role Metamodel::TypePretense { }
Warning: this role is part of the Rakudo implementation, and is not a part of the language specification.
Any role will type-check as Mu
, Any
, and Cool
, but don't actually have these classes as parents:
class Class { } role Role { } say Role ~~ Mu; # OUTPUT: «True» say Role ~~ Any; # OUTPUT: «True» say Role ~~ Cool; # OUTPUT: «True» say Class.^parents(:all).map(*.^name); # OUTPUT: «(Any Mu)» say Role.^pun.^parents(:all).map(*.^name); # OUTPUT: «()»
Metamodel::TypePretense
is the metarole that's responsible for this behavior. Using the metamethods this provides, types can pretend to be
other types, i.e. types can type-check as other types. This can be useful when implementing types that should not store parent types through Metamodel::MultipleInheritance
, but should still type-check like other types somehow.
All this metarole does is provide an interface for storing type objects in a HOW and provide a default type_check
method that allows types to type-check as the types they're pretending to be if no other type-checking behavior for it exists; any other behavior related to type pretenses are left up to the metaclasses that do this metarole to implement themselves.
Because type pretenses are a property of the metaclass for a HOW, not HOWs themselves, the pretend_to_be
and pretending_to_be
metamethods this metarole provides must be invoked directly through a metaclass or HOW, not with .^
syntax:
say Role.HOW.pretending_to_be.map(*.^name); # OUTPUT: «(Cool Any Mu)»
This metarole is commonly used in combination with Metamodel::MethodDelegation
.
Methods§
method pretend_to_be§
method pretend_to_be(@types)
Makes all types for a type of HOW pretend to be any of the type objects in @types
.
method pretending_to_be§
method pretending_to_be()
Returns the type objects this type of HOW is pretