Sub::Exporter::Tutorial(3) User Contributed Perl Documentation
NAME
Sub::Exporter::Tutorial - a friendly guide to exporting with
Sub::Exporter
VERSION
version 0.990
DESCRIPTION
What's an Exporter?
When you "use" a module, first it is required, then its "import" method
is called. The Perl documentation tells us that the following two
lines are equivalent:
use Module LIST;
BEGIN { require Module; Module->import(LIST); }
The method named "import" is the module's exporter, it exports
functions and variables into its caller's namespace.
The Basics of Sub::Exporter
Sub::Exporter builds a custom exporter which can then be installed into
your module. It builds this method based on configuration passed to
its "setup_exporter" method.
A very basic use case might look like this:
package Addition;
use Sub::Exporter;
Sub::Exporter::setup_exporter({ exports => [ qw(plus) ]});
sub plus { my ($x, $y) = @_; return $x + $y; }
This would mean that when someone used your Addition module, they could
have its "plus" routine imported into their package:
use Addition qw(plus);
my $z = plus(2, 2); # this works, because now plus is in the main package
That syntax to set up the exporter, above, is a little verbose, so for
the simple case of just naming some exports, you can write this:
use Sub::Exporter -setup => { exports => [ qw(plus) ] };
...which is the same as the original example -- except that now the
exporter is built and installed at compile time. Well, that and you
typed less.
Using Export Groups
You can specify whole groups of things that should be exportable
together. These are called groups. Exporter calls these tags. To
specify groups, you just pass a "groups" key in your exporter
configuration:
package Food;
use Sub::Exporter -setup => {
exports => [ qw(apple banana beef fluff lox rabbit) ],
groups => {
fauna => [ qw(beef lox rabbit) ],
flora => [ qw(apple banana) ],
}
};
Now, to import all that delicious foreign meat, your consumer needs
only to write:
use Food qw(:fauna);
use Food qw(-fauna);
Either one of the above is acceptable. A colon is more traditional,
but barewords with a leading colon can't be enquoted by a fat arrow.
We'll see why that matters later on.
Groups can contain other groups. If you include a group name (with the
leading dash or colon) in a group definition, it will be expanded
recursively when the exporter is called. The exporter will not recurse
into the same group twice while expanding groups.
There are two special groups: "all" and "default". The "all" group is
defined for you and contains all exportable subs. You can redefine it,
if you want to export only a subset when all exports are requested.
The "default" group is the set of routines to export when nothing
specific is requested. By default, there is no "default" group.
Renaming Your Imports
Sometimes you want to import something, but you don't like the name as
which it's imported. Sub::Exporter can rename your imports for you.
If you wanted to import "lox" from the Food package, but you don't like
the name, you could write this:
use Food lox => { -as => 'salmon' };
Now you'd get the "lox" routine, but it would be called salmon in your
package. You can also rename entire groups by using the "prefix"
option:
use Food -fauna => { -prefix => 'cute_little_' };
Now you can call your "cute_little_rabbit" routine. (You can also call
"cute_little_beef", but that hardly seems as enticing.)
When you define groups, you can include renaming.
use Sub::Exporter -setup => {
exports => [ qw(apple banana beef fluff lox rabbit) ],
groups => {
fauna => [ qw(beef lox), rabbit => { -as => 'coney' } ],
}
};
A prefix on a group like that does the right thing. This is when it's
useful to use a dash instead of a colon to indicate a group: you can
put a fat arrow between the group and its arguments, then.
use Food -fauna => { -prefix => 'lovely_' };
eat( lovely_coney ); # this works
Prefixes also apply recursively. That means that this code works:
use Sub::Exporter -setup => {
exports => [ qw(apple banana beef fluff lox rabbit) ],
groups => {
fauna => [ qw(beef lox), rabbit => { -as => 'coney' } ],
allowed => [ -fauna => { -prefix => 'willing_' }, 'banana' ],
}
};
...
use Food -allowed => { -prefix => 'any_' };
$dinner = any_willing_coney; # yum!
