refactor docs

[p5sagit/Import-Into.git] / lib / Import / Into.pm

package Import::Into;

use strict;
use warnings FATAL => 'all';

our $VERSION = '1.001001'; # 1.1.1

sub _prelude {
  my $target = shift;
  my ($package, $file, $line, $level)
    = ref $target         ? @{$target}{qw(package filename line)}
    : $target =~ /[^0-9]/ ? ($target)
                          : (undef, undef, undef, $target);
  if (defined $level) {
    my ($p, $fn, $ln) = caller($level + 2);
    $package ||= $p;
    $file    ||= $fn;
    $line    ||= $ln;
  }
  qq{package $package;\n}
    . ($file ? "#line $line \"$file\"\n" : '')
}

sub _make_action {
  my ($action, $target) = @_;
  my $version = ref $target && $target->{version};
  my $ver_check = $version ? '$_[0]->VERSION($version);' : '';
  eval _prelude($target).qq{sub { $ver_check shift->$action(\@_) }}
    or die "Failed to build action sub to ${action} for ${target}: $@";
}

sub import::into {
  my ($class, $target, @args) = @_;
  _make_action(import => $target)->($class, @args);
}

sub unimport::out_of {
  my ($class, $target, @args) = @_;
  _make_action(unimport => $target)->($class, @args);
}

1;
 
=head1 NAME

Import::Into - import packages into other packages

=head1 SYNOPSIS

  package My::MultiExporter;

  use Import::Into;

  use Thing1 ();
  use Thing2 ();

  # simple
  sub import {
    Thing1->import::into(scalar caller);
  }

  # multiple
  sub import {
    my $target = caller;
    Thing1->import::into($target);
    Thing2->import::into($target, qw(import arguments));
  }

  # by level
  sub import {
    Thing1->import::into(1);
  }

  # with exporter
  use base qw(Exporter);
  sub import {
    shift->export_to_level(1);
    Thing1->import::into(1);
  }

  # no My::MultiExporter == no Thing1
  sub unimport {
    Thing1->unimport::out_of(scalar caller);
  }

You don't need to do anything more clever than this provided you
document that people wanting to re-export your module should also be using
L<Import::Into>.

Note: You do B<not> need to make ayny changes to Thing1 to be able to call
C<import::into> on it. This is a global method, and is callable on any
package (and in fact on any object as well, although it's rarer that you'd
want to do that).

=head1 DESCRIPTION

Writing exporters is a pain. Some use L<Exporter>, some use L<Sub::Exporter>,
some use L<Moose::Exporter>, some use L<Exporter::Declare> ... and some things
are pragmas.

Exporting on someone else's behalf is harder.  The exporters don't provide a
consistent API for this, and pragmas need to have their import method called
directly, since they effect the current unit of compilation.

C<Import::Into> provides global methods to make this painless.

=head1 METHODS

=head2 $package->import::into( $target, @arguments );

A global method, callable on any package.  Imports the given package into
C<$target>.  C<@arguments> are passed along to the package's import method.

C<$target> can be an package name to export to, an integer for the caller level to export to, or a hashref with the following options:

=over 4

=item package

The target package to export to.

=item filename

The apparent filename to export to.  Some exporting modules, such as L<autodie> or L<strictures>, care about the filename they are being imported to.

=item line

The apparent line number to export to.  To be combined with the C<filename> option.

=item level

The caller level to export to.  This will automatically populate the C<package>, C<filename>, and C<line> options, making it the easiest most constent option.

=item version

A version number to check for the module.  The equivalent of specifying the version number on a C<use> line.

=back

=head2 $package->unimport::out_of( $target, @arguments );

Equivalent to C<import::into>, but dispatches to C<$package>'s C<unimport> method instead of C<import>.

=head1 WHY USE THIS MODULE

The APIs for exporting modules aren't consistent.  L<Exporter> subclasses
provide export_to_level, but if they overrode their import method all bets
are off.  L<Sub::Exporter> provides an into parameter but figuring out
something used it isn't trivial. Pragmas need to have their C<import> method
called directly since they affect the current unit of compilation.

It's ... annoying.

However, there is an approach that actually works for all of these types.

  eval "package $target; use $thing;"

will work for anything checking caller, which is everything except pragmas.
But it doesn't work for pragmas - pragmas need:

  $thing->import;

because they're designed to affect the code currently being compiled - so
within an eval, that's the scope of the eval itself, not the module that
just C<use>d you - so

  sub import {
    eval "use strict;"
  }

doesn't do what you wanted, but

  sub import {
    strict->import;
  }

will apply L<strict> to the calling file correctly.

Of course, now you have two new problems - first, that you still need to
know if something's a pragma, and second that you can't use either of
these approaches alone on something like L<Moose> or L<Moo> that's both
an exporter and a pragma.

So, a solution for that is:

  my $sub = eval "package $target; sub { shift->import(\@_) }";
  $sub->($thing, @import_args);

which means that import is called from the right place for pragmas to take
effect, and from the right package for caller checking to work - and so
behaves correctly for all types of exporter, for pragmas, and for hybrids.

Additionally, some import routines check the filename they are being imported
to.  This can be dealt with by generating a L<#line directive|perlsyn/Plain
Old Comments (Not!)> in the eval, which will change what C<caller> reports for
the filename when called in the importer. The filename and line number to use
in the directive then need to be fetched using C<caller>:

  my ($target, $file, $line) = caller(1);
  my $sub = eval qq{
    package $target;
  #line $line "$file"
    sub { shift->import(\@_) }
  };
  $sub->($thing, @import_args);

Remembering all this, however, is excessively irritating. So I wrote a module
so I didn't have to anymore. Loading L<Import::Into> creates a global method
C<import::into> which you can call on any package to import it into another
package. So now you can simply write:

  use Import::Into;

  $thing->import::into($target, @import_args);

This works because of how perl resolves method calls - a call to a simple
method name is resolved against the package of the class or object, so

  $thing->method_name(@args);

is roughly equivalent to:

  my $code_ref = $thing->can('method_name');
  $code_ref->($thing, @args);

while if a C<::> is found, the lookup is made relative to the package name
(i.e. everything before the last C<::>) so

  $thing->Package::Name::method_name(@args);

is roughly equivalent to:

  my $code_ref = Package::Name->can('method_name');
  $code_ref->($thing, @args);

So since L<Import::Into> defines a method C<into> in package C<import>
the syntax reliably calls that.

For more craziness of this order, have a look at the article I wrote at
L<http://shadow.cat/blog/matt-s-trout/madness-with-methods> which covers
coderef abuse and the C<${\...}> syntax.

Final note: You do still need to ensure that you already loaded C<$thing> - if
you're receiving this from a parameter, I recommend using L<Module::Runtime>:

  use Import::Into;
  use Module::Runtime qw(use_module);

  use_module($thing)->import::into($target, @import_args);

And that's it.

=head1 ACKNOWLEDGEMENTS

Thanks to Getty for asking "how can I get C<< use strict; use warnings; >>
turned on for all consumers of my code?" and then "why is this not a
module?!".

=head1 AUTHOR

mst - Matt S. Trout (cpan:MSTROUT) <mst@shadowcat.co.uk>

=head1 CONTRIBUTORS

haarg - Graham Knop (cpan:HAARG) <haarg@haarg.org>

=head1 COPYRIGHT

Copyright (c) 2012 the Import::Into L</AUTHOR> and L</CONTRIBUTORS>
as listed above.

=head1 LICENSE

This library is free software and may be distributed under the same terms
as perl itself.