isa.pm

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#!/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
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<[e-mail]>

=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__