Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates subtypes and coercion use HTTP-related classes |
Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates subtypes and coercion use HTTP-related classes (Request, Protocol, etc.)
version 2.1605
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 } );
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 { $_->isa('HTTP::Headers') };
Internally, Moose creates a type constraint for each Moose-using class, but for non-Moose classes, the type must be declared explicitly.
We could go ahead and use this new type directly:
has 'headers' => ( is => 'rw', isa => 'My::Types::HTTP::Headers', default => sub { HTTP::Headers->new } );
This creates a simple attribute which defaults to an empty instance of the HTTP::Headers manpage.
The constructor for the HTTP::Headers manpage accepts a list of key-value pairs
representing the HTTP header fields. In Perl, such a list could be
stored in an ARRAY or HASH reference. We want our headers
attribute
to accept those data structures instead of an HTTP::Headers
instance, and just do the right thing. This is exactly what coercion
is for:
coerce 'My::Types::HTTP::Headers' => from 'ArrayRef' => via { HTTP::Headers->new( @{$_} ) } => 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 the HTTP::Headers manpage 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 the Params::Coerce manpage.
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 the Params::Coerce manpage do its magic, and we just use its
return value.
If the Params::Coerce manpage 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.
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
.
EvenElementArrayRef
type, and then coerce from that type, as
opposed to a plain ArrayRef
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.
Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates subtypes and coercion use HTTP-related classes |