 | |
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267 | =back |
268 | |
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269 | Failure to find a next method will result in an |
270 | exception being thrown (see below for alternatives). |
271 | |
272 | This is substantially different than the behavior |
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273 | of C<SUPER> under complex multiple inheritance. |
274 | (This becomes obvious when one realizes that the |
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275 | common superclasses in the C3 linearizations of |
276 | a given class and one of its parents will not |
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277 | always be ordered the same for both.) |
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278 | |
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279 | B<Caveat>: Calling C<next::method> from methods defined outside the class: |
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280 | |
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281 | There is an edge case when using C<next::method> from within a subroutine |
282 | which was created in a different module than the one it is called from. It |
283 | sounds complicated, but it really isn't. Here is an example which will not |
284 | work correctly: |
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285 | |
286 | *Foo::foo = sub { (shift)->next::method(@_) }; |
287 | |
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288 | The problem exists because the anonymous subroutine being assigned to the |
289 | C<*Foo::foo> glob will show up in the call stack as being called |
290 | C<__ANON__> and not C<foo> as you might expect. Since C<next::method> uses |
291 | C<caller> to find the name of the method it was called in, it will fail in |
292 | this case. |
293 | |
294 | But fear not, there's a simple solution. The module C<Sub::Name> will |
295 | reach into the perl internals and assign a name to an anonymous subroutine |
296 | for you. Simply do this: |
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297 | |
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298 | use Sub::Name 'subname'; |
299 | *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) }; |
300 | |
301 | and things will Just Work. |
302 | |
303 | =head2 next::can |
304 | |
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305 | This is similar to C<next::method>, but just returns either a code |
306 | reference or C<undef> to indicate that no further methods of this name |
307 | exist. |
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308 | |
309 | =head2 maybe::next::method |
310 | |
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311 | In simple cases, it is equivalent to: |
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312 | |
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313 | $self->next::method(@_) if $self->next::can; |
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314 | |
315 | But there are some cases where only this solution |
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316 | works (like C<goto &maybe::next::method>); |
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317 | |
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318 | =head1 SEE ALSO |
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319 | |
320 | =head2 The original Dylan paper |
321 | |
322 | =over 4 |
323 | |
324 | =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html> |
325 | |
326 | =back |
327 | |
328 | =head2 The prototype Perl 6 Object Model uses C3 |
329 | |
330 | =over 4 |
331 | |
332 | =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/> |
333 | |
334 | =back |
335 | |
336 | =head2 Parrot now uses C3 |
337 | |
338 | =over 4 |
339 | |
340 | =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631> |
341 | |
342 | =item L<http://use.perl.org/~autrijus/journal/25768> |
343 | |
344 | =back |
345 | |
346 | =head2 Python 2.3 MRO related links |
347 | |
348 | =over 4 |
349 | |
350 | =item L<http://www.python.org/2.3/mro.html> |
351 | |
352 | =item L<http://www.python.org/2.2.2/descrintro.html#mro> |
353 | |
354 | =back |
355 | |
356 | =head2 C3 for TinyCLOS |
357 | |
358 | =over 4 |
359 | |
360 | =item L<http://www.call-with-current-continuation.org/eggs/c3.html> |
361 | |
362 | =back |
363 | |
364 | =head2 Class::C3 |
365 | |
366 | =over 4 |
367 | |
368 | =item L<Class::C3> |
369 | |
370 | =back |
371 | |
372 | =head1 AUTHOR |
373 | |
374 | Brandon L. Black, E<lt>blblack@gmail.comE<gt> |
375 | |
376 | Based on Stevan Little's L<Class::C3> |
377 | |
378 | =cut |