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in reply to Neat Debugger tricks

Something that's missing in the debugger is "set next statement", something that some c and Java debuggers have. And differential compilation, but that's probably even more difficult. Eg. 

# ...
107    sub foo {
108        my $x;
109        $x = 43;
110 =>     die "invalid x!" unless $x == 42;
111        print "Yay!"
112        # ...

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At this point, I notice I have a bug so I edit line 109 (the source code on disk changes): 

109        $x = 43;
           # When I hit "save", foo is recompiled. If line numbers
           # have changed, breakpoints are updated.

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And set the next instruction to it again. Then I hit s/n/c/whatever and keep going.

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Re^2: Neat Debugger tricks
by osunderdog (Deacon) on Nov 17, 2005 at 12:59 UTC

    I understand what you are trying to do...identify a bug, modify the code, and continue debugging with the modified code. That would be very nice.

    I think I can point out that you could at least change it for this instance and continue debugging. For example, usin g your code: 

    # ...
107    sub foo {
108        my $x;
109        $x = 43;
110 =>     die "invalid x!" unless $x == 42;
111        print "Yay!"
112        # ...

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    At this point you could set the value of $x before the unless expression is evaluated: 

    DB<6> $x = 43;

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    Of course, you would have to remember to make the same modification in code...

    As an interesting alternative, you could redefine the function on the fly... This wouldn't work in your example because it dies on a fail case. However after a first run through the function, cut the function from an editor and paste it into the debugger with the change you desire. Subsequent runs restarts through the debugger would have the modified version of the function.

    For example: 

    use strict;

foo();
foo();

sub foo { my  $x; $x = 43; if($x==42){print "DANGER Will  Robinson!\n"
+}else{print "Yay!\n";}}

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    Then I run this in the debugger... After the first execution of foo Cut and paste the modified subroutine in to the debugger and execute it. This redefines the foo function. 

    main::(perldebugexample2.pl:3): foo();
  DB<1> n
Yay!
main::(perldebugexample2.pl:4): foo();
<$x = 42; if($x==42){print "DANGER Will Robinson!\n"}else{print "Yay!\
+n";}}   

  DB<2> n
DANGER Will  Robinson!

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    I'm not sure if this is practicle, but it is interesting... :)

    Hazah! I'm Employed!

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[d/l]
[select]
      I understand what you are trying to do...identify a bug, modify the code, and continue debugging with the modified code. That would be very nice.

      Yes, yes, yes, and yes :-)

      Microsoft Visual Studio does this, at least for c, and Eclipse does it for Java. These are languages where you'd expect these things to be waaaay harder to get away with than Perl. (Well, except for "set next instruction", which in c is relatively easy.)

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        Your wish is granted (in 5.8.5): 
        $ perl -de0

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::(-e:1):   0
  DB<1> h rerun
rerun           Rerun session to current position.
rerun n         Rerun session to numbered command.
rerun -n        Rerun session to number'th-to-last command.

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        rerun reloads the program and applies all commands you've executed in the debugger to (theoretically) get you back to the same point.
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[d/l]

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