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Postings from November 2016
On 28 November 2016 at 13:48, Aristotle Pagaltzis <pagaltzis@gmx.de> wrote:
>> * update $# on each loop iteration, which would slow down all loop
>> iterations, or
>> * give $# get magic that searches the context stack for a loop context,
>> then retrieves the current loop index stored in there. That wouldn't
>> slow down normal loops, but accessing $# itself might be quite slow.
There's a third option, its not well employed, but it could be:
At parse time, the Perl compiler parsing the for { } block *lexically*
parses for a variable ( we'll just say $# for now ).
And then upon detecting that variable's usage in its block, then instruments
the for loop to behave differently.
This would mean *no* performance impact for existing code at runtime,
and the associations of loops and variables would be compile-time bound.
Though it could be tricky.
for ( ... ) {
for ( ... ) {
$#
}
}
Here $# is only instrumenting its direct parent.
Logically, it would be like imagining there was a builtin called
"for_idx" , which looked as follows:
for_idx my($#, $value) ( .... ) {
}
And I think that latter form is quite easy to reason about.
You don't have to worry about distant subs secretly poking at $# and
sniffing your index and all those kinds of spooky side-effects.
Also, my memory tells me lexicals are rather fast in comparison to
globals, but my memory could be wrong.
--
Kent
KENTNL - https://metacpan.org/author/KENTNL
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