Moose::Cookbook::BasicUserMoose::Cookbook::Basics::HTTP_SubtypesAndCoercion(3)NAMEMoose::Cookbook::Basics::HTTP_SubtypesAndCoercion - Demonstrates
subtypes and coercion use HTTP-related classes (Request, Protocol,
etc.)
VERSION
version 2.1005
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"
AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many
contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for
details.
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 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 v5.18.1Moose::Cookbook::Basics::HTTP_SubtypesAndCoercion(3)
