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

Hi,

I am writing a simple plugin system. I'm not sure whether I'm going to use it, but it's more like a way to explore Perl.

So, I have an array which contains packages names. Those packages should have a sub 'ReceiveMessage'.
A package called Communications contains a sub 'SendMessage'.
This SendMessage loops over the array to check which packages do have a ReceiveMessage and send a message to it. Currently, the array consists of one item: "Communications".

package Communications;

sub SendMessage {
    my $msg = join("", @_);
    foreach my $cpack (@main::plugpack) {
        if (defined eval "${cpack}::ReceiveMessage") {
            eval "\&${cpack}::ReceiveMessage('$msg')";
        }
    }
}

sub ReceiveMessage {
    print "Communications::ReceiveMessage('$_[0]')\n";
}

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Of course I call the SendMessage sub once: &Communications::SendMessage("Hello, World!");
The strange thing is, I get output like I called SendMessage twice: 
Use of uninitialized value in concatenation (.) or string at D:\device
+\modem\internet\Xitami\cgi-bin\index.pl line 48.
Communications::ReceiveMessage('')
Communications::ReceiveMessage('Hello, World!')

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It appears to me that ReceiveMessage is not only called when the message is sent, but also when SendMessage checks it's existance. How can I prevent this?

Update: just clarified the last paragraph a little
Update: removed line of code which was coded in a inconsistent style and only was there for debug purposes :)

Comment on See if a certain package has a certain sub, without executing that sub
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Re: See if a certain package has a certain sub, without executing that sub
by gmax (Abbot) on Mar 20, 2004 at 15:27 UTC

    Use the can method (it's a method of the UNIVERSAL package, so every class in Perl has access to it).

    #!/usr/bin/perl

use strict;
use warnings;

package Communications;

sub SendMessage {
    my $packages = shift;
    my $msg = shift;
    for (@$packages) {
        if ($_->can('ReceiveMessage') ) {
            $_->ReceiveMessage($msg);
        }
        else {
            print "$_ can't ReceiveMessage\n"
        }
    }
}

sub ReceiveMessage {
    print __PACKAGE__, "::ReceiveMessage('$_[1]')\n";
}

package Second;

sub ReceiveMessage {
    print __PACKAGE__, "::ReceiveMessage('$_[1]')\n";
}

package Other;

sub nothing {
    print "\n";
}

package main;

my @packages = qw(Communications Other Second);

Communications::SendMessage( \@packages, 'hello');
__END__
output:
Communications::ReceiveMessage('hello')
Other can't ReceiveMessage
Second::ReceiveMessage('hello')

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[reply]
[d/l]
      Wow! This one is great! It's also nice to read:
      "if it can receive message" :)
      Thank you a lot!
[reply]

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