Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - phpMan

NAME
    Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates
    subtypes and coercion use HTTP-related classes (Request, Protocol, etc.)

VERSION
    version 2.2200

SYNOPSIS
      package Request;
      use Moose;
      use Moose::Util::TypeConstraints;

      use HTTP::Headers  ();
      use Params::Coerce ();
      use URI            ();

      subtype 'My::Types::HTTP::Headers' => as class_type('HTTP::Headers');

      coerce 'My::Types::HTTP::Headers'
          => from 'ArrayRef'
              => via { HTTP::Headers->new( @{$_} ) }
          => from 'HashRef'
              => via { HTTP::Headers->new( %{$_} ) };

      subtype 'My::Types::URI' => as class_type('URI');

      coerce 'My::Types::URI'
          => from 'Object'
              => via { $_->isa('URI')
                       ? $_
                       : Params::Coerce::coerce( 'URI', $_ ); }
          => from 'Str'
              => via { URI->new( $_, 'http' ) };

      subtype 'Protocol'
          => as 'Str'
          => where { /^HTTP\/[0-9]\.[0-9]$/ };

      has 'base' => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
      has 'uri'  => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
      has 'method'   => ( is => 'rw', isa => 'Str' );
      has 'protocol' => ( is => 'rw', isa => 'Protocol' );
      has 'headers'  => (
          is      => 'rw',
          isa     => 'My::Types::HTTP::Headers',
          coerce  => 1,
          default => sub { HTTP::Headers->new }
      );

DESCRIPTION
    This recipe introduces type coercions, which are defined with the
    "coerce" sugar function. Coercions are attached to existing type
    constraints, and define a (one-way) transformation from one type to
    another.

    This is very powerful, but it can also have unexpected consequences, so
    you have to explicitly ask for an attribute to be coerced. To do this,
    you must set the "coerce" attribute option to a true value.

    First, we create the subtype to which we will coerce the other types:

      subtype 'My::Types::HTTP::Headers' => as class_type('HTTP::Headers');

    We are creating a subtype rather than using "HTTP::Headers" as a type
    directly. The reason we do this is that coercions are global, and a
    coercion defined for "HTTP::Headers" in our "Request" class would then
    be defined for *all* Moose-using classes in the current Perl
    interpreter. It's a best practice to avoid this sort of namespace
    pollution.

    The "class_type" sugar function is simply a shortcut for this:

      subtype 'HTTP::Headers'
          => as 'Object'
          => where { $_->isa('HTTP::Headers') };

    Internally, Moose creates a type constraint for each Moose-using class,
    but for non-Moose classes, the type must be declared explicitly.

    We could go ahead and use this new type directly:

      has 'headers' => (
          is      => 'rw',
          isa     => 'My::Types::HTTP::Headers',
          default => sub { HTTP::Headers->new }
      );

    This creates a simple attribute which defaults to an empty instance of
    HTTP::Headers.

    The constructor for HTTP::Headers accepts a list of key-value pairs
    representing the HTTP header fields. In Perl, such a list could be
    stored in an ARRAY or HASH reference. We want our "headers" attribute to
    accept those data structures instead of an HTTP::Headers instance, and
    just do the right thing. This is exactly what coercion is for:

      coerce 'My::Types::HTTP::Headers'
          => from 'ArrayRef'
              => via { HTTP::Headers->new( @{$_} ) }
          => from 'HashRef'
              => via { HTTP::Headers->new( %{$_} ) };

    The first argument to "coerce" is the type *to* which we are coercing.
    Then we give it a set of "from"/"via" clauses. The "from" function takes
    some other type name and "via" takes a subroutine reference which
    actually does the coercion.

    However, defining the coercion doesn't do anything until we tell Moose
    we want a particular attribute to be coerced:

      has 'headers' => (
          is      => 'rw',
          isa     => 'My::Types::HTTP::Headers',
          coerce  => 1,
          default => sub { HTTP::Headers->new }
      );

    Now, if we use an "ArrayRef" or "HashRef" to populate "headers", it will
    be coerced into a new HTTP::Headers instance. With the coercion in
    place, the following lines of code are all equivalent:

      $foo->headers( HTTP::Headers->new( bar => 1, baz => 2 ) );
      $foo->headers( [ 'bar', 1, 'baz', 2 ] );
      $foo->headers( { bar => 1, baz => 2 } );

    As you can see, careful use of coercions can produce a very open
    interface for your class, while still retaining the "safety" of your
    type constraint checks. (1)

    Our next coercion shows how we can leverage existing CPAN modules to
    help implement coercions. In this case we use Params::Coerce.

    Once again, we need to declare a class type for our non-Moose URI class:

      subtype 'My::Types::URI' => as class_type('URI');

    Then we define the coercion:

      coerce 'My::Types::URI'
          => from 'Object'
              => via { $_->isa('URI')
                       ? $_
                       : Params::Coerce::coerce( 'URI', $_ ); }
          => from 'Str'
              => via { URI->new( $_, 'http' ) };

    The first coercion takes any object and makes it a "URI" object. The
    coercion system isn't that smart, and does not check if the object is
    already a URI, so we check for that ourselves. If it's not a URI
    already, we let Params::Coerce do its magic, and we just use its return
    value.

    If Params::Coerce didn't return a URI object (for whatever reason),
    Moose would throw a type constraint error.

    The other coercion takes a string and converts it to a URI. In this
    case, we are using the coercion to apply a default behavior, where a
    string is assumed to be an "http" URI.

    Finally, we need to make sure our attributes enable coercion.

      has 'base' => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );
      has 'uri'  => ( is => 'rw', isa => 'My::Types::URI', coerce => 1 );

    Re-using the coercion lets us enforce a consistent API across multiple
    attributes.

CONCLUSION
    This recipe showed the use of coercions to create a more flexible and
    DWIM-y API. Like any powerful feature, we recommend some caution.
    Sometimes it's better to reject a value than just guess at how to DWIM.

    We also showed the use of the "class_type" sugar function as a shortcut
    for defining a new subtype of "Object".

FOOTNOTES
    (1) This particular example could be safer. Really we only want to
        coerce an array with an *even* number of elements. We could create a
        new "EvenElementArrayRef" type, and then coerce from that type, as
        opposed to a plain "ArrayRef"

AUTHORS
    *   Stevan Little <stevan AT cpan.org>

    *   Dave Rolsky <autarch AT urth.org>

    *   Jesse Luehrs <doy AT cpan.org>

    *   Shawn M Moore <sartak AT cpan.org>

    *   יובל קוג'מן (Yuval Kogman) <nothingmuch AT woobling.org>

    *   Karen Etheridge <ether AT cpan.org>

    *   Florian Ragwitz <rafl AT debian.org>

    *   Hans Dieter Pearcey <hdp AT cpan.org>

    *   Chris Prather <chris AT prather.org>

    *   Matt S Trout <mstrout AT cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2006 by Infinity Interactive, Inc.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.