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

I'd like to be able to do this:
use $foo;
where $foo contains some module name. But when I try, I get a syntax error. Is such a thing even possible?

---
A fair fight is a sign of poor planning.

Replies are listed 'Best First'.
Re: Using arbitrary packages
by fergal (Chaplain) on Jul 12, 2004 at 21:49 UTC 
    
eval "use $foo";
die $@ if $@ # die with error message if it failed

    [download]
    will do it but it will do it at runtime, whereas a genuine use will do it at compile time. If you do 
    BEGIN
{
  my $foo = get_a_module_name();
  eval "use $foo";
  die $@ if $@
}

    [download]
    it will happen at compile time, you just need to be careful that $foo has a value at compile time.
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[d/l]
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Re: Using arbitrary packages
by ysth (Canon) on Jul 12, 2004 at 21:52 UTC
    If $foo is trusted data, you can say eval "use $foo";, but that won't take effect at compile time. Starting with 5.6.2, you can say 
       use if 1, $foo;

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    but $foo has to already be set at compile time, e.g. 
       my $foo;
   BEGIN { $foo = $ENV{SERIALIZER} || "Storable" }
   use if 1, $foo;

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    but even that IMO is more clearly written as 
       BEGIN { eval "use ".($ENV{SERIALIZER}||"Storable"); }

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    What exactly are you trying to accomplish? There's probably a better way.
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      In a nutshell, I'd like to have a config file that contains module names and function names. I want to read the config file, 'use' all the modules, and then make the function calls as given in the config file.

      It's all trusted data, since it's all being run locally.

      ---
      A fair fight is a sign of poor planning.

[reply]
        Perhaps you should use 
        do $filename;

        [download]
        and have actual real Perl in the file that looks like 
        use Module1;
use Module2;
function1();
function2();

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        it will run faster. No point in inventing your own mini perl parser. Especially if in the future you decide you need to start adding arguments to your functions etc.
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      Starting with 5.6.2, you can say
      Its more like starting with perl 5 (if its on CPAN, you can get it from CPAN) :D

      MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
      I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
      ** The third rule of perl club is a statement of fact: pod is sexy.

[reply]
Re: Using arbitrary packages
by duff (Parson) on Jul 13, 2004 at 03:49 UTC
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