Warning: this role is part of the Rakudo implementation, and is not a part of the language specification.
Using the does
and but
infix operators, mixins of a base object and an arbitrary number of roles (or another object) can be created. These are objects whose types have properties of both operands' types. Respectively, these rebless the existing object to have the generated mixin type and clone said object with said mixin type:
my Str =my Str =my Billboard .= new: :;say eq ; # OUTPUT: «True»my Billboard = but Billboard::Vandalized[:];say eq ; # OUTPUT: «True»does Billboard::Vandalized[:];say eq ; # OUTPUT: «True»
Optionally, mixins may have a mixin attribute. This occurs when only one role having only one public attribute gets mixed into an object. If a mixin attribute exists on a resulting mixin's type, it can be initialized by but
or does
using its value
named parameter. This makes it possible for mixins to not only have composable methods, but composable state as well. Using this feature, the example above can be rewritten so billboards can be vandalized more than once without needing to generate more mixins by making Billboard::Vandalism
's $vandalism
named parameter a rw mixin attribute instead:
my Str =my Str =my Str = Qs:to/FALSE-ALARM/.chomp;$vandalism⬆️ This is just one of our namesakes we at Brilliant Solutions have beenhelping people like you create since 1972!FALSE-ALARMmy Billboard:D .= new: :;say eq ; # OUTPUT: «True»does Billboard::Vandalized :value();say eq ; # OUTPUT: «True».vandalism = ;say eq ; # OUTPUT: «True»
Metamodel::Mixins
is the metarole that implements the behavior of said mixins. Formally, mixins are objects whose HOW inherits from a base composable metaobject and applies an arbitrary number of roles, resulting in an object whose HOW has a combination of their properties. In particular, the metamethods this metarole provides are used to implement the behavior of the but
and does
infix operators, but these also support introspection related to mixins. For example, the work done by but
when invoked with an object and one role can be written explicitly using the mixin
metamethod provided:
say Foo.new but Bar; # OUTPUT: «Foo+{Bar}.new»say Foo.new.^mixin(Bar); # OUTPUT: «Foo+{Bar}.new»
Methods§
method set_is_mixin§
method set_is_mixin()
Marks $obj
as being a mixin.
method is_mixin§
method is_mixin()
Returns 1
If $obj
has been marked as being a mixin with set_is_mixin
, otherwise returns 0
.
method set_mixin_attribute§
method set_mixin_attribute(, )
Sets the mixin attribute for $obj
to $attr
(which should be an Attribute
instance).
method mixin_attribute§
method mixin_attribute()
Returns the mixin attribute for $obj
set with set_mixin_attribute
.
method setup_mixin_cache§
method setup_mixin_cache()
Sets up caching of mixins for $obj
. After this metamethod has been called, calls to mixin
will not create a new type for mixins of $obj
given the same list of roles more than once. This should be called at some point before composition.
method flush_cache§
method flush_cache()
No-op.
method generate_mixin§
method generate_mixin(, )
Creates a new mixin metaobject that inherits from $obj
and does each of the roles in @roles
. This is then composed and has its mixin attribute set (if any exists) before getting returned.
While this generates a new mixin type, this doesn't actually mix it into $obj
; if that is what you intend to do, use the mixin metamethod instead.
method mixin§
method mixin(, *, :)
Generates a new mixin type by calling generate_mixin
with $obj
and @roles
. If $obj
is composed, the mixin cache of $obj
will be checked for any existing mixin for these beforehand. If $obj
is an instance of a type, this will return $obj
reblessed with the mixin generated, otherwise this will return the mixin itself.
If $needs-mixin-attribute
is True
, this will throw an exception if no mixin attribute exists on the mixin generated before returning.