Re^2: Using the Perl Debugger (-d)

[sgifford]

Debug prints such as yours entail a runtime hit, which is almost always negligible; but they also clutter the code, which often isn't.

If you use something like this:

use constant DEBUG => 1;
print "Some useful information"
  if (DEBUG);

the compiler should optimize it away, which will remove even the small performance hit (or at least relegate it to the one-time compilation of the code).

[MaxKlokan]

I have used your same approach sometimes. The problem I hit is that if your program has been migrated to a production environment where the script is read-only, you cannot turn on/off the switch for debugging purposes.
That is why I prefer to add a command-line switch to turn on debugging:

#!/usr/bin/perl -s
use warings
use strict;

use vars $d;

use vars qw/$d/;

print "Debugging message..." if $d;

Update
To run the script you would use perl myscript.pl -d

[bart]

I'm sure it must be feasable to set the value for the constant based on a command line switch. I'm not sure if one can just plug in use a module like Getopt::Std or Getopt::Long for this — they probably work at runtime, and might interfere with your own use of command line switches. Hence the handcrafted code, in the example:

{
   my $debug;
   BEGIN {
       if(grep { $_ eq '-d' } @ARGV) {          # look for switch
           $debug = 1;
           @ARGV = grep { $_ ne '-d' } @ARGV;  # remove switch
       }
   }
   use constant DEBUG => $debug;
}

print "Debug is on" if DEBUG;

Does anybody else get a "Useless use of a constant in void context" warning for the print line if DEBUG is off, or is it my old Perl version (5.6.1)? -MO=Deparse tells me it got replaced by the statement '???';, which might explain it. It's still silly.

[BrowserUk]

Did you see Re: conditional debug call perf?