Module setup style and BEGIN blocks

[tim.largy@gmail.com]

When writing Perl modules, there are several templates floating around
(in books, man pages, and so on) and one major difference between them
is in the use of BEGIN blocks. I don't understand why some templates
put Exporter-related things in the BEGIN block, and others do not.
Consider the template provided by the perlmod manpage :

package Some::Module; # assumes Some/Module.pm

use strict;
use warnings;

BEGIN {
use Exporter ();
our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);

# set the version for version checking
$VERSION = 1.00;
# if using RCS/CVS, this may be preferred
$VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~
/(\d+)/g;

@ISA = qw(Exporter);
@EXPORT = qw(&func1 &func2 &func4);
%EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2!
],

# your exported package globals go here,
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit &func3);
}
our @EXPORT_OK;

[remainder clipped]

Why put all of these things in the BEGIN block instead of outside, like
the template in the _Perl Cookbook_, 2nd Edition (O'Reilly)?

Tim

[Eric R. Meyers]

[email]tim.largy@gmail.com[/email] wrote:

> When writing Perl modules, there are several templates floating around
> (in books, man pages, and so on) and one major difference between them
> is in the use of BEGIN blocks. I don't understand why some templates
> put Exporter-related things in the BEGIN block, and others do not.
> Consider the template provided by the perlmod manpage :
>
> package Some::Module; # assumes Some/Module.pm
>
> use strict;
> use warnings;
>
> BEGIN {
> use Exporter ();
> our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
>
> # set the version for version checking
> $VERSION = 1.00;
> # if using RCS/CVS, this may be preferred
> $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~
> /(\d+)/g;
>
> @ISA = qw(Exporter);
> @EXPORT = qw(&func1 &func2 &func4);
> %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2!
> ],
>
> # your exported package globals go here,
> # as well as any optionally exported functions
> @EXPORT_OK = qw($Var1 %Hashit &func3);
> }
> our @EXPORT_OK;
>
> [remainder clipped]
>
> Why put all of these things in the BEGIN block instead of outside, like
> the template in the _Perl Cookbook_, 2nd Edition (O'Reilly)?
>
> Tim

Hi Tim,

First things first.

Everything you do should be Focused on Function(ality), Readability and
Maintainability, with a little Class, Humor and Style thrown into your code
to make it enjoyable Function and Edutainment for yourself and others. I
don't believe that Good Style is as important to emulate, as is Good
Function and Good Timing, so forget the Style for the most part. If you
like the compact K&R Style, then God Bless you, but I like the better
Readability of the ANSI Style.

If you look at 'perldoc -f use,' the use just means BEGIN { require Module;
import Module LIST; }. So, BEGIN { use Exporter qw() }, just means BEGIN
{ BEGIN{ require Exporter; import Exporter qw() } }.

If you specify just 'require Exporter;,' then you just need to access
Exporter components as 'Exporter::component,' rather than just specifying
'component,' if you had just specified 'import Exporter qw(component);.'

Just got that?

Except, and there is only one reason for an except, the BEGIN or END.

BEGIN {
##
## K&R Style here, not ANSI. Sometimes BEGIN/END only Function this way.
##

##
## In the Beginning.
##
## God ("Mr./The" Exporter) may have said something like:
##

require Exporter; import Exporter;

##
## This is my Dawning Moment of Creation. I BEGIN! Thanks God!
##
## I hope that I may get Blessed someday, and someone uses me for Good.
##
## I'm now using all of the Exporter components that I've been given.
##
## I require Exporter, so that I get it from @INC, and I import Exporter
## to get to access its components on a first name basis.
##
## Everything in @Exporter::EXPORT I got by name with the import.
## Everything in @Exporter::EXPORT_OK I'll get by name, as needed.
##

##
## Exporter, VERSION or VCS related stuff here.
##
## I'm becoming another version of an Exporter, and
## I'll have a real VERSION someday, because I'll be Complex.
##
## If I'm not Complex, I'll have one.
##

} ## end BEGIN

END
{ ##<-- this could be a Dis-Functional Style of END {} / BEGIN {}.
die( "Sorry, no will:$!" );
} ## end END

That is perfect! I was so nice to it. But the code Style above was faulty.

You should use 'my' instead of 'our,' because these are 'my' components and
no one else's by a Gentlemen's Agreement not to invade each others
namespace. Except, if you are Agreegating intentionally.

You should not have anything in @EXPORT, but the things that you just feel
compelled to force into another person's namespace. It's forcing yourself.
Instead the @EXPORT_OK, let's us take what we want, as we need it from you.

@EXPORT_TAGS lets you define classes of imported components, when asked
specifically for a specific class of components, like ':all.' When the
namespace above asks for ':all,' then you didn't force your components upon
their namespace, because the asked for ':all' of them.

I hope this helps you to understand it.

May God Bless You,

Eric
An Exporter.