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For an iterator to work in Perl one needs to initialize it properly

use strict; use warnings; use Data::Dump qw/pp dd/; for my $limit (reverse 1..5) { for ( my $iter = countdown($limit); $iter->(my $a) ; ) { print "$a: "; } print "\n"; } sub countdown{ my $val = shift; my $iter = sub { if ($val--) { $_[0]=$val; return 1; } return; # stop iteration }; return $iter; }
4: 3: 2: 1: 0: 3: 2: 1: 0: 2: 1: 0: 1: 0: 0:

(I used c-style for here for clarity*)

But I'd rather like to stay DRY and to write something like

while ( countdown $limit => my $a ) { .... }

Where countdown can elaborate if the loop is (re)entered and does the init step automatically.

NB: Other languages have this feature for so called iterator objects.

I'm wondering if this could be tricked into Perl without XS wizardy ...

Approaches ...
  • I tried %^H , but this seems to be a compile time effect only.
  • Blessing a DESTROY method to \$a doesn't help, because it's triggered for all iterations not only the first.
  • I was thinking to use PadWalker to inject a var with a DESTROY hook into closed_over , but I doubt this has a chance to work.
  • UPDATE: Well with Keyword::Simple I could define a new loop keyword which expands into for(;;;)
  • UPDATE: there is some magic connected to the diamond operator <$obj> which can be overloaded, but you can't use it with functions because the syntax is convoluted with file-glob.
IDEAS?

Cheers Rolf
(addicted to the Perl Programming Language and ☆☆☆☆ :)
Wikisyntax for the Monastery

*) please note that while(CODE){} and for(;CODE;){} have the same effect.

) many issues could be solved like this...

update

The countdown iterator was used for demonstration only, I now plenty of ways to countdown. Iterators are a general issue.


In reply to Can I check if a loop's scope is entered for the first time? by LanX

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