How to export a package sitting lower in a module?

[HelenCr]

Dear esteemed PerlMonks

I am wrestinlg with the following issue (which may be a trivial one): how do you export functions from a package, which is not the first one in a module file? I've been searching the documentation and the Net, and can't seem to find an answer.
Here is an example:

# Name of this file: pack_A.pm
package pack_A;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {first second};

sub first {
}
sub second {
}
1;
#end package pack_A;

package pack_B;

sub third {
}
sub fourth {
}
1;
#end package pack_B;

In the main script, I would like to use package pack_B, which sits lower in the file: pack_A.pm.
It turns out that it's not that simple. If you add the subroutines 'third' and 'fourth' to @EXPORT,

@EXPORT = qw {first second third fourth};

then in your main script, when you go:

use pack_A;

and call the 'third' sub, it will complain:

undefined subroutine &pack_A::third ...etc

(You can't go:
use pack_B;
since the interpreter looks for the file specified in "use". In other words, it will look for the module pack_B.pm, which does not exist).

Your advice will be appreciated.
Many TIA - Helen

[kennethk]

use is an execution of require and import wrapped in a BEGIN block. Because you have package pack_B in the same file as package pack_A, you are failing the require. So the solution is to add the export text (as Corion says) and then invoke import directly, i.e. pack_B->import.

The better way (IMHO) to do this is to actually respect the package system intent and put pack_B in a different file, and don't surprise your maintainers.

[HelenCr]

Corion and kennethk:

It works best if you put separate packages into separate files.

Sometimes (especially if you have an object-oriented project, with many little packages), it's not convenient to have each package in a speparate file.

So the solution is to add the export text (as Corion says) and then invoke import directly, i.e. pack_B->import.

Please note that this still doesn't work: the .pm module fails, complaining that the second @ISA is a bareword.

Main script:

use pack_A;
pack_B->import;
third();

The packages:

# Name of this file: pack_A.pm
package pack_A;
use strict;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {first second};

sub first {
}
sub second {
}
1;
#end package pack_A;

package pack_B;
use strict;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {third fourth};
sub third {
}
sub fourth {
}
1;
#end package pack_B;
#end of file pack_A.pm

[kennethk]

Sometimes (especially if you have an object-oriented project, with many little packages), it's not convenient to have each package in a speparate file.

If you are doing OO programming in Perl, you don't need to import anything. Using packages as libraries and exporting subroutines into a user space is actually a direct contradiction with a rigid OO model.

Please note that this still doesn't work: the .pm module fails, complaining that the second @ISA is a bareword.

That's because you're violating strict. Try our:

package pack_B;
use strict;
require Exporter;
our @ISA = qw {Exporter};
our @EXPORT = qw {third fourth};
sub third {
}
sub fourth {
}
1;

[Corion]

You try to export things from pack_A by adding them to @pack_A::EXPORT. But some of these things do not live in pack_A. You will have to export things from their respective packages. In your case, that would be pack_B for third and fourth.

[HelenCr]

Corion: can you please show how to do it?

(You can't go:
use pack_B;
since the interpreter looks for the file specified in "use". In other words, it will look for the module pack_B.pm, which does not exist).

[Corion]

I'm not sure how to explain "You will have to export things from their respective packages." in different words, so I'll do it by example:

package pack_B;
require Exporter;
@ISA = qw {Exporter};
@EXPORT = qw {third fourth};
...

This is identical to the respective parts of pack_A, except with the obvious changes for pack_B.