syntax question: conditional 'use' statement?

[Anonymous Monk]

Fellow Monks,
Is there anything wrong with using this syntax?

my $debug = 1;
use warnings     if ($debug == 1);
use Data::Dumper if ($debug == 1);

Thanks,
Jonathan

2006-09-20 Retitled by planetscape, as per Monastery guidelines

( keep:0 edit:30 reap:0 )

Original title: 'syntax question'

[Fletch]

You want the if pragma.

Update: Not to mention you'll have problems because the compilation phase use lines would be happening before the runtime initialization of your variable. You'd need to stick that in a BEGIN block.

[hgolden]

Hey Jonathan,

The problem is that use statements are evaluated before the program is run, so the program can't evaluate the conditional when it needs to figure out whether to use warnings and use Data::Dumper. Consider commenting out those two lines, except when you're debugging. (Or consider always using warnings).

Hays

[Tanktalus]

This is just like the following "C" code:

bool debug = true;  // I think bool is a new C keyword...
if (debug)
{
    #include <assert.h>
    #include "mydebugstuff.h"
}

The difference is that in C, you can more easily tell what is compile-time and what is run-time based on that hashmark. In Perl, we have the BEGIN block, but most of our compile-time code is actually executed via use.

You're mixing compile-time and run-time code, then, so it doesn't actually work. Someone mentioned the if pragma - but it's not really needed here.

Simply remove the if's. There's nothing wrong with warnings being used at all times. There's also nothing really wrong with loading modules you don't need - slows down compilation, but not much. If's usage is better suited for modules that may not be there than debugging statements that do no harm if they're left in, IMO. Since Data::Dumper is always installed, it's pretty safe to always load.

That said, if you want to avoid loading Data::Dumper, you can learn how to use require - then you will only load it when you need it - at runtime, not compile-time.

[ptum]

Personally, I tend to 'use warnings' unconditionally in my production code, not only in debug mode. If I was too lazy to stamp out all the warnings before going to production, I deserve whatever happens to me. If something later goes wrong, I like to know about it as early as possible. Just my $0.02 worth.

Update: Blush. I didn't try it until now ... but I get a syntax error if I try to use warnings conditionally:

#!/usr/local/bin/perl
my $debug = 1;
use warnings if ($debug == 1);
use Data::Dumper if ($debug == 1);

syntax error at ./test8.pl line 3, near "use warnings if"
Execution of ./test8.pl aborted due to compilation errors.

[mda2]

I'm according to Monsignor Tanktalus, but when you really need to do it, think to use constant to enable the debug flag:

use constant DEBUG => 2;
use if DEBUG==2, warnings;

print ">> $debug $D <<\n\n";

I'm really known if pragma now, and like it!

20060920 Janitored by Corion: Linkified links as per What shortcuts can I use for linking to other information?

[caelifer]

Of course you can always use require for Data::Dumper module, i.e. as in

BEGIN {
   use constant DEBUG => 1;
   if (DEBUG) {
      require Data::Dumper;
   }
}

However, this would not work for warnings since it works during compilation time. Moreover, as a rule of thumb I would recommend to always include both use strict and use warnings in every perl program. Believe it or not, it will catch a lot of errors and typos and reduce the debugging time greatly. Even for the advance programming you can always localize the effect by temporarily switching off both warnings and strict with no warnings and no strict directives.

[ikegami]

You forgot to import.

BEGIN {
   use constant DEBUG => 1;
   if (DEBUG) {
      require Data::Dumper;
      Data::Dumper->import() if Data::Dumper->can('import');
   }
}