source: trunk/essentials/dev-lang/perl/lib/Class/ISA.pm@ 3184

#!/usr/local/bin/perl
# Time-stamp: "2004-12-29 20:01:02 AST" -*-Perl-*-

package Class::ISA;
require 5;
use strict;
use vars qw($Debug $VERSION);
$VERSION = '0.33';
$Debug = 0 unless defined $Debug;

=head1 NAME

Class::ISA -- report the search path for a class's ISA tree

=head1 SYNOPSIS

  # Suppose you go: use Food::Fishstick, and that uses and
  # inherits from other things, which in turn use and inherit
  # from other things.  And suppose, for sake of brevity of
  # example, that their ISA tree is the same as:

  @Food::Fishstick::ISA = qw(Food::Fish  Life::Fungus  Chemicals);
  @Food::Fish::ISA = qw(Food);
  @Food::ISA = qw(Matter);
  @Life::Fungus::ISA = qw(Life);
  @Chemicals::ISA = qw(Matter);
  @Life::ISA = qw(Matter);
  @Matter::ISA = qw();

  use Class::ISA;
  print "Food::Fishstick path is:\n ",
        join(", ", Class::ISA::super_path('Food::Fishstick')),
        "\n";

That prints:

  Food::Fishstick path is:
   Food::Fish, Food, Matter, Life::Fungus, Life, Chemicals

=head1 DESCRIPTION

Suppose you have a class (like Food::Fish::Fishstick) that is derived,
via its @ISA, from one or more superclasses (as Food::Fish::Fishstick
is from Food::Fish, Life::Fungus, and Chemicals), and some of those
superclasses may themselves each be derived, via its @ISA, from one or
more superclasses (as above).

When, then, you call a method in that class ($fishstick->calories),
Perl first searches there for that method, but if it's not there, it
goes searching in its superclasses, and so on, in a depth-first (or
maybe "height-first" is the word) search.  In the above example, it'd
first look in Food::Fish, then Food, then Matter, then Life::Fungus,
then Life, then Chemicals.

This library, Class::ISA, provides functions that return that list --
the list (in order) of names of classes Perl would search to find a
method, with no duplicates.

=head1 FUNCTIONS

=over

=item the function Class::ISA::super_path($CLASS)

This returns the ordered list of names of classes that Perl would
search thru in order to find a method, with no duplicates in the list.
$CLASS is not included in the list.  UNIVERSAL is not included -- if
you need to consider it, add it to the end.


=item the function Class::ISA::self_and_super_path($CLASS)

Just like C<super_path>, except that $CLASS is included as the first
element.

=item the function Class::ISA::self_and_super_versions($CLASS)

This returns a hash whose keys are $CLASS and its
(super-)superclasses, and whose values are the contents of each
class's $VERSION (or undef, for classes with no $VERSION).

The code for self_and_super_versions is meant to serve as an example
for precisely the kind of tasks I anticipate that self_and_super_path
and super_path will be used for.  You are strongly advised to read the
source for self_and_super_versions, and the comments there.

=back

=head1 CAUTIONARY NOTES

* Class::ISA doesn't export anything.  You have to address the
functions with a "Class::ISA::" on the front.

* Contrary to its name, Class::ISA isn't a class; it's just a package.
Strange, isn't it?

* Say you have a loop in the ISA tree of the class you're calling one
of the Class::ISA functions on: say that Food inherits from Matter,
but Matter inherits from Food (for sake of argument).  If Perl, while
searching for a method, actually discovers this cyclicity, it will
throw a fatal error.  The functions in Class::ISA effectively ignore
this cyclicity; the Class::ISA algorithm is "never go down the same
path twice", and cyclicities are just a special case of that.

* The Class::ISA functions just look at @ISAs.  But theoretically, I