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Index Attribute
Maintainer: David Nicol <perl6rfc@davidnicol.com> Date: 19 Sep 2000 Mailing List: perl6-language@perl.org Number: 262 Version: 1 Status: Developing
An attribute :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 foreach statement, it also may be used inside container-like
abstractions
which dynamicly generate their contents.
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
When this was suggested on the language-objects list,
there was discussion of the name of the attribute, with :i being
suggested as well as :index
We also need access to attributes of variables as a standard language feature, that is implicit here.
Another possibility would be to use $[ for the attribute, which
makes sense if we allow $[ to be set individually per container.
Use of brackets in variable names is a Big Hassle for parsing:
print "$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
Within looping constructs, we only need to track this if it is used.
We may be using an implied c-like 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.)
John McNamara left this radical suggestion out of RFC 120.