Named anonymous subs

Here's an example of an anonymous sub generator:

use strict;
use warnings;
use Carp 'cluck';

sub make_incrementor {
   my ($initial_value, $reset_value) = @_;

   my $i = $initial_value;
   sub {
      if ($i == $reset_value) {
         cluck('Iterator overuse');
         $i = $initial_value;
      }  
      return $i++;
   }
}

sub test_incrementor {
   my $foo_incrementor = make_incrementor(0,2);
   print "call $_: got ", $foo_incrementor->(), "\n" for 1..3;
}

test_incrementor;
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When you are debugging it, the debugger shows the anonymous sub as a stringified code ref, which can be less than helpful if you are stepping through the code and have a number of different kinds of anonymous subs.

$ perl -d inc.pl

Loading DB routines from perl5db.pl version 1.19
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(inc.pl:23):      test_incrementor;
  DB<1> c 11
call 1: got 0
call 2: got 1
main::CODE(0x10326b1c)(inc.pl:11):               cluck('Iterator overuse');
  DB<2>

Fortunately, there is a way to tell the debugger a name to show for an anonymous sub. Adding local $DB::sub = 'iterator' at the top of the anonymous sub (also having another mention somewhere of $DB::sub to prevent a warning when not run with -d) gives the debugger a name for it:

main::incrementor(inc.pl:13):            cluck('Iterator overuse');

But when using stack traces for debugging (e.g. cluck), it calls it just __ANON__.

call 1: got 0
call 2: got 1
Iterator overuse at inc.pl line 13
        main::__ANON__() called at inc.pl line 22
        main::test_incrementor() called at inc.pl line 25
call 3: got 0

A little exploration of perl internals shows an undocumented feature. When you create an anonymous sub actually creates an *__ANON__ glob in the package and uses that for the name. This means you can add local *__ANON__ = "incrementor" in the sub {} to give it a real name while it is running.

What does that buy you, since it already says "at inc.pl line 22"? Well, if the sub{} is in a string eval, you won't have an actual code line. Also, you can include extra information for closures like local *__ANON__ = "incrementor_init${initial_value}_reset$reset_value"

Note that this is an example of a harmless use of an undocumented feature. You use it for debugging only, and if perl stops working this way, the *__ANON__ setting does nothing harmful.

(Given time, I will post more later about my attempts to attach a different *__ANON__ to each sub {} at compile time, which led to this question. For those who experiment, I will note now that the sub doesn't hold a refcnt on the *__ANON__ glob, so you have to arrange some other way to keep it allocated.)

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