Perl Module Basics

When my fellow BOA developers see this message, they'll likely jump to the 

conclusion that there's something wrong with BOA::Logger. They'll send me angry 

e mails and I'll be forced to sign them up for spam.

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 Nobody wants that, and 

thankfully the situation can be avoided. The Carp module, which comes with Perl, 

can be used to place the blame where it belongs. Here's a new version of the module 

header and open_log() using Carp:

package BOA::Logger;

use Carp qw(croak);

sub open_log {

   my $filename = shift;

   open(LOG_FILE, ">>$filename") or croak("Unable to open $filename : $!");

   print LOG_FILE "BOA log started: " . localtime(time) . "\n";

}

Now the blame is properly placed and the resulting error is

Unable to open /path/to/log : No such file or directory at caller.pl line 5

The croak() routine provides a die() replacement that assigns blame to the 

caller of the subroutine. The Carp module also provides a warn() replacement 

called carp(), as well as routines to generate full back traces. See the Carp docu 

mentation for more details; you can access it with the command perldoc Carp.

Object Oriented Modules

As previously mentioned, BOA is a big ol' application. In fact, it's so big that just 

one log file will not be enough. There are several subsystems (GUI, Network, Database, 

and so on) that each need their own log files with independent log levels. One way to 

address this would be to create a new package for each log file and copy and 

paste the code from BOA::Logger into each one creating BOA::Logger::GUI, 

BOA::Logger::Network, and so on. This approach has some obvious drawbacks 

the code becomes harder to maintain since a change in once place has to be care 

fully replicated in each copy. Also, it would be difficult to use multiple BOA::Logger 

clones at the same time they all want to export write_log(), so you'd have to 

forgo exporting and type the whole package name for every call.

There is an easier way. Instead of creating a new package just to hold some 

state information, you'll create an object oriented module that provides an object 

for each log file. These objects will contain the state necessary to support a single 

11. I recommend Oprah's book club mailing list.

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