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Moose::Cookbook::Basics::Recipe5(3)  User Contributed Perl Documentation Moose::Cookbook::Basics::Recipe5(3)



NAME
       Moose::Cookbook::Basics::Recipe5 - More subtypes, coercion in a Request class

VERSION
       version 2.0205

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
       Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE
       This software is copyright (c) 2011 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 v5.12.5                                     2011-09-06              Moose::Cookbook::Basics::Recipe5(3)