=head1 TITLE

Index Attribute

=head1 VERSION

  Maintainer: David Nicol <perl6rfc@davidnicol.com>
  Date: 19 Sep 2000
  Mailing List: perl6-language@perl.org
  Number: 262
  Version: 1
  Status: Developing

=head1 ABSTRACT


An attribute C<:n> is defined which means the index or key of a
contained item.  This attribute is useful when an index number is needed
within a C<foreach> statement, it also may be used inside container-like
abstractions 
which dynamicly generate their contents.


=head1 DESCRIPTION


	foreach $item (@items){
		#print "$item was at location ",$item:n,"\n";
		print "$item was at location ${item:n}\n";
	};


Given acceptance of several other iffy proposals,

	@Nplus1 = lazy sub { return $_:1 + 1 }; # or something like this



=head1 DISCUSSION

When this was suggested on the language-objects list,
there was discussion of the name of the attribute, with C< :i > being
suggested as well as C< :index >

We also need access to attributes of variables as a standard language
feature, that is implicit here.

Another possibility would be to use C< $[ > for the attribute, which
makes sense if we allow C< $[ > to be set individually per container.

Use of brackets in variable names is a Big Hassle for parsing:

		print "$item was at location ${item:[}\n";

is parseable if colon is elevated to the same level of immediacy,
in terms of eating subsequent tokens, as dollar-sign now has.

Use of left-bracket as the index attribute matches very well with
the situation of pulling sequential items out of a function pretending
to be a list, though, as internal to the code
we can refer to $[, "The index of the first item", to get the index
of the current item after several items have been shifted off (and
the specific $[ attribute incremented each time.)

	@Nplus1 = lazy sub { return $[ + 1 }; # or something like this


=head1 IMPLEMENTATION

Within looping constructs, we only need to track this if it is used.
We may be using an implied c-like C<for> when we do a looping construct
anyway, in which case we already have the information, and just need some
Perl syntax for sharing it with user code.  In linked-list kinds of
situations, we will need to identify the need to track this info and
then track it for the user.

In terms of $[ we would make the familiar and vaguely deprecable $[ be
the global to which the one specific to an array instance defaults to
or starts at.  (which may deserve it's own RFC it is true.)

=head1 REFERENCES

John McNamara left this radical suggestion out of RFC 120.