NAME¶
Sub::Exporter::Cookbook - useful, demonstrative, or stupid Sub::Exporter tricks
VERSION¶
version 0.986
OVERVIEW¶
Sub::Exporter is a fairly simple tool, and can be used to achieve some very
simple goals. Its basic behaviors and their basic application (that is,
"traditional" exporting of routines) are described in
Sub::Exporter::Tutorial and Sub::Exporter. This document presents applications
that may not be immediately obvious, or that can demonstrate how certain
features can be put to use (for good or evil).
THE RECIPES¶
Exporting Methods as Routines¶
With Exporter.pm, exporting methods is a non-starter. Sub::Exporter makes it
simple. By using the "curry_method" utility provided in
Sub::Exporter::Util, a method can be exported with the invocant built in.
package Object::Strenuous;
use Sub::Exporter::Util 'curry_method';
use Sub::Exporter -setup => {
exports => [ objection => curry_method('new') ],
};
With this configuration, the importing code may contain:
my $obj = objection("irrelevant");
...and this will be equivalent to:
my $obj = Object::Strenuous->new("irrelevant");
The built-in invocant is determined by the invocant for the "import"
method. That means that if we were to subclass Object::Strenuous as follows:
package Object::Strenuous::Repeated;
@ISA = 'Object::Strenuous';
...then importing "objection" from the subclass would build-in that
subclass.
Finally, since the invocant can be an object, you can write something like this:
package Cypher;
use Sub::Exporter::Util 'curry_method';
use Sub::Exporter -setup => {
exports => [ encypher => curry_method ],
};
with the expectation that "import" will be called on an instantiated
Cypher object:
BEGIN {
my $cypher = Cypher->new( ... );
$cypher->import('encypher');
}
Now there is a globally-available "encypher" routine which calls the
encypher method on an otherwise unavailable Cypher object.
Exporting Methods as Methods¶
While exporting modules usually export subroutines to be called as subroutines,
it's easy to use Sub::Exporter to export subroutines meant to be called as
methods on the importing package or its objects.
Here's a trivial (and naive) example:
package Mixin::DumpObj;
use Data::Dumper;
use Sub::Exporter -setup => {
exports => [ qw(dump) ]
};
sub dump {
my ($self) = @_;
return Dumper($self);
}
When writing your own object class, you can then import "dump" to be
used as a method, called like so:
$object->dump;
By assuming that the importing class will provide a certain interface, a
method-exporting module can be used as a simple plugin:
package Number::Plugin::Upto;
use Sub::Exporter -setup => {
into => 'Number',
exports => [ qw(upto) ],
groups => [ default => [ qw(upto) ] ],
};
sub upto {
my ($self) = @_;
return 1 .. abs($self->as_integer);
}
The "into" line in the configuration says that this plugin will
export, by default, into the Number package, not into the "use"-ing
package. It can be exported anyway, though, and will work as long as the
destination provides an "as_integer" method like the one it expects.
To import it to a different destination, one can just write:
use Number::Plugin::Upto { into => 'Quantity' };
Mixing-in Complex External Behavior¶
When exporting methods to be used as methods (see above), one very powerful
option is to export methods that are generated routines that maintain an
enclosed reference to the exporting module. This allows a user to import a
single method which is implemented in terms of a complete, well-structured
package.
Here is a very small example:
package Data::Analyzer;
use Sub::Exporter -setup => {
exports => [ analyze => \'_generate_analyzer' ],
};
sub _generate_analyzer {
my ($mixin, $name, $arg, $col) = @_;
return sub {
my ($self) = @_;
my $values = [ $self->values ];
my $analyzer = $mixin->new($values);
$analyzer->perform_analysis;
$analyzer->aggregate_results;
return $analyzer->summary;
};
}
If imported by any package providing a "values" method, this plugin
will provide a single "analyze" method that acts as a simple
interface to a more complex set of behaviors.
