Alas, watch-expression not obvious to this one

throop has asked for the wisdom of the Perl Monks concerning the following question:

Brethern, perlman debug sez, concerning the 'watch expression':

w expr: Add a global watch-expression. We hope you know what one of these is, because they're supposed to be obvious.

Alas, I don't understand. I'm not even using it right when I try to watch a single variable.

 w $negatePrefix
[download]

But what I really want to do is to watch a particular place within a HoH:

 w $onto{'PROBLEM'}{'mappings'}
[download]

Can some one give me a more full example of how to use this powerful feature?

throop

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Re: Alas, watch-expression not obvious to this one
by almut (Canon) on Feb 05, 2007 at 20:31 UTC

What problems do you have? For me, it seems to work as specified:

Assuming the following program

#!/usr/bin/perl
for my $c (1..1000) {
    $onto{'PROBLEM'}{'mappings'} = "foo" if $c == 99;
}
[download]

here's an example session:

$ perl -d watchpoint-test.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

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

main::(watchpoint-test.pl:2):        for my $c (1..1000) {
  DB<1> w $onto{'PROBLEM'}{'mappings'}
  DB<2> c
Watchpoint 0:   $onto{'PROBLEM'}{'mappings'} changed:
    old value:  ''
    new value:  'foo'
main::(watchpoint-test.pl:3):            $onto{'PROBLEM'}{'mappings'} 
+= "foo" if $c == 99;
  DB<2> p $c
100
  DB<3> c
Watchpoint 0:   $onto{'PROBLEM'}{'mappings'} changed:
    old value:  'foo'
    new value:  ''
Debugged program terminated.  Use q to quit or R to restart,
  use o inhibit_exit to avoid stopping after program termination,
  h q, h R or h o to get additional info.
  DB<3> q
[download]

Maybe you're using an old version of Perl - I dimly remember that commands were slightly different in 5.6.x (I think 'w' was 'W', but I could be wrong).

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