Groups can also be passed a "-suffix" argument.
Finally, if the "-as" argument to an exported routine is a reference to
a scalar, a reference to the routine will be placed in that scalar.
Building Subroutines to Order
Sometimes, you want to export things that you don't have on hand. You
might want to offer customized routines built to the specification of
your consumer; that's just good business! With Sub::Exporter, this is
easy.
To offer subroutines to order, you need to provide a generator when you
set up your exporter. A generator is just a routine that returns a new
routine. perlref is talking about these when it discusses closures and
function templates. The canonical example of a generator builds a
unique incrementor; here's how you'd do that with Sub::Exporter;
package Package::Counter;
use Sub::Exporter -setup => {
exports => [ counter => sub { my $i = 0; sub { $i++ } } ],
groups => { default => [ qw(counter) ] },
};
Now anyone can use your Package::Counter module and he'll receive a
"counter" in his package. It will count up by one, and will never
interfere with anyone else's counter.
This isn't very useful, though, unless the consumer can explain what he
wants. This is done, in part, by supplying arguments when importing.
The following example shows how a generator can take and use arguments:
package Package::Counter;
sub _build_counter {
my ($class, $name, $arg) = @_;
$arg ||= {};
my $i = $arg->{start} || 0;
return sub { $i++ };
}
use Sub::Exporter -setup => {
exports => [ counter => \'_build_counter' ],
groups => { default => [ qw(counter) ] },
};
Now, the consumer can (if he wants) specify a starting value for his
counter:
use Package::Counter counter => { start => 10 };
Arguments to a group are passed along to the generators of routines in
that group, but Sub::Exporter arguments -- anything beginning with a
dash -- are never passed in. When groups are nested, the arguments are
merged as the groups are expanded.
Notice, too, that in the example above, we gave a reference to a method
name rather than a method implementation. By giving the name rather
than the subroutine, we make it possible for subclasses of our
"Package::Counter" module to replace the "_build_counter" method.
When a generator is called, it is passed four parameters:
o the invocant on which the exporter was called
o the name of the export being generated (not the name it's being
installed as)
o the arguments supplied for the routine
o the collection of generic arguments
The fourth item is the last major feature that hasn't been covered.
Argument Collectors
Sometimes you will want to accept arguments once that can then be
available to any subroutine that you're going to export. To do this,
you specify collectors, like this:
package Menu::Airline
use Sub::Exporter -setup => {
exports => ... ,
groups => ... ,
collectors => [ qw(allergies ethics) ],
};
Collectors look like normal exports in the import call, but they don't
do anything but collect data which can later be passed to generators.
If the module was used like this:
use Menu::Airline allergies => [ qw(peanuts) ], ethics => [ qw(vegan) ];
...the consumer would get a salad. Also, all the generators would be
passed, as their fourth argument, something like this:
{ allerges => [ qw(peanuts) ], ethics => [ qw(vegan) ] }
Generators may have arguments in their definition, as well. These must
be code refs that perform validation of the collected values. They are
passed the collection value and may return true or false. If they
return false, the exporter will throw an exception.
Generating Many Routines in One Scope
Sometimes it's useful to have multiple routines generated in one scope.
This way they can share lexical data which is otherwise unavailable.
To do this, you can supply a generator for a group which returns a
hashref of names and code references. This generator is passed all the
usual data, and the group may receive the usual "-prefix" or "-suffix"
arguments.
PERL VERSION
This library should run on perls released even a long time ago. It
should work on any version of perl released in the last five years.
Although it may work on older versions of perl, no guarantee is made
that the minimum required version will not be increased. The version
may be increased for any reason, and there is no promise that patches
will be accepted to lower the minimum required perl.
SEE ALSO
o Sub::Exporter(3) for complete documentation and references to
other exporters
AUTHOR
Ricardo Signes <cpan@semiotic.systems>
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 v5.34.1 2023-07-22 Sub::Exporter::Tutorial(3)
sub-exporter 0.990.0 - Generated Sun Aug 20 16:32:22 CDT 2023