mro.3

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.\" ========================================================================
.\"
.IX Title "mro 3"
.TH mro 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
mro \- Method Resolution Order
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  use mro; # enables next::method and friends globally
\&
\&  use mro \*(Aqdfs\*(Aq; # enable DFS MRO for this class (Perl default)
\&  use mro \*(Aqc3\*(Aq; # enable C3 MRO for this class
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \*(L"mro\*(R" namespace provides several utilities for dealing
with method resolution order and method caching in general.
.PP
These interfaces are only available in Perl 5.9.5 and higher.
See MRO::Compat on \s-1CPAN\s0 for a mostly forwards compatible
implementation for older Perls.
.SH "OVERVIEW"
.IX Header "OVERVIEW"
It's possible to change the \s-1MRO\s0 of a given class either by using \f(CW\*(C`use
mro\*(C'\fR as shown in the synopsis, or by using the \*(L"mro::set_mro\*(R" function
below.  The functions in the mro namespace do not require loading the
\&\f(CW\*(C`mro\*(C'\fR module, as they are actually provided by the core perl interpreter.
.PP
The special methods \f(CW\*(C`next::method\*(C'\fR, \f(CW\*(C`next::can\*(C'\fR, and
\&\f(CW\*(C`maybe::next::method\*(C'\fR are not available until this \f(CW\*(C`mro\*(C'\fR module
has been loaded via \f(CW\*(C`use\*(C'\fR or \f(CW\*(C`require\*(C'\fR.
.SH "The C3 MRO"
.IX Header "The C3 MRO"
In addition to the traditional Perl default \s-1MRO\s0 (depth first
search, called \f(CW\*(C`DFS\*(C'\fR here), Perl now offers the C3 \s-1MRO\s0 as
well.  Perl's support for C3 is based on the work done in
Stevan Little's module Class::C3, and most of the C3\-related
documentation here is ripped directly from there.
.Sh "What is C3?"
.IX Subsection "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"\s-1SEE\s0 \s-1ALSO\s0\*(R" section), and then
later adopted as the preferred \s-1MRO\s0 (Method Resolution Order) for the
new-style classes in Python 2.3. Most recently it has been adopted as the
\&\*(L"canonical\*(R" \s-1MRO\s0 for Perl 6 classes, and the default \s-1MRO\s0 for Parrot objects
as well.
.Sh "How does C3 work"
.IX Subsection "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:
.PP
.Vb 5
\&     <A>
\&    /   \e
\&  <B>   <C>
\&    \e   /
\&     <D>
.Ve
.PP
The standard Perl 5 \s-1MRO\s0 would be (D, B, A, C). The result being that \fBA\fR
appears before \fBC\fR, even though \fBC\fR is the subclass of \fBA\fR. The C3 \s-1MRO\s0
algorithm however, produces the following order: (D, B, C, A), which does
not have this issue.
.PP
This example is fairly trivial; for more complex cases and a deeper
explanation, see the links in the \*(L"\s-1SEE\s0 \s-1ALSO\s0\*(R" section.
.SH "Functions"
.IX Header "Functions"
.ie n .Sh "mro::get_linear_isa($classname[, $type])"
.el .Sh "mro::get_linear_isa($classname[, \f(CW$type\fP])"
.IX Subsection "mro::get_linear_isa($classname[, $type])"
Returns an arrayref which is the linearized \s-1MRO\s0 of the given class.
Uses whichever \s-1MRO\s0 is currently in effect for that class by default,
or the given \s-1MRO\s0 (either \f(CW\*(C`c3\*(C'\fR or \f(CW\*(C`dfs\*(C'\fR if specified as \f(CW$type\fR).
.PP
The linearized \s-1MRO\s0 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.
.PP
If the requested class doesn't yet exist, this function will still
succeed, and return \f(CW\*(C`[ $classname ]\*(C'\fR
.PP
Note that \f(CW\*(C`UNIVERSAL\*(C'\fR (and any members of \f(CW\*(C`UNIVERSAL\*(C'\fR's \s-1MRO\s0) are not
part of the \s-1MRO\s0 of a class, even though all classes implicitly inherit