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Author image Anthony Brummett
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UR::Role - Roles in UR, an alternative to inheritance


  package My::Role;
  role My::Role {
      id_by => [
          role_id_property => { is => 'String' },
      has => [
          role_property => { is => 'String' },
          another_prop  => { is => 'Integer' },
      requires => ['class_method'],
      excludes => ['Bad::Role'],
  sub role_method { ... }

  package My::Class;
  class My::Class {
      has => [
          class_property => { is => 'Integer ' },
      roles => ['My::Role'],
  sub class_method { ... }

  my $obj = My::Class->new();
  $obj->does('My::Role');  # true


Roles are used to encapsulate a piece of behavior to be used in other classes. They have properties and methods that get melded into any class that composes them. A Role can require any composing class to implement a list of methods or properties.

Roles are not classes. They can not be instantiated or inherited from. They are composed into a class by listing their names in the roles attribute of a class definition.

Defining a Role

Roles are defined with the role keyword. Their definition looks very similar to a class definition as described in UR::Object::Type::Initializer. In particular, Roles have a has section to define properties, and accept many class-meta attributes such as 'id_generator', 'valid_signals', and 'doc'.

Roles may implement operator overloading via the 'use overload' mechanism.

Roles also have unique attributes to declare restrictions on their use.


A listref of property and method names that must appear in any class composing the Role. Properties and methods defined in other roles or parent classes can satisfy a requirement.


A listref of Role names that may not be composed together with this Role. This is useful to declare incompatibilities between roles.

Composing a Role

Compose one or more Roles into a class using the 'roles' attribute in a class definition.

  class My::Class {
      roles => ['My::Role', 'Other::Role'],
      is => ['Parent::Class'],
      has => ['prop_a','prop_b'],

Properties and meta-attributes from the Roles get copied into the composing class. Subroutines defined in the Roles' namespaces are imported into the class's namespace. Operator overloads defined in the Roles are applied to the class.

Property and meta-attribute conflicts

An exception is thrown if multiple Roles are composed together that define the same property, even if the composing class defines the same property in an attempt to override them.

A class may declare a property with the same name that a role also declares. The definition in the class overrides whatever appears in the role. An exception is thrown if a role declares an ID property in the 'id_by' section and the consuming class redeclares it in the 'has' section as a normal property.

Method conflicts

An exception is thrown if multiple Roles are composed together that define the same subroutine, or if the composing class (or any of its parent classes) defines the same subroutine as any of the roles.

If the class wants to override a subroutine defined in one of its roles, the override must be declared with the "Overrides" attribute.

  sub overridden_method : Overrides(My::Role, Other::Role) { ... }

All the conflicting role names must be listed in the override, separated by commas. The class will probably implement whatever behavior is required, maybe by calling one role's method or the other, both methods, neither, or anything else.

To call a function in a role, the function's fully qualified name, including the role's package, must be used.

Overload conflicts

Like with method conflicts, an exception is thrown if multiple Roles are composed together that overload the same operator unless the composing class also overloads that same operator.

An exception is also thrown if composed roles define incompatible 'fallback' behavior. If a role does not specify 'fallback', or explicitly sets it to undef, it is compatible with other values. A Role that sets its 'fallback' value to true or false is only compatible with other roles' values of undef or the same true or false value.


Each time a Role is composed into a class, its __import__() method is called. __import__() is passed two arguments:

  • The name of the role

  • The class metadata object composing the role.

This happens after the class is completely constructed.

Parameterized Roles

Scalar variables with the RoleParam attribute are designated as role params. Values can be supplied when a role composes the role as a means to provide more flexibility and genericity for a role.

  package ObjectDisplayer;
  use ProjectNamespace;

  our $target_type : RoleParam(target_type);
  role ObjectDisplayer {
      has => [
          target_object => { is => $target_type },

  package ShowCars;
  class ShowCars {
      roles => [ ObjectDisplayer->create(target_type => 'Car' ],

When the role is composed, the call to create() in the class definition creates a UR::Role::Instance to represent the ObjectDisplayer role being composed into the ShowCars class with the params { target_type = 'car' }>. Values for the role param values in the role definition are swapped out with the provided values as the role's properties are composed into the class.

At run-time, these role param variables are tied with the UR::Role::Param class. Its FETCH method searches the call stack for the first function whose invocant composes the role where the variable's value is being fetched from. The proper param value is returned.

An exception is thrown if a class composes a role and either provides unknown role params or omits values for existing params.

Method Modifiers

Roles can hook into methods defined in consuming classes by using the "before", "after" and "around" method modifiers.

  use UR;
  package RoleWithModifiers;
  use UR::Role qw(before after around);
  role RoleWithModifiers { };
  before 'do_something' => sub {
      my($self, @params) = @_;
      print "Calling do_something with params ",join(',',@params),"\n";
  after 'do_something' => sub {
      my($rv, $self, @params) = @_;
      print "Result from do_something: $rv\n";
  around 'do_something' => sub {
      my($orig, $self, @params) = @_;
      print "Wrapped call to do_something params ",join(',',@params),"\n";
      my $rv = $self->$orig(@params);
      print "The wrapped call to do_something returned $rv\n";
      return 123;

  package ClassUsingRole;
  class ClassUsingRole { roles => 'RoleWithModifiers' };
  sub do_something {
      print "In original do_something\n";
      return 'abc';

  my $rv = ClassUsingRole->create()->do_something();
  print "The call to do_something returned $rv\n";

Running this code will generate the following output:

  Wrapped call to do_something params
  Calling do_something with params
  In original do_something
  Result from do_something: abc
  The wrapped call to do_something returned abc
  The call to do_something returned 123

Method modifiers are applied in the order they appear in the role's implementation.


A before modifier runs before the named method. It receives all the arguments and wantarray context as the original method call. It cannot affect the parameters to the original method call, and its return value is ignored.

after($rv, @params)

The first argument to an after modifier is the return value of the original method call, the remaining arguments and wantarray context are the same as the original method call. If the original method was called in list context, then $rv will be an arrayref containing the list of return values. This modifier's return value is ignored.

around($orig, @params)

An around modifier is run in place of the original method, and receives a coderef of the original method as its first argument. Around modifiers can munge arguments and return values, and control when and whether the original method is called.


UR, UR::Object::Type::Initializer, UR::Role::Instance, UR::Role::Param