a few questions about __PACKAGE_

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

Hi, keepers of Perl knowledge!

I have code where I am constructing the name of a .conf file based on the location of the module:

package Module;
use warnings;
BEGIN {
    my $pathname = $INC{$\(__PACKAGE__)}.pm"};
    $pathname =~ s/\.pm$/.conf/;
}
1;
[download]

When I check the syntax via the interpreter, I get a warning:

$ perl -c Module.pm
Use of uninitialized value in substitution (s///) at Module.pm line 8.
Module.pm syntax OK
$
[download]

If I comment out the use warnings; pragma, the warning goes away.

I'm assuming that __PACKAGE__ is not defined until a script loads the module, so this might be an explanation. perlmod states that using __PACKAGE__ can be thorny when creating variable names, so I thought I would pose the question to you. Should I be looking at another method to construct a variable name?

Thanks!

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Re: a few questions about __PACKAGE__ by ikegami (Patriarch) on Feb 10, 2012 at 20:11 UTC
What you have there does not compile `syntax error at Module.pm line 5, near "$\(" BEGIN not safe after errors--compilation aborted at Module.pm line 5.` [download] I presume your code is closer to `package Module; use strict; use warnings; BEGIN { my $pathname = $INC{"$\(__PACKAGE__)}.pm"}; $pathname =~ s/\.pm$/.conf/; } 1;` [download] If so, the problem is that `"$\(__PACKAGE__)}.pm"` [download] is the same as `$\ . "(__PACKAGE__)}.pm"` [download] because `$\` is a variable. You want `my $pathname = $INC{__PACKAGE__ . ".pm"};` [download] Except that will fail if your module name has more than one part (e.g. "Foo::Bar"), and it's more complicated that it needs to be. You really just want `my $pathname = __FILE__;` [download]	[reply] [d/l] [select]
Re: a few questions about __PACKAGE__ by Eliya (Vicar) on Feb 10, 2012 at 20:00 UTC
I think you want `$INC{__PACKAGE__.".pm"};` Also note that the lexical variable `my $pathname` would go out of scope at the end of the BEGIN block.	[reply] [d/l] [select]
Re: a few questions about __PACKAGE__ by Anonymous Monk on Feb 10, 2012 at 19:58 UTC
The error message you posted is on line 8, but the code you posted does not have 8 lines The code you posted does not compile (typo) You're also missing a pair of braces in the interpolation, to dereference the reference you're taking The syntax you're looking for, the common idiom, is `print "@{[ 1,2,'Q', rand ]}\n"; print "@{[ __PACKAGE__ ]}\n"; __END__ 1 2 Q 0.770751953125 main` [download] references quick reference	[reply] [d/l]
Re^2: a few questions about __PACKAGE__ by Anonymous Monk on Feb 11, 2012 at 15:36 UTC
I think the form of that idiom the OP had seen was probably $ perl -wE 'print "${\__PACKAGE__}\n";' main (or `"${\(...)}"`) which is a little less unsightly, if the expression returns a scalar.	[reply] [d/l]
Re: a few questions about __PACKAGE__ by JavaFan (Canon) on Feb 12, 2012 at 10:54 UTC
I'm assuming that __PACKAGE__ is not defined until a script loads the module, That's not your problem. The problem is that `$INC{"Module.pm"}` only gets set after successful complication, and you haven't finished compiling yet.	[reply] [d/l]