stop the redefine warnings?

[gitmo/Class-C3.git] / README

NAME
    Class::C3 - A pragma to use the C3 method resolution order algortihm

SYNOPSIS
        package A;
        use Class::C3;     
        sub hello { 'A::hello' }

        package B;
        use base 'A';
        use Class::C3;     

        package C;
        use base 'A';
        use Class::C3;     

        sub hello { 'C::hello' }

        package D;
        use base ('B', 'C');
        use Class::C3;    

        # Classic Diamond MI pattern
        #    <A>
        #   /   \
        # <B>   <C>
        #   \   /
        #    <D>

        package main;
        
    # initializez the C3 module 
        # (formerly called in INIT)
        Class::C3::initialize();  

        print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A

        print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
        
    D->can('hello')->();          # can() also works correctly
        UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()

DESCRIPTION
    This is pragma to change Perl 5's standard method resolution order from
    depth-first left-to-right (a.k.a - pre-order) to the more sophisticated
    C3 method resolution order.

  What is C3?
    C3 is the name of an algorithm which aims to provide a sane method
    resolution order under multiple inheritence. It was first introduced in
    the langauge Dylan (see links in the "SEE ALSO" section), and then later
    adopted as the prefered 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 precendence ordering. This
    essentially means that no class will appear before any of it's
    subclasses. Take the classic diamond inheritence pattern for instance:

         <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 MRO (D, B, C, A), which does
    not have this same issue.

    This example is fairly trival, for more complex examples and a deeper
    explaination, see the links in the "SEE ALSO" section.

  How does this module work?
    This module uses a technique similar to Perl 5's method caching. When
    "Class::C3::initialize" is called, this module calculates the MRO of all
    the classes which called "use Class::C3". It then gathers information
    from the symbol tables of each of those classes, and builds a set of
    method aliases for the correct dispatch ordering. Once all these
    C3-based method tables are created, it then adds the method aliases into
    the local classes symbol table.

    The end result is actually classes with pre-cached method dispatch.
    However, this caching does not do well if you start changing your @ISA
    or messing with class symbol tables, so you should consider your classes
    to be effectively closed. See the CAVEATS section for more details.

OPTIONAL LOWERCASE PRAGMA
    This release also includes an optional module c3 in the opt/ folder. I
    did not include this in the regular install since lowercase module names
    are considered *"bad"* by some people. However I think that code looks
    much nicer like this:

      package MyClass;
      use c3;

    The the more clunky:

      package MyClass;
      use Class::C3;

    But hey, it's your choice, thats why it is optional.

FUNCTIONS
    calculateMRO ($class)
        Given a $class this will return an array of class names in the
        proper C3 method resolution order.

    initialize
        This must be called to initalize the C3 method dispatch tables, this
        module will not work if you do not do this. It is advised to do this
        as soon as possible after loading any classes which use C3. Here is
        a quick code example:

          package Foo;
          use Class::C3;
          # ... Foo methods here
          
  package Bar;
          use Class::C3;
          use base 'Foo';
          # ... Bar methods here
          
  package main;
          
  Class::C3::initialize(); # now it is safe to use Foo and Bar

        This function used to be called automatically for you in the INIT
        phase of the perl compiler, but that lead to warnings if this module
        was required at runtime. After discussion with my user base (the
        DBIx::Class folks), we decided that calling this in INIT was more of
        an annoyance than a convience. I apologize to anyone this causes
        problems for (although i would very suprised if I had any other
        users other than the DBIx::Class folks). The simplest solution of
        course is to define your own INIT method which calls this function.

        NOTE:

        If "initialize" detects that "initialize" has already been executed,
        it will "uninitialize" and clear the MRO cache first.

    uninitialize
        Calling this function results in the removal of all cached methods,
        and the restoration of the old Perl 5 style dispatch order
        (depth-first, left-to-right).

    reinitialize
        This is an alias for "initialize" above.

METHOD REDISPATCHING
    It is always useful to be able to re-dispatch your method call to the
    "next most applicable method". This module provides a pseudo package
    along the lines of "SUPER::" or "NEXT::" which will re-dispatch the
    method along the C3 linearization. This is best show with an examples.

