mro(3perl) Perl Programmers Reference Guide mro(3perl)
NAME
mro - Method Resolution Order
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
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
MRO::Compat on CPAN for a mostly forwards compatible implementation for
older Perls.
OVERVIEW
It's possible to change the MRO of a given class either by using "use
mro" as shown in the synopsis, or by using the "mro::set_mro" function
below.
The special methods "next::method", "next::can", and
"maybe::next::method" are not available until this "mro" module has
been loaded via "use" or "require".
The C3 MRO
In addition to the traditional Perl default MRO (depth first search,
called "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 Class::C3,
and most of the C3-related documentation here is ripped directly from
there.
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 "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.
How does C3 work
C3 works by always preserving local precedence 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 A
appears before C, even though C is the subclass of 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 "SEE ALSO" section.
Functions
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 "c3" or "dfs" if specified as $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 "[ $classname ]"
Note that "UNIVERSAL" (and any members of "UNIVERSAL"'s MRO) are not
part of the MRO of a class, even though all classes implicitly inherit
methods from "UNIVERSAL" and its parents.
mro::set_mro ($classname, $type)
Sets the MRO of the given class to the $type argument (either "c3" or
"dfs").
mro::get_mro($classname)
Returns the MRO of the given class (either "c3" or "dfs").
mro::get_isarev($classname)
Gets the "mro_isarev" for this class, returned as an arrayref of class
names. These are every class that "isa" the given class name, even if
the isa relationship is indirect. This is used internally by the MRO
code to keep track of method/MRO cache invalidations.
As with "mro::get_linear_isa" above, "UNIVERSAL" is special.
"UNIVERSAL" (and parents') isarev lists do not include every class in
existence, even though all classes are effectively descendants for
method inheritance purposes.
mro::is_universal($classname)
Returns a boolean status indicating whether or not the given classname
is either "UNIVERSAL" itself, or one of "UNIVERSAL"'s parents by @ISA
inheritance.
Any class for which this function returns true is "universal" in the
sense that all classes potentially inherit methods from it.
mro::invalidate_all_method_caches()
Increments "PL_sub_generation", which invalidates method caching in all
packages.
mro::method_changed_in($classname)
Invalidates the method cache of any classes dependent on the given
class. This is not normally necessary. The only known case where pure
perl code can confuse the method cache is when you manually install a
new constant subroutine by using a readonly scalar value, like the
internals of constant do. If you find another case, please report it
so we can either fix it or document the exception here.
mro::get_pkg_gen($classname)
Returns an integer which is incremented every time a real local method
in the package $classname changes, or the local @ISA of $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/@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 1 when a package stash
is instantiated. Calling it on packages whose stashes do not exist at
all will return 0. If a package stash is completely deleted (not a
normal occurrence, but it can happen if someone does something like
"undef %PkgName::"), the number will be reset to either 0 or 1,
depending on how completely the package was wiped out.
next::method
This is somewhat like "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, "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:
1. First, it determines the linearized C3 MRO of the object or class
it is being called on.
2. Then, it determines the class and method name of the context it was
invoked from.
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.
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 "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.)
Caveat: Calling "next::method" from methods defined outside the class:
There is an edge case when using "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 *Foo::foo glob will show up in the call stack as being called
"__ANON__" and not "foo" as you might expect. Since "next::method" uses
"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 "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.
next::can
This is similar to "next::method", but just returns either a code
reference or "undef" to indicate that no further methods of this name
exist.
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 "goto
&maybe::next::method");
SEE ALSO
The original Dylan paper
<http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.19.3910&rep=rep1&type=pdf>
Pugs
The Pugs prototype Perl 6 Object Model uses C3
Parrot
Parrot now uses C3
<http://use.perl.org/~autrijus/journal/25768>
Python 2.3 MRO related links
<http://www.python.org/2.3/mro.html>
<http://www.python.org/2.2.2/descrintro.html#mro>
Class::C3
Class::C3
AUTHOR
Brandon L. Black, <blblack@gmail.com>
Based on Stevan Little's Class::C3
perl v5.30.3 2020-06-07 mro(3perl)