class Metamodel::EnumHOW

Metaobject representing a Raku enum.

class Metamodel::EnumHOW
    does Metamodel::Naming
    does Metamodel::Documenting
    does Metamodel::Stashing
    does Metamodel::AttributeContainer
    does Metamodel::MethodContainer
    does Metamodel::MultiMethodContainer
    does Metamodel::RoleContainer
    does Metamodel::BaseType
    does Metamodel::MROBasedMethodDispatch
    does Metamodel::MROBasedTypeChecking
    does Metamodel::BUILDPLAN
    does Metamodel::BoolificationProtocol
    does Metamodel::REPRComposeProtocol
    does Metamodel::InvocationProtocol
    does Metamodel::Mixins
        { }

Warning: this class is part of the Rakudo implementation, and is not a part of the language specification.

Metamodel::EnumHOW is the metaclass behind the enum keyword.

enum Numbers <1 2>;
say Numbers.HOW ~~ Metamodel::EnumHOW; # OUTPUT: «True␤»

The following enum declaration:

our Int enum Error <Warning Failure Exception Sorrow Panic>;

Is roughly equivalent to this code using Metamodel::EnumHOW's methods:

BEGIN {
    my constant Error = Metamodel::EnumHOW.new_type: :name<Error>, :base_type(Int);
    Error.^add_role: Enumeration;
    Error.^add_role: NumericEnumeration;
    Error.^compose;
    for <Warning Failure Exception Sorrow Panic>.kv -> Int $v, Str $k {
        # Note: Enumeration.pred and .succ will not work when adding enum 
        # values as pairs. They should be instances of the enum itself, but 
        # this isn't possible to do without nqp. 
        Error.^add_enum_value: $k => $v;
        OUR::{$k} := Error.^enum_from_value: $v;
    }
    Error.^compose_values;
    OUR::<Error> := Error;
}

Methods

method new_type

method new_type(:$name!, :$base_type?, :$repr = 'P6opaque', :$is_mixin)

Creates a new type object for an enum. $name is the enum name, $base_type is the type given when the enum is declared using a scoped declaration (if any), and $repr is the type representation passed to the enum using the repr trait. $is_mixin is unused.

method add_parent

method add_parent($obj, $parent)

Sets the base type of an enum. This can only be used if no base type was passed to .new_type.

method set_export_callback

method set_export_callback($obj, $callback)

Sets the enum's export callback, which is invoked when calling .compose_values. This is called when applying the export trait to an enum. $callback should be a routine of some sort, taking no arguments, that handles exporting the enum's values.

method export_callback

method export_callback($obj)

Returns the export callback set by .set_export_callback.

method compose

method compose($obj, :$compiler_services)

Completes a type object for an enum. This is when any roles done by the enum are mixed in. This needs to be called before any enum values can be added using .add_enum_value.

method is_composed

method is_composed($obj)

Returns 1 if the enum is composed, otherwise returns 0.

method compose_values

method compose_values($obj)

Calls the export callback set by .set_export_callback and removes it from state. This should be called after adding the enum's values using .add_enum_value.

method set_composalizer

method set_composalizer($c)

Sets the composalizer for an enum, which produces a type that can be mixed in with another. $c should be a routine of some that has the following signature:

:($type, $name, @enum_values)

method composalizer

method composalizer($obj)

Returns the composalizer set by .set_composalizer.

method add_enum_value

method add_enum_value($obj, $value)

Adds a value to this enum. $value should be an instance of the enum itself, as type Enumeration.

method enum_values

method enum_values($obj)

Returns the values for the enum.

enum Numbers <10 20>;
say Numbers.^enum_values;                   # OUTPUT: {10 => 0, 20 => 1}

method elems

method elems($obj)

Returns the number of values.

enum Numbers <10 20>;
say Numbers.^elems;                         # OUTPUT: 2

method enum_from_value

method enum_from_value($obj, $value)

Given a value of the enum's base type, return the corresponding enum.

enum Numbers <10 20>;
say Numbers.^enum_from_value(0);            # OUTPUT: 10

method enum_value_list

method enum_value_list($obj)

Returns a list of the enum values.

enum Numbers <10 20>;
say Numbers.^enum_value_list;               # OUTPUT: (10 20)