allow optional renaming of exported generator

[p5sagit/Package-Variant.git] / lib / Package / Variant.pm

package Package::Variant;

use strictures 1;

our %Variable;

sub import {
  my $target = caller;
  my $me = shift;
  my $last = (split '::', $target)[-1];
  my $anon = 'A000';
  my $variable = $target;
  my %args = @_;
  no strict 'refs';
  $Variable{$variable} = {
    anon => $anon,
    args => \%args,
    subs => {
      map +($_ => sub {}), @{$args{subs}||[]},
    },
  };
  *{"${target}::import"} = sub {
    my $target = caller;
    my (undef, %arg) = @_;
    my $as = defined($arg{as}) ? $arg{as} : $last;
    no strict 'refs';
    *{"${target}::${as}"} = sub {
      $me->build_variant_of($variable, @_);
    };
  };
  my $subs = $Variable{$variable}{subs};
  foreach my $name (keys %$subs) {
    *{"${target}::${name}"} = sub {
      goto &{$subs->{$name}}
    };
  }
  *{"${target}::install"} = sub {
    goto &{$Variable{$variable}{install}};
  }
}

sub build_variant_of {
  my ($me, $variable, @args) = @_;
  my $variant_name = "${variable}::_Variant_".++$Variable{$variable}{anon};
  my $import = $Variable{$variable}{args}{importing} || {};
  my $setup = join("\n",
    "package ${variant_name};",
    (map sprintf(
      q!use %s @{$import->{'%s'}||[]};!, $_, quotemeta($_),
    ), keys %$import),
    "1;",
  );
  eval $setup
    or die "evaling ${setup} failed: $@";
  my $subs = $Variable{$variable}{subs};
  local @{$subs}{keys %$subs} = map $variant_name->can($_), keys %$subs;
  local $Variable{$variable}{install} = sub {
    my ($name, $ref) = @_;
    no strict 'refs';
    *{"${variant_name}::${name}"} = $ref;
  };
  $variable->make_variant($variant_name, @args);
  return $variant_name;
}

1;

__END__

=head1 NAME

Package::Variant - Parameterizable packages

=head1 SYNOPSIS

  # declaring a variable Moo role
  package My::Role::ObjectAttr;
  use strictures 1;
  use Package::Variant
    # what modules to 'use'
    importing => { 'Moo::Role' => [] },
    # proxied subroutines
    subs => [qw( has around before after extends )],

  sub make_variant {
    my ($class, $target_package, %arguments) = @_;
    # access arguments
    my $name = $arguments{name};
    # use proxied 'has' to add an attribute
    has $name => (is => 'lazy');
    # install a builder method
    install "_build_${name}" => sub {
      return $arguments{class}->new;
    };
  }

  # using the role
  package My::Class::WithObjectAttr;
  use strictures 1;
  use Moo;
  use My::Role::ObjectAttr;

  with ObjectAttr(name => 'some_obj', class => 'Some::Class');

  # using our class
  my $obj = My::Class::WithObjectAttr->new;
  $obj->some_obj; # returns a Some::Class instance

=head1 DESCRIPTION

This module allows you to build packages that return different variations
depending on what parameters are given.

Users of your package will receive a subroutine able to take parameters
and return the name of a suitable variant package. The implmenetation does
not care about what kind of package it builds.

=head2 Declaring a variable package

There are two important parts to creating a variable package. You first
have to give C<Package::Variant> some basic information about what kind of
package you want to provide, and how. The second part is implementing a
method receiving the user's arguments and generating your variants.

=head3 Setting up the environment for building variations

When you C<use Package::Variant>, you pass along some arguments that
describe how you intend to build your variations.

  use Package::Variant
    importing => { $package => \@import_arguments, ... },
    subs      => [ @proxied_subroutine_names ];

The L</importing> option needs to be a hash reference with package names
to be C<use>d as keys, and array references containing the import
arguments as values. These packages will be imported into every new
variant, and need to set up every declarative subroutine you require to
build your variable package. The next option will allow you to use these
functions.

The L</subs> option is an array reference of subroutine names that are
exported by the packages specified with L</importing>. These subroutines
will be proxied from your declaration package to the variant to be
generated.

