Sub::Exporter::CookbooUsermContributed Perl DocumeSub::Exporter::Cookbook(3pm)NAMESub::Exporter::Cookbook - useful, demonstrative, or stupid
Sub::Exporter tricks
VERSION
version 0.987
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.
perl v5.26.1 2018-01-01 Sub::Exporter::Cookbook(3pm)
