--- /dev/null
+#!/usr/local/bin/perl
+# Time-stamp: "2000-05-13 20:03:22 MDT" -*-Perl-*-
+
+package Class::ISA;
+require 5;
+use strict;
+use vars qw($Debug $VERSION);
+$VERSION = 0.32;
+$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 $VERSION).
+
+The code for self_and_super_versions is meant to serve as an example
+for precisely the kind of tasks I anticipate that self_and_super_path
+and super_path will be used for. You are strongly advised to read the
+source for self_and_super_versions, and the comments there.
+
+=back
+
+=head1 CAUTIONARY NOTES
+
+* Class::ISA doesn't export anything. You have to address the
+functions with a "Class::ISA::" on the front.
+
+* Contrary to its name, Class::ISA isn't a class; it's just a package.
+Strange, isn't it?
+
+* Say you have a loop in the ISA tree of the class you're calling one
+of the Class::ISA functions on: say that Food inherits from Matter,
+but Matter inherits from Food (for sake of argument). If Perl, while
+searching for a method, actually discovers this cyclicity, it will
+throw a fatal error. The functions in Class::ISA effectively ignore
+this cyclicity; the Class::ISA algorithm is "never go down the same
+path twice", and cyclicities are just a special case of that.
+
+* The Class::ISA functions just look at @ISAs. But theoretically, I
+suppose, AUTOLOADs could bypass Perl's ISA-based search mechanism and
+do whatever they please. That would be bad behavior, tho; and I try
+not to think about that.
+
+* If Perl can't find a method anywhere in the ISA tree, it then looks
+in the magical class UNIVERSAL. This is rarely relevant to the tasks
+that I expect Class::ISA functions to be put to, but if it matters to
+you, then instead of this:
+
+ @supers = Class::Tree::super_path($class);
+
+do this:
+
+ @supers = (Class::Tree::super_path($class), 'UNIVERSAL');
+
+And don't say no-one ever told ya!
+
+* When you call them, the Class::ISA functions look at @ISAs anew --
+that is, there is no memoization, and so if ISAs change during
+runtime, you get the current ISA tree's path, not anything memoized.
+However, changing ISAs at runtime is probably a sign that you're out
+of your mind!
+
+=head1 COPYRIGHT
+
+Copyright (c) 1999, 2000 Sean M. Burke. All rights reserved.
+
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself.
+
+=head1 AUTHOR
+
+Sean M. Burke C<sburke@cpan.org>
+
+=cut
+
+###########################################################################
+
+sub self_and_super_versions {
+ no strict 'refs';
+ map {
+ $_ => (defined(${"$_\::VERSION"}) ? ${"$_\::VERSION"} : undef)
+ } self_and_super_path($_[0])
+}
+
+# Also consider magic like:
+# no strict 'refs';
+# my %class2SomeHashr =
+# map { defined(%{"$_\::SomeHash"}) ? ($_ => \%{"$_\::SomeHash"}) : () }
+# Class::ISA::self_and_super_path($class);
+# to get a hash of refs to all the defined (and non-empty) hashes in
+# $class and its superclasses.
+#
+# Or even consider this incantation for doing something like hash-data
+# inheritance:
+# no strict 'refs';
+# %union_hash =
+# map { defined(%{"$_\::SomeHash"}) ? %{"$_\::SomeHash"}) : () }
+# reverse(Class::ISA::self_and_super_path($class));
+# Consider that reverse() is necessary because with
+# %foo = ('a', 'wun', 'b', 'tiw', 'a', 'foist');
+# $foo{'a'} is 'foist', not 'wun'.
+
+###########################################################################
+sub super_path {
+ my @ret = &self_and_super_path(@_);
+ shift @ret if @ret;
+ return @ret;
+}
+
+#--------------------------------------------------------------------------
+sub self_and_super_path {
+ # Assumption: searching is depth-first.
+ # Assumption: '' (empty string) can't be a class package name.
+ # Note: 'UNIVERSAL' is not given any special treatment.
+ return () unless @_;
+
+ my @out = ();
+
+ my @in_stack = ($_[0]);
+ my %seen = ($_[0] => 1);
+
+ my $current;
+ while(@in_stack) {
+ next unless defined($current = shift @in_stack) && length($current);
+ print "At $current\n" if $Debug;
+ push @out, $current;
+ no strict 'refs';
+ unshift @in_stack,
+ map
+ { my $c = $_; # copy, to avoid being destructive
+ substr($c,0,2) = "main::" if substr($c,0,2) eq '::';
+ # Canonize the :: -> main::, ::foo -> main::foo thing.
+ # Should I ever canonize the Foo'Bar = Foo::Bar thing?
+ $seen{$c}++ ? () : $c;
+ }
+ @{"$current\::ISA"}
+ ;
+ # I.e., if this class has any parents (at least, ones I've never seen
+ # before), push them, in order, onto the stack of classes I need to
+ # explore.
+ }
+
+ return @out;
+}
+#--------------------------------------------------------------------------
+1;
+
+__END__
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