With L</importing> initializing your package and L</subs> declaring what
subroutines you want to use to build a variant, you can now write a
L</make_variant> method building your variants.

=head3 Declaring a method to produce variants

Every time a user requests a new variant a method named L</make_variant>
will be called with the name of the target package and the arguments from
the user.

It can then use the proxied subroutines declared with L</subs> to
customize the new package. An L</install> subroutine is exported as well
allowing you to dynamically install methods into the new package. If these
options aren't flexible enough, you can use the passed name of the new
package to do any other kind of customizations.

  sub make_variant {
    my ($class, $target, @arguments) = @_;
    # ...
    # customization goes here
    # ...
  }

When the method is finished, the user will receive the name of the new
package variant you just set up.

=head2 Using variable packages

After your variable package is L<created|/Declaring a variable package>
your users can get a variant generating subroutine by simply importing
your package.

  use My::Variant;
  my $new_variant_package = Variant( @variant_arguments );

The package is now fully initialized and used. You can import the
subroutine under a different name by specifying an C<as> argument.

=head2 Dynamic creation of variant packages

For regular uses, the L<normal import|/Using variable packages> provides
more than enough flexibility. However, if you want to create variations of
dynamically determined packages, you can use the L</build_variation_of>
method.

You can use this to create variations of other packages and pass arguments
on to them to allow more modular and extensible variations.

=head1 OPTIONS

These are the options that can be passed when importing
C<Package::Variant>. They describe the environment in which the variants
are created.

  use Package::Variant
    importing => { $package => \@import_arguments, ... },
    subs      => [ @proxied_subroutines ];

=head2 importing

This option is a hash reference mapping package names to array references
containing import arguments. The packages will be C<use>d with the given
arguments by every variation before the L</make_variant> method is asked
to create the package.

=head2 subs

An array reference of strings listing the names of subroutines that should
be proxied. These subroutines are expected to be installed into the new
variant package by the modules imported with L</importing>. Subroutines
with the same name will be availabe in your declaration package, and will
proxy through to the newly created package when used within
L</make_variant>.

=head1 VARIABLE PACKAGE METHODS

These are methods on the variable package you declare when you import
C<Package::Variant>.

=head2 make_variant

  Some::Variant::Package->make_variant( $target, @arguments );

B<You need to provide this method.> This method will be called for every
new variant of your package. This method should use the subroutines
declared in L</subs> to customize the new variant package.

This is a class method receiving the C<$target> package and the
C<@arguments> defining the requested variant.

=head2 import

  use Some::Variant::Package;
  my $variant_package = Package( @arguments );

This method is provided for you. It will allow a user to C<use> your
package and receive a subroutine taking C<@arguments> defining the variant
and returning the name of the newly created variant package.

The following options can be specified when importing:

=over

=item * B<as>

  use Some::Variant::Package as => 'Foo';
  my $variant_package = Foo( @arguments );

Exports the generator subroutine under a different name than the default.

=back

=head1 C<Package::Variant> METHODS

These methods are available on C<Package::Variant> itself.

=head2 build_variation_of

  my $variant_package = Package::Variant
    ->build_variation_of( $variable_package, @arguments );

This is the dynamic method of creating new variants. It takes the
C<$variable_package>, which is a pre-declared variable package, and a set
of C<@arguments> passed to the package to generate a new
C<$variant_package>, which will be returned.

=head2 import

  use Package::Variant @options;

Sets up the environment in which you declare the variants of your
packages. See L</OPTIONS> for details on the available options and
L</EXPORTS> for a list of exported subroutines.

=head1 EXPORTS

Additionally to the proxies for subroutines provided in L</subs>, the
following exports will be available in your variable package:

=head2 install

  install( $method_name, $code_reference );

Installs a method with the given C<$method_name> into the newly created
variant package. The C<$code_reference> will be used as the body for the
method.

=head1 AUTHOR

=over

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

=back

=head1 COPYRIGHT

Copyright (c) 2010-2011 the C<Package::Stash> L</AUTHOR> as listed above.

=head1 LICENSE

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

=cut