Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl: the Markov chain saw

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
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.

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...


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

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or or How to display code and escape characters are good places to start.
Log In?

What's my password?
Create A New User
Domain Nodelet?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others scrutinizing the Monastery: (3)
As of 2022-10-07 19:34 GMT
Find Nodes?
    Voting Booth?
    My preferred way to holiday/vacation is:

    Results (30 votes). Check out past polls.