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

Update: resolved and further explored here http://perlmonks.org/index.pl?node_id=1076241

I'm trying to insert a little pre-prompt script into the Perl debugger which makes my source editor follow along with its cursor in the open source file window while I'm tracing the source code on the command line.

In my case the editor is BBEdit on the Mac, so I'm using this command to update the cursor position from Perl, which already works so far from a regular script: 

system("osascript", "-e", "tell application \"BBEdit\"
    tell the text of the front window
        select insertion point before line $line
    end tell
end tell");

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I've already verified that it works as a pre-prompt command in Perl debug:

DB<34> < system("osascript", "-e", "tell application \"BBEdit\"\n    tell the text of the front window\n        select insertion point before line $line\n    end tell\nend tell");

Whenever I change $line in the debugger, the editor cursor jumps to its new line number. So almost all of it already works.

(It executes an AppleScript snippet from the command line which moves the cursor in the open BBEdit GUI application.)

The thing is that I would need to know where to get the current tracing line number from within the debugger, so I can insert this instead of $line as a pre-prompt Perl command which should then slave the editor cursor to the current tracing location in the debugger.

I've just not found such a debugger variable yet; The special Perl variable listings here and on perldocs don't seem to indicate such a variable as far as I can see thus far.

Any ideas? Have I just overlooked such a variable or do I have to patch the debugger to get it?

  • Comment on Update: homespun GUI Wrapper for the Perl Debugger with an auto-refreshing editor (BBEdit on Mac OS X) (was: Perl Debugger: Is there a variable with the current source line number being traced?)
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Re: Perl Debugger: Is there a variable with the current source line number being traced?
by Anonymous Monk on Feb 25, 2014 at 08:29 UTC
    Devel::Trace deals with line-by-line tracing :) surely you can learn something from its source 
    # This is the important part.  The rest is just fluff.
sub DB::DB {
  return unless $TRACE;
  my ($p, $f, $l) = caller;
  my $code = \@{"::_<$f"};
  print STDERR ">> $f:$l: $code->[$l]";
}

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      Thanks for the encouragement...! 8-)

      I've indeed looked into the debugger (perl5db.pl) where it's reading the caller info (near the beginning of sub DB) and I've patched my script into it right there. I didn't even have to change the $line variable as it's present with that name there already. ;-)

      Of course it's still a dirty hack in its current form, but it suits me for the moment: The editor cursor indeed follows the debugger in lockstep, now even without having to insert a pre-prompt command.

      Cool! :-)

      Especially since the debugger now always points to the code in the editor right where it's ready to get fixed right away, just like a regular IDE. (I still have to switch windows in the editor when tracing into a different file for now, but the application window always remains in the background and updates there while I'm tracing Perl in the Terminal, so that's bearable for the moment.)

      At a cursory glance I haven't seen an obvious "official" hook for this purpose in the debugger source yet, so even if my immediate need has now been satisfied, a cleaner way would still be interesting to know if somebody should happen to know one.

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Re: Perl Debugger: Is there a variable with the current source line number being traced?
by LanX (Saint) on Feb 25, 2014 at 17:12 UTC
    is $DB::line what you are looking for?

    It should essentially always be set to (caller)[2].

    Did you already have a look at perldebguts?

    perl5db.pl is also heavily PODed.

    This thread might also be of interest Make debugger break on source lines matching a pattern.

    HTH! :)

    Cheers Rolf

    ( addicted to the Perl Programming Language)

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      yes, $DB::line was the one I was looking for originally. Thanks.

      The perl5db.pl documentation was most helpful to me, but actually the in-place comments more than the POD, at least for what I wanted right now. perldebguts is also interesting.

      I've also begun to use the explicit debugger invocation from the source in a few interesting places.

      My current state is this (to give everyone some feedback):

      I'm working on a Mac and I have employed BBEdit as debugging output viewer.

      I have made just some minor modifications to perl5db.pl to the following effects:

      • whenever the debugger detects a switch between sources (notably including evals as well!), it writes out the current source to the file ~/t/Perl/thesource.pl

      • The debugger pushes a cursor position update on every tracing step to the BBEdit window named thesource.pl via AppleScript (called osascript on the command-line gateway):

      system("osascript", "-e",
"tell application \"BBEdit\"
    tell the text of the window \"thesource.pl\"
        select insertion point before line $line
    end tell
end tell");

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      • I have augmented the debugger command "|" with an optional parameter which is appended to the pager option string before execution. I'm using it like this:

      o pager=cat>~/t/Perl/
{|(vars.pl)y
{{|(stack.txt)T
{{|(package_vars.pl)X

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      |(vars.pl)y actually executes the y command and pipes it to the effective "pager" cat>~/t/Perl/vars.pl, writing it to that file on every prompt.

      So the pre-prompt auto commands keep updating the following files in ~/t/Perl/:

      vars.pl: dump of local variables
      stack.txt: stack backtrace
      package_vars.pl: package variables (I keep this off much of the time because in my project the amount can be huge)

      In the GUI editor BBEdit I keep these three files and thesource.pl open besides my working sources and data files. BBEdit automatically detects any files being updated and reloads them transparently while preserving their scrolled positions. And perl5db.pl also keeps updating the cursor in thesource.pl.

      This is so fast that it is an actually usable replacement for a regular GUI debugger.

      And when I occasionally want to display additional information, I can just push it to additional windows as needed like this:

      |(more.pl)x %mhash

      For me it's quite nice that syntax colouring and all the GUI niceties of my regular source editor are just seamlessly available and I have large, independent windows for debugger output. That the debugger still needs to be operated on the command line in a Terminal window is not an issue in this context since BBEdit in the background keeps updating its windows while I keep going.

      And this is what it looks like while debugging Perl code:

      <img src="http://i.imgur.com/zSWB3wO.png" title="Hosted by imgur.com" />

      I'm aware that this is kind of freaky (which is half the fun of it!), but it works quite well and is much more practical than the command-line-only debugger on its own. I'm aware that there are some other GUI wrappers around, but they come with their own quirks again, so I actually prefer it this way for the moment.

      The same principle could certainly be applied to other environments than OS X and BBEdit as well, so maybe it helps anyone else along as well.

      If anyone is interested, I can of course provide the (pretty raw) modifications I've made to perl5db.pl.

      I've just needed to get ahead with the actual debugging in some relatively complex code, so right now functionality goes before beauty for me.

      One known TODO would be to run the tracing cursor in the actual source files and switching between those as needed instead of using the saved thesource.pl for everything (except for evals where that's the only way), but I may get around to that some time later.

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        tl;dr .... :-|

        Unfortunately I don't have the time to go into details ATM, but I think many modifications to perl5db.pl where unnecessary.

        For instance you could have used persistent aliases instead of manipulating commands. The debugger has also an IDE interface to direct outputs to pipes.

        If you can't get ptkdb running on your Mac because of TK version problems (which is really weird and merits a thread of its own) try giving emacs a try, which has more or less the reference implementation of debugger integration.

        With Aquamacs you'll even get standard Mac behaviour.

        Try M-x perldb.

        For more tricks of how to patch the debugger please see the slides I linked in the thread I linked to before.

        HTH :)

        Cheers Rolf

        ( addicted to the Perl Programming Language)

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