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Params::Coerce - Allows your classes to do coercion of parameters


version 0.15


  # Coerce a object of class Foo to a Bar
  my $bar = Params::Coerce::coerce('Bar', $Foo)

  # Create a coercion param function
  use Params::Coerce '_Bar' => 'Bar';
  my $bar = _Bar($Foo);

  # Usage when Bar has a 'from' method
  my $bar = Bar->from($Foo);

Real world example using HTML::Location.

  # My class needs a URI
  package Web::Spider;

  use URI;
  use Params::Coerce 'coerce';

  sub new {
      my $class = shift;

      # Where do we start spidering
      my $start = coerce('URI', shift) or die "Wasn't passed a URI";

      bless { root => $start }, $class;

  # Now we can do the following

  # Pass a URI as normal
  my $URI     = URI->new('');
  my $Spider1 = Web::Spider->new( $URI );

  # We can also pass anything that can be coerced into being a URI
  my $Website = HTML::Location->new( '/home/adam/public_html', '' );
  my $Spider2 = Web::Spider->new( $Website );


A big part of good API design is that we should be able to be flexible in the ways that we take parameters.

Params::Coerce attempts to encourage this, by making it easier to take a variety of different arguments, while adding negligible additional complexity to your code.

What is Coercion

"Coercion" in computing terms generally refers to "implicit type conversion". This is where data and object are converted from one type to another behind the scenes, and you just just magically get what you need.

The overload pragma, and its string overloading is the form of coercion you are most likely to have encountered in Perl programming. In this case, your object is automatically (within perl itself) coerced into a string.

Params::Coerce is intended for higher-order coercion between various types of different objects, for use mainly in subroutine and (mostly) method parameters, particularly on external APIs.

__as_Another_Class Methods

At the heart of Params::Coerce is the ability to transform objects from one thing to another. This can be done by a variety of different mechanisms.

The preferred mechanism for this is by creating a specially named method in a class that indicates it can be coerced into another type of object.

As an example, HTML::Location provides an object method that returns an equivalent URI object.

  # In the package HTML::Location

  # Coerce to a URI
  sub __as_URI {
        my $self = shift;
        return URI->new( $self->uri );

__from_Another_Class Methods

From version 0.04 of Params::Coerce, you may now also provide __from_Another_Class methods as well. In the above example, rather then having to define a method in HTML::Location, you may instead define one in URI. The following code has an identical effect.

  # In the package URI

  # Coerce from a HTML::Location
  sub __from_HTML_Location {
        my $Location = shift;
        return URI->new( $Location->uri );

Params::Coerce will only look for the __from method, if it does not find a __as method.

Loading Classes

One thing to note with the __as_Another_Class methods is that you are not required to load the class you are converting to in the class you are converting from.

In the above example, HTML::Location does not have to load the URI class. The need to load the classes for every object we might some day need to be coerced to would result in highly excessive resource usage.

Instead, Params::Coerce guarantees that the class you are converting to will be loaded before it calls the __as_Another_Class method. Of course, in most situations you will have already loaded it for another purpose in either the From or To classes and this won't be an issue.

If you make use of some class other than the class you are being coerced to in the __as_Another_Class method, you will need to make sure that is loaded in your code, but it is suggested that you do it at run-time with a require if you are not using it already elsewhere.

Coercing a Parameter

The most explicit way of accessing the coercion functionality is with the Params::Coerce::coerce function. It takes as its first argument the name of the class you wish to coerce to, followed by the parameter to which you wish to apply the coercion.

  package My::Class;

  use URI ();
  use Params::Coerce '_URI' => 'URI';

  sub new {
        my $class = shift;

        # Take a URI argument
        my $URI = Params::Coerce::coerce('URI', shift) or return;


For people doing procedural programming, you may also import this function.

  # Import the coerce function
  use Params::Coerce 'coerce';

Please note that the coerce function is the only function that can be imported, and that the two argument pragma (or the passing of two or more arguments to ->import) means something different entirely.

Importing Parameter Coercion Methods

The second way of using Params::Coerce, and the more common one for Object-Oriented programming, is to create method specifically for taking parameters in a coercing manner.

  package My::Class;

  use URI ();
  use Params::Coerce '_URI' => 'URI';

  sub new {
        my $class = shift;

        # Take a URI as parameter
        my $URI1 = $class->_URI(shift) or return;
        my $URI2 = _URI(shift) or return;

The from Constructor

From version 0.11 of Params::Coerce, an additional mechanism is available with the importable from constructor.

  package My::Class;

  use Params::Coerce 'from';

  package Other::Class;

  sub method {
        my $self = shift;
        my $My   = My::Class->from(shift) or die "Bad param";

This is mainly a convenience. The above is equivalent to

  package My::Class;

  use Params::Coerce 'from' => 'Params::Coerce';

In future versions, this ->from syntax may also tweak the resolution order of the coercion.

Chained Coercion

While it is intended that Params::Coerce will eventually support coercion using multiple steps, like <Foo::Bar-__as_HTML_Location->__as_URI>>, it is not currently capable of this. At this time only a single coercion step is supported.



coerce $class, $param

The coerce function takes a class name and a single parameter and attempts to coerce the parameter into the intended class, or one of its subclasses.

Please note that it is the responsibility of the consuming class to ensure that the class you wish to coerce to is loaded. coerce will check this and die is it is not loaded.

Returns an instance of the class you specify, or one of its subclasses. Returns undef if the parameter cannot be coerced into the class you wish.


- Write more unit tests

- Implement chained coercion

- Provide a way to coerce to string, int, etc that is compatible with overload and other types of things.


Bugs may be submitted through the RT bug tracker (or


Adam Kennedy <>


  • Karen Etheridge <>

  • Adam Kennedy <>


This software is copyright (c) 2004 by Adam Kennedy.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.