Recipe 12.6 Determining the Caller's Package

12.6.1 Problem

You need to find out the current or calling package.

12.6.2 Solution

To find the current package:

$this_pack = _ _PACKAGE_ _;

To find the caller's package:

$that_pack = caller( );

12.6.3 Discussion

The _ _PACKAGE_ _ symbol returns the package that the code is currently being compiled into. This doesn't interpolate into double-quoted strings:

print "I am in package _ _PACKAGE_ _\n";              # WRONG!
I am in package _ _PACKAGE_ _

Needing to figure out the caller's package arose more often in older code that received as input a string of code to be evaluated, or a filehandle, format, or directory handle name. Consider a call to a hypothetical runit function:

package Alpha;
runit('$line = <TEMP>');

package Beta;
sub runit {
    my $codestr = shift;
    eval $codestr;
    die if $@;
}

Because runit was compiled in a different package than was currently executing, when the eval runs, it acts as though it were passed $Beta::line and Beta::TEMP. The old workaround was to include your caller's package first:

package Beta;
sub runit {
    my $codestr = shift;
    my $hispack = caller;
    eval "package $hispack; $codestr";
    die if $@;
}

That approach works only when $line is a global variable. If it's lexical, that won't help at all. Instead, arrange for runit to accept a reference to a subroutine:

package Alpha;
runit( sub { $line = <TEMP> } );

package Beta;
sub runit {
    my $coderef = shift;
    &$coderef( );
}

This not only works with lexicals, but has the added benefit of checking the code's syntax at compile time, which is a major win.

If all that's being passed in is a filehandle, it's more portable to use the Symbol::qualify function. This function takes a name and package to qualify the name into. If the name needs qualification, it fixes it; otherwise, it's left alone. But that's considerably less efficient than a * prototype.

Here's an example that reads and returns n lines from a filehandle. The function qualifies the handle before working with it.

open (FH, "<", "/etc/termcap")        or die "can't open /etc/termcap: $!";
($a, $b, $c) = nreadline(3, "FH");

use Symbol ( );
use Carp;
sub nreadline {
    my ($count, $handle) = @_;
    my(@retlist,$line);

    croak "count must be > 0" unless $count > 0;
    $handle = Symbol::qualify($handle, (caller( ))[0]);
    croak "need open filehandle" unless defined fileno($handle);

    push(@retlist, $line) while defined($line = <$handle>) && $count--;
    return @retlist;
}

If everyone who called your nreadline function passed the filehandle as a typeglob *FH, as a glob reference *FH, or using FileHandle or IO::Handle objects, you wouldn't need to do this. It's only the possibility of a bare "FH" string that requires qualification.

12.6.4 See Also

The documentation for the standard Symbol module, also found in Chapter 32 of Programming Perl; the descriptions of the special symbols _ _FILE_ _, _ _LINE_ _, and _ _PACKAGE_ _ in perldata(1); Recipe 12.14; Recipe 7.6