Even more importantly, because the $mixin value will be the invocant on which
the "import" was actually called, one can subclass
"Data::Analyzer" and replace only individual pieces of the complex
behavior, making it easy to write complex, subclassable toolkits with simple
single points of entry for external interfaces.
Exporting Constants¶
While Sub::Exporter isn't in the constant-exporting business, it's easy to
export constants by using one of its sister modules, Package::Generator.
package Important::Constants;
use Sub::Exporter -setup => {
collectors => [ constants => \'_set_constants' ],
};
sub _set_constants {
my ($class, $value, $data) = @_;
Package::Generator->assign_symbols(
$data->{into},
[
MEANING_OF_LIFE => \42,
ONE_TRUE_BASE => \13,
FACTORS => [ 6, 9 ],
],
);
return 1;
}
Then, someone can write:
use Important::Constants 'constants';
print "The factors @FACTORS produce $MEANING_OF_LIFE in $ONE_TRUE_BASE.";
(The constants must be exported via a collector, because they are effectively
altering the importing class in a way other than installing subroutines.)
Altering the Importer's @ISA¶
It's trivial to make a collector that changes the inheritance of an importing
package:
use Sub::Exporter -setup => {
collectors => { -base => \'_make_base' },
};
sub _make_base {
my ($class, $value, $data) = @_;
my $target = $data->{into};
push @{"$target\::ISA"}, $class;
}
Then, the user of your class can write:
use Some::Class -base;
and become a subclass. This can be quite useful in building, for example, a
module that helps build plugins. We may want a few utilities imported, but we
also want to inherit behavior from some base plugin class;
package Framework::Util;
use Sub::Exporter -setup => {
exports => [ qw(log global_config) ],
groups => [ _plugin => [ qw(log global_config) ]
collectors => { '-plugin' => \'_become_plugin' },
};
sub _become_plugin {
my ($class, $value, $data) = @_;
my $target = $data->{into};
push @{"$target\::ISA"}, $class->plugin_base_class;
push @{ $data->{import_args} }, '-_plugin';
}
Now, you can write a plugin like this:
package Framework::Plugin::AirFreshener;
use Framework::Util -plugin;
Eating Exporter.pm's Brain¶
You probably shouldn't actually do this in production. It's offered more as a
demonstration than a suggestion.
sub exporter_upgrade {
my ($pkg) = @_;
my $new_pkg = "$pkg\::UsingSubExporter";
return $new_pkg if $new_pkg->isa($pkg);
Sub::Exporter::setup_exporter({
as => 'import',
into => $new_pkg,
exports => [ @{"$pkg\::EXPORT_OK"} ],
groups => {
%{"$pkg\::EXPORT_TAG"},
default => [ @{"$pkg\::EXPORTS"} ],
},
});
@{"$new_pkg\::ISA"} = $pkg;
return $new_pkg;
}
This routine, given the name of an existing package configured to use
Exporter.pm, returns the name of a new package with a Sub::Exporter-powered
"import" routine. This lets you import
"Toolkit::exported_sub" into the current package with the name
"foo" by writing:
BEGIN {
require Toolkit;
exporter_upgrade('Toolkit')->import(exported_sub => { -as => 'foo' })
}
If you're feeling particularly naughty, this routine could have been declared in
the UNIVERSAL package, meaning you could write:
BEGIN {
require Toolkit;
Toolkit->exporter_upgrade->import(exported_sub => { -as => 'foo' })
}
The new package will have all the same exporter configuration as the original,
but will support export and group renaming, including exporting into scalar
references. Further, since Sub::Exporter uses "can" to find the
routine being exported, the new package may be subclassed and some of its
exports replaced.
AUTHOR¶
Ricardo Signes <rjbs@cpan.org>
COPYRIGHT AND LICENSE¶
This software is copyright (c) 2007 by Ricardo Signes.
This is free software; you can redistribute it and/or modify it under the same
terms as the Perl 5 programming language system itself.