      # a classic diamond MI pattern ...
         <A>
        /   \
      <B>   <C>
        \   /
         <D>
      
  package A;
      use c3; 
      sub foo { 'A::foo' }       
     
  package B;
      use base 'A'; 
      use c3;     
      sub foo { 'B::foo => ' . (shift)->next::method() }       
     
  package B;
      use base 'A'; 
      use c3;    
      sub foo { 'C::foo => ' . (shift)->next::method() }   
     
  package D;
      use base ('B', 'C'); 
      use c3; 
      sub foo { 'D::foo => ' . (shift)->next::method() }   
      
  print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"

    A few things to note. First, we do not require you to add on the method
    name to the "next::method" call (this is unlike "NEXT::" and "SUPER::"
    which do require that). This helps to enforce the rule that you cannot
    dispatch to a method of a different name (this is how "NEXT::" behaves
    as well).

    The next thing to keep in mind is that you will need to pass all
    arguments to "next::method" it can not automatically use the current @_.

    If "next::method" cannot find a next method to re-dispatch the call to,
    it will throw an exception. You can use "next::can" to see if
    "next::method" will succeed before you call it like so:

      $self->next::method(@_) if $self->next::can;

    Additionally, you can use "maybe::next::method" as a shortcut to only
    call the next method if it exists. The previous example could be simply
    written as:

      $self->maybe::next::method(@_);

    There are some caveats about using "next::method", see below for those.

CAVEATS
    This module used to be labeled as *experimental*, however it has now
    been pretty heavily tested by the good folks over at DBIx::Class and I
    am confident this module is perfectly usable for whatever your needs
    might be.

    But there are still caveats, so here goes ...

    Use of "SUPER::".
        The idea of "SUPER::" under multiple inheritence is ambigious, and
        generally not recomended anyway. However, it's use in conjuntion
        with this module is very much not recommended, and in fact very
        discouraged. The recommended approach is to instead use the supplied
        "next::method" feature, see more details on it's usage above.

    Changing @ISA.
        It is the author's opinion that changing @ISA at runtime is pure
        insanity anyway. However, people do it, so I must caveat. Any
        changes to the @ISA will not be reflected in the MRO calculated by
        this module, and therefor probably won't even show up. If you do
        this, you will need to call "reinitialize" in order to recalulate
        all method dispatch tables. See the "reinitialize" documentation and
        an example in t/20_reinitialize.t for more information.

    Adding/deleting methods from class symbol tables.
        This module calculates the MRO for each requested class by
        interogatting the symbol tables of said classes. So any symbol table
        manipulation which takes place after our INIT phase is run will not
        be reflected in the calculated MRO. Just as with changing the @ISA,
        you will need to call "reinitialize" for any changes you make to
        take effect.

    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 glob *Foo::foo 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 is 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. Of course this is not always possible to
        do, but to be honest, I just can't manage to find a workaround for
        it, so until someone gives me a working patch this will be a known
        limitation of this module.

COMPATIBILITY
    If your software requires Perl 5.9.5 or higher, you do not need
    Class::C3, you can simply "use mro 'c3'", and not worry about
    "initialize()", avoid some of the above caveats, and get the best
    possible performance. See mro for more details.

    If your software is meant to work on earlier Perls, use Class::C3 as
    documented here. Class::C3 will detect Perl 5.9.5+ and take advantage of
    the core support when available.

Class::C3::XS
    This module will load Class::C3::XS if it's installed and you are
    running on a Perl version older than 5.9.5. Installing this is
    recommended when possible, as it results in significant performance
    improvements (but unlike the 5.9.5+ core support, it still has all of
    the same caveats as Class::C3).

CODE COVERAGE
    Devel::Cover was reporting 94.4% overall test coverage earlier in this
    module's life. Currently, the test suite does things that break under
    coverage testing, but it is fair to assume the coverage is still close
    to that value.

SEE ALSO
  The original Dylan paper
    <http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>

  The prototype Perl 6 Object Model uses C3
    <http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>

  Parrot now uses C3
    <http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
    <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>

  C3 for TinyCLOS
    <http://www.call-with-current-continuation.org/eggs/c3.html>

ACKNOWLEGEMENTS
    Thanks to Matt S. Trout for using this module in his module DBIx::Class
    and finding many bugs and providing fixes.
    Thanks to Justin Guenther for making "next::method" more robust by
    handling calls inside "eval" and anon-subs.
    Thanks to Robert Norris for adding support for "next::can" and
    "maybe::next::method".

AUTHOR
    Stevan Little, <stevan@iinteractive.com>

    Brandon L. Black, <blblack@gmail.com>

COPYRIGHT AND LICENSE
    Copyright 2005, 2006 by Infinity Interactive, Inc.

    <http://www.iinteractive.com>

    This library is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.