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Manual Reference Pages  -  PACKAGE::VARIANT (3)

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Package::Variant - Parameterizable packages



Creation of anonymous variants:

  # declaring a variable Moo role
  package My::VariableRole::ObjectAttr;
  use strictures 2;
  use Package::Variant
    # what modules to use
    importing => [Moo::Role],
    # proxied subroutines
    subs => [ qw(has around before after with) ];

  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 2;
  use Moo;
  use My::VariableRole::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

And the same thing, only with named variants:

  # declaring a variable Moo role that can be named
  package My::VariableRole::ObjectAttrNamed;
  use strictures 2;
  use Package::Variant importing => [Moo::Role],
    subs => [ qw(has around before after with) ];
  use Module::Runtime module_notional_filename; # only if you need protection

  # this method is run at variant creation time to determine its custom
  # package name. it can use the arguments or do something entirely else.
  sub make_variant_package_name {
    my ($class, $package, %arguments) = @_;
    $package = "Private::$package"; # you can munge the input here if you like
    # only if you *need* protection
    die "Wont clobber $package" if $INC{module_notional_filename $package};
    return $package;

  # same as in the example above, except for the argument list. in this example
  # $package is the user input, and
  # $target_package is the actual package in which the variant gets installed
  sub make_variant {
    my ($class, $target_package, $package, %arguments) = @_;
    my $name = $arguments{name};
    has $name => (is => lazy);
    install "_build_${name}" => sub {return $arguments{class}->new};

  # using the role
  package My::Class::WithObjectAttr;
  use strictures 2;
  use Moo;
  use My::VariableRole::ObjectAttrNamed;

  # create the role under a specific name
  ObjectAttrNamed "My::Role" => (name => some_obj, class => Some::Class);
  # and use it
  with "Private::My::Role";

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


This module allows you to build a variable package that contains a package template and can use it to build variant packages at runtime.

Your variable package will export a subroutine which will build a variant package, combining its arguments with the template, and return the name of the new variant package.

The implementation does not care about what kind of packages it builds, be they simple function exporters, classes, singletons or something entirely different.

    Declaring a variable package

There are two important parts to creating a variable package. You first have to give Package::Variant some basic information about what kind of variant packages you want to provide, and how. The second part is implementing a method which builds the components of the variant packages that use the user’s arguments or cannot be provided with a static import.

Setting up the environment for building variants

When you use Package::Variant, you pass along some arguments that describe how you intend to build your variants.

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

The importing option needs to be a hash or array reference with package names to be used as keys, and array references containing the import arguments as values. These packages will be imported into every new variant package, to provide static functionality of the variant packages and to set up every declarative subroutine you require to build variants package components. The next option will allow you to use these functions. See importing for more options. You can omit empty import argument lists when passing an array reference.

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

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

Declaring a method to produce variants

Every time a user requests a new variant, a method named 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 subs to customize the variant package. An install subroutine is exported as well allowing you to dynamically install methods into the variant package. If these options aren’t flexible enough, you can use the passed name of the variant 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 variant package you just set up.

    Using variable packages

After your variable package is created your users can get a variant generator subroutine by simply importing your package.

  use My::Variant;
  my $new_variant_package = Variant(@variant_arguments);
  # the variant package is now fully initialized and used

You can import the subroutine under a different name by specifying an as argument.

    Dynamic creation of variant packages

For regular uses, the normal import provides more than enough flexibility. However, if you want to create variants of dynamically determined packages, you can use the build_variant_of method.

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


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

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


This option is a hash reference mapping package names to array references containing import arguments. The packages will be imported with the given arguments by every variant before the make_variant method is asked to create the package (this is done using Import::Into).

If import order is important to you, you can also pass the importing arguments as a flat array reference:

  use Package::Variant
    importing => [ PackageA, PackageB ];

  # same as
  use Package::Variant
    importing => [ PackageA => [], PackageB => [] ];

  # or
  use Package::Variant
    importing => { PackageA => [], PackageB => [] };

The import method will be called even if the list of import arguments is empty or not specified,

If you just want to import a single package’s default exports, you can also pass a string instead:

  use Package::Variant importing => Package;


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 importing. Subroutines with the same name will be available in your variable package, and will proxy through to the newly created package when used within make_variant.


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


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

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

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


  Some::Variant::Package->make_variant_package_name( @arguments );

<B>You may optionally provide this method.B> If present, this method will be used to determine the package name for a particular variant being constructed.

If you do not implement it, a unique package name something like


will be created for you.


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

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

The following options can be specified when importing:
o <B>asB>

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

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


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

This method is provided for you. It will generate a variant package and return its name, just like the generator sub provided by import. This allows you to avoid importing anything into the consuming package.

Package::Variant METHODS

These methods are available on Package::Variant itself.


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

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


  use Package::Variant @options;

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


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


  install($method_name, $code_reference);

Installs a method with the given $method_name into the newly created variant package. The $code_reference will be used as the body for the method, and if Sub::Name is available the coderef will be named. If you want to name it something else, then use:

  install($method_name, $name_to_use, $code_reference);


mst - Matt S. Trout (cpan:MSTROUT) <>


phaylon - Robert Sedlacek (cpan:PHAYLON) <>

haarg - Graham Knop (cpan:HAARG) <>


Copyright (c) 2010-2012 the Package::Variant AUTHOR and CONTRIBUTORS as listed above.


This library is free software and may be distributed under the same terms as perl itself.
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perl v5.20.3 PACKAGE::VARIANT (3) 2015-07-25

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