NEXT.pm - Provide a pseudo-class NEXT that allows method redispatch

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If a method <code>m</code> calls <code>$self-</code>NEXT::m()>, the call to <code>m</code> is redispatched as if the calling method had not originally been found.</p> <p>In other words, a call to <code>$self-</code>NEXT::m()> resumes the depth-first, left-to-right search of <code>$self</code>'s class hierarchy that resulted in the original call to <code>m</code>.</p> <p>Note that this is not the same thing as <code>$self-</code>SUPER::m()>, which begins a new dispatch that is restricted to searching the ancestors of the current class. <code>$self-</code>NEXT::m()> can backtrack past the current class -- to look for a suitable method in other ancestors of <code>$self</code> -- whereas <code>$self-</code>SUPER::m()> cannot.</p> <p>A typical use would be in the destructors of a class hierarchy, as illustrated in the synopsis above. Each class in the hierarchy has a DESTROY method that performs some class-specific action and then redispatches the call up the hierarchy. As a result, when an object of class D is destroyed, the destructors of <i>all</i> its parent classes are called (in depth-first, left-to-right order).</p> <p>Another typical use of redispatch would be in <code>AUTOLOAD</code>'ed methods. If such a method determined that it was not able to handle a particular call, it might choose to redispatch that call, in the hope that some other <code>AUTOLOAD</code> (above it, or to its left) might do better.</p> <p>By default, if a redispatch attempt fails to find another method elsewhere in the objects class hierarchy, it quietly gives up and does nothing (but see <a href="#Enforcing-redispatch">"Enforcing redispatch"</a>). This gracious acquiesence is also unlike the (generally annoying) behaviour of <code>SUPER</code>, which throws an exception if it cannot redispatch.</p> <p>Note that it is a fatal error for any method (including <code>AUTOLOAD</code>) to attempt to redispatch any method that does not have the same name. For example:</p> <pre><code>sub D::oops { print "oops!\n"; $_[0]->NEXT::other_method() }</code></pre> <h2 id="Enforcing-redispatch"><a class="permalink" href="#Enforcing-redispatch">#</a><a id="Enforcing"></a>Enforcing redispatch</h2> <p>It is possible to make <code>NEXT</code> redispatch more demandingly (i.e. like <code>SUPER</code> does), so that the redispatch throws an exception if it cannot find a "next" method to call.</p> <p>To do this, simple invoke the redispatch as:</p> <pre><code>$self->NEXT::ACTUAL::method();</code></pre> <p>rather than:</p> <pre><code>$self->NEXT::method();</code></pre> <p>The <code>ACTUAL</code> tells <code>NEXT</code> that there must actually be a next method to call, or it should throw an exception.</p> <p><code>NEXT::ACTUAL</code> is most commonly used in <code>AUTOLOAD</code> methods, as a means to decline an <code>AUTOLOAD</code> request, but preserve the normal exception-on-failure semantics:</p> <pre><code>sub AUTOLOAD { if ($AUTOLOAD =~ /foo|bar/) { # handle here } else { # try elsewhere shift()->NEXT::ACTUAL::AUTOLOAD(@_); } }</code></pre> <p>By using <code>NEXT::ACTUAL</code>, if there is no other <code>AUTOLOAD</code> to handle the method call, an exception will be thrown (as usually happens in the absence of a suitable <code>AUTOLOAD</code>).</p> <h2 id="Avoiding-repetitions"><a class="permalink" href="#Avoiding-repetitions">#</a><a id="Avoiding"></a>Avoiding repetitions</h2> <p>If <code>NEXT</code> redispatching is used in the methods of a "diamond" class hierarchy:</p> <pre><code># A B # / \ / # C D # \ / # E use NEXT; package A; sub foo { print "called A::foo\n"; shift->NEXT::foo() } package B; sub foo { print "called B::foo\n"; shift->NEXT::foo() } package C; @ISA = qw( A ); sub foo { print "called C::foo\n"; shift->NEXT::foo() } package D; @ISA = qw(A B); sub foo { print "called D::foo\n"; shift->NEXT::foo() } package E; @ISA = qw(C D); sub foo { print "called E::foo\n"; shift->NEXT::foo() } E->foo();</code></pre> <p>then derived classes may (re-)inherit base-class methods through two or more distinct paths (e.g. in the way <code>E</code> inherits <code>A::foo</code> twice -- through <code>C</code> and <code>D</code>). In such cases, a sequence of <code>NEXT</code> redispatches will invoke the multiply inherited method as many times as it is inherited. For example, the above code prints:</p> <pre><code class="plaintext">called E::foo called C::foo called A::foo called D::foo called A::foo called B::foo</code></pre> <p>(i.e. <code>A::foo</code> is called twice).</p> <p>In some cases this <i>may</i> be the desired effect within a diamond hierarchy, but in others (e.g. for destructors) it may be more appropriate to call each method only once during a sequence of redispatches.</p> <p>To cover such cases, you can redispatch methods via:</p> <pre><code>$self->NEXT::UNSEEN::method();</code></pre> <p>rather than:</p> <pre><code>$self->NEXT::method();</code></pre> <p>This causes the redispatcher to skip any classes in the hierarchy that it has already visited in an earlier redispatch. So, for example, if the previous example were rewritten:</p> <pre><code>package A; sub foo { print "called A::foo\n"; shift->NEXT::UNSEEN::foo() } package B; sub foo { print "called B::foo\n"; shift->NEXT::UNSEEN::foo() } package C; @ISA = qw( A ); sub foo { print "called C::foo\n"; shift->NEXT::UNSEEN::foo() } package D; @ISA = qw(A B); sub foo { print "called D::foo\n"; shift->NEXT::UNSEEN::foo() } package E; @ISA = qw(C D); sub foo { print "called E::foo\n"; shift->NEXT::UNSEEN::foo() } E->foo();</code></pre> <p>then it would print:</p> <pre><code class="plaintext">called E::foo called C::foo called A::foo called D::foo called B::foo</code></pre> <p>and omit the second call to <code>A::foo</code>.</p> <p>Note that you can also use:</p> <pre><code>$self->NEXT::UNSEEN::ACTUAL::method();</code></pre> <p>or:</p> <pre><code>$self->NEXT::ACTUAL::UNSEEN::method();</code></pre> <p>to get both unique invocation <i>and</i> exception-on-failure.</p> <h1 id="AUTHOR"><a class="permalink" href="#AUTHOR">#</a>AUTHOR</h1> <p>Damian Conway (damian@conway.org)</p> <h1 id="BUGS-AND-IRRITATIONS"><a class="permalink" href="#BUGS-AND-IRRITATIONS">#</a><a id="BUGS"></a>BUGS AND IRRITATIONS</h1> <p>Because it's a module, not an integral part of the interpreter, NEXT.pm has to guess where the surrounding call was found in the method look-up sequence. In the presence of diamond inheritance patterns it occasionally guesses wrong.</p> <p>It's also too slow (despite caching).</p> <p>Comment, suggestions, and patches welcome.</p> <h1 id="COPYRIGHT"><a class="permalink" href="#COPYRIGHT">#</a>COPYRIGHT</h1> <pre><code class="plaintext">Copyright (c) 2000-2001, Damian Conway. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself.</code></pre> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p> <p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the <a href="https://github.com/Perl/perl5/issues">Perl issue tracker</a>, the <a href="https://lists.perl.org/list/perl5-porters.html">mailing list</a>, or <a href="https://kiwiirc.com/client/irc.perl.org/p5p">IRC</a> to report any issues with the contents or format of the documentation.</p> </div> </div> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.slim.min.js" integrity="sha512-/DXTXr6nQodMUiq+IUJYCt2PPOUjrHJ9wFrqpJ3XkgPNOZVfMok7cRw6CSxyCQxXn6ozlESsSh1/sMCTF1rL/g==" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script> <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js" integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV" crossorigin="anonymous"></script> <script src="/js/highlight.pack.js"></script> <script>hljs.highlightAll();</script> </body> </html>

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NEXT.pm - Provide a pseudo-class NEXT that allows method redispatch - Perldoc Browser