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

In the following case, why is it that I'm able to 'use' the module, yet am unable to call its exported function in the 'main' namespace? I know how to work around this, but I'm trying to understand why this doesn't work the way it is.

[test]$ cat test.pl 
#!/usr/bin/perl -w

use strict;

use dir::mod;

hello("world");
[test]$ cat dir/mod.pm 
package mod;

use strict;
use warnings;

use Exporter;
our @ISA = qw(Exporter);

our @EXPORT = qw(hello);

sub hello {
  print "hello " . shift() . "\n";
}

1;
[test]$ ./test.pl 
Undefined subroutine &main::hello called at ./test.pl line 7.
[test]$

[download]

Thanks!

Replies are listed 'Best First'.
Re: Unabled to use exported routine
by repellent (Priest) on Apr 02, 2009 at 20:57 UTC
    The full package name has to be defined in mod.pm:
    package dir::mod;

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    Please use standard conventions to name packages. Browse CPAN for some examples.
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Re: Unabled to use exported routine
by Fletch (Bishop) on Apr 02, 2009 at 20:59 UTC

    First thing that jumps out is that your package directive is just mod not dir::mod.

    The cake is a lie.
    The cake is a lie.
    The cake is a lie.

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      I know that will make it work, but in the way I had it, why was it even able to 'use' the module? There must be something different about how functions are exported than how modules are accessed that I don't understand. It's that understanding that I'm looking for :)
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        There's nothing strictly speaking which directly ties the package namespace the contents of a module defines with the filename containing it. You could have a file Fred/Ethyl/Mertzomatic.pm which contained a line package Ricky::Lucy::Recardotron; and use Fred::Ethyl::Mertzomatic; would succeed fine so long as it parsed and returned a true value. You can also have code in several different namespaces in the same file if you like (often used to declare subclasses or subsidiary packages which are pulled in by using the top level module).

        However use will look and see if the package corresponding to the name it was given has an import method and will call that method if it exists; since your example doesn't define a method dir::mod->import (directly or through inheritance; you do declare, implicitly through inheritance, mod->import though) Exporter's import is never called and so the assignment of \&mod::hello into *main::hello never occurs and you get the undefined sub error at runtime.

        That clear things up?

        The cake is a lie.
        The cake is a lie.
        The cake is a lie.

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        Well, I am not so familiar with the internal Perl actions, but I assume the problem is something like this:

        use dir::mod; is internally a combination of require dir::mod; and import dir::mod;.

        require dir::mod works, as it is translated into a file path dir/mod.pm and that file is found and read. Then it tries to import the exported stuff from the dir::mod namespace. But this is not defined as your package name is mod, and not dir::mod.

        If you'd call mod::hello() your script should work.

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