in reply to debugging eval'd code with source (SOLVED)

Each time you have an eval step inside the debugger, it replies with s.t. like: 
main::((eval 8)[evals.pl:15]:1):

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The string (eval 8) is the "file" name of the eval. You can use that name to list the eval.

The following example with the debugger illustrates the point. I will use this listing: 

pp2@nereida:~/src/perl/testing$ cat -n evals.pl
     1  use strict;
     2  use warnings;
     3
     4  my ($a, $b) = (1, 9);
     5
     6  my $x = eval q {
     7    $a = 2;
     8    $b = 3;
     9    print "$b\n";
    10    $a+$b;
    11  };
    12
    13
    14
    15  my $y = eval '$a+$b';
    16
    17  print "$y\n";

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See how the evals produce the "virtual filenames": 
pp2@nereida:~/src/perl/testing$ perl -wd evals.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::(evals.pl:4):     my ($a, $b) = (1, 9);
  DB<1> c 6
main::(evals.pl:6):     my $x = eval q {
main::(evals.pl:7):       $a = 2;
main::(evals.pl:8):       $b = 3;
main::(evals.pl:9):       print "$b\n";
main::(evals.pl:10):      $a+$b;
  DB<2> n
main::((eval 6)[evals.pl:6]:2):   $a = 2;
  DB<2> l (eval 6)
1
2==>      $a = 2;
3:        $b = 3;
4:        print "$b\n";
5:        $a+$b;
6
7       ;

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See it?. Now I try to stop at line 15 
  DB<3> c 15
Line 15 not breakable.

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I fail because I am in file eval not in the main file: 
  DB<4> f evals.pl
1:      use strict;
2:      use warnings;
3
4:      my ($a, $b) = (1, 9);
5
6:      my $x = eval q {
7         $a = 2;
8         $b = 3;
9         print "$b\n";
10        $a+$b;
  DB<5> c 15
3
main::(evals.pl:15):    my $y = eval '$a+$b';
  DB<6> n
main::((eval 8)[evals.pl:15]:1):        $a+$b
  DB<6> l (eval 8)
1==>    $a+$b
2       ;
  DB<7> c
5
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<7>

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Replies are listed 'Best First'.
Re^2: debugging eval'd code with source
by sharkey (Scribe) on Jul 09, 2008 at 18:01 UTC
    Thanks for your answer. Here's a more concrete example to show the problem. 
    $ cat -n tt
     1  #!/usr/bin/perl
     2
     3  my $perl = <<'EOF';
     4  sub {
     5  my $x = 123;
     6  $x += shift;
     7  print "x is $x\n"
     8  }
     9  EOF
    10
    11  my $cf = eval $perl;
    12
    13  $cf->(4);

$ perl -d tt

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::(tt:3):   my $perl = <<'EOF';
main::(tt:4):   sub {
main::(tt:5):   my $x = 123;
main::(tt:6):   $x += shift;
main::(tt:7):   print "x is $x\n"

DB<8> s
main::(tt:11):  my $cf = eval $perl;

DB<8> s
main::((eval 20)[tt:11]:7):     ;

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    Now we are inside the eval, and we can see the code... 
    DB<8> p $#{"_<(eval 20)[tt:11]"}
7

DB<9> l (eval 20)
1       sub {
2:      my $x = 123;
3:      $x += shift;
4:      print "x is $x\n"
5       }
6 
7==>    ;

    [download]
    But as soon as we step out of the eval, the code is gone... 
    DB<10> s
main::(tt:13):  $cf->(4);

DB<10> p $#{"_<(eval 20)[tt:11]"}
-1

    [download]
    And yet, there are other evals out there, for the code stays around... 
    DB<11> p join "\n", map { "$_   $#{$_}" } grep /_<\(/, keys %::
_<(eval 20)[tt:11]   -1
_<(eval 22)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   6
_<(eval 11)[/usr/lib/perl5/5.8.8/SelfLoader.pm:111]   1
_<(eval 12)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   40
_<(eval 5)[/usr/lib/perl5/5.8.8/Term/ReadLine.pm:306]   -1
_<(eval 21)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   10
_<(eval 23)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38]   20
_<(eval 26)[/usr/lib/perl5/5.8.8/perl5db.pl:628]   4

DB<13> f eval 22
Choosing (eval 22)[/usr/lib/perl5/5.8.8/SelfLoader.pm:38] matching `ev
+al 22':
1       package readline; sub F_PreviousHistory {
2:          &get_line_from_history($rl_HistoryIndex - shift);
3       }
4 
5 
6       ;

    [download]
    I no longer can get to my eval'd code... 
    DB<14> f eval 20
Choosing (eval 20)[tt:11] matching `eval 20':


DB<15> l

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    And neither can the debugger... 
    DB<15> s
main::CODE(0x97b7d28)((eval 20)[tt:11]:2):
2:

DB<15> 
main::CODE(0x97b7d28)((eval 20)[tt:11]:3):
3:

DB<15> 
main::CODE(0x97b7d28)((eval 20)[tt:11]:4):
4:

DB<15> 
x is 127

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.

    [download]
    So the question remains, why does my eval'd source code get discarded, while other eval'd source code stays in memory? And what can I do to convince perl to keep my eval'd source code?
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