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.
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