Provided by: libmoose-perl_2.1604-1build1_amd64 
NAME
Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates subtypes and coercion use
HTTP-related classes (Request, Protocol, etc.)
VERSION
version 2.1604
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.little@iinteractive.com>
• Dave Rolsky <autarch@urth.org>
• Jesse Luehrs <doy@tozt.net>
• Shawn M Moore <code@sartak.org>
• יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
• Karen Etheridge <ether@cpan.org>
• Florian Ragwitz <rafl@debian.org>
• Hans Dieter Pearcey <hdp@weftsoar.net>
• Chris Prather <chris@prather.org>
• 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.
perl v5.22.1 Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion(3pm)