GSP
Quick Navigator

Search Site

Unix VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Support
Contact Us
Online Help
Handbooks
Domain Status
Man Pages

FAQ
Virtual Servers
Pricing
Billing
Technical

Network
Facilities
Connectivity
Topology Map

Miscellaneous
Server Agreement
Year 2038
Credits
 

USA Flag

 

 

Man Pages


Manual Reference Pages  -  MOOSE::COOKBOOK::BASICS::HTTP_SUBTYPESANDCOERCION (3)

.ds Aq ’

NAME

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

CONTENTS

VERSION

version 2.1605

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 <B>HTTP::HeadersB> 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

o Stevan Little <stevan.little@iinteractive.com>
o Dave Rolsky <autarch@urth.org>
o Jesse Luehrs <doy@tozt.net>
o Shawn M Moore <code@sartak.org>
o XXXX XXXXX (Yuval Kogman) <nothingmuch@woobling.org>
o Karen Etheridge <ether@cpan.org>
o Florian Ragwitz <rafl@debian.org>
o Hans Dieter Pearcey <hdp@weftsoar.net>
o Chris Prather <chris@prather.org>
o Matt S Trout <mst@shadowcat.co.uk>

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.

Search for    or go to Top of page |  Section 3 |  Main Index


perl v5.20.3 MOOSE::COOKBOOK::BASICS::HTTP_SUBTYPESANDCOERCION (3) 2016-02-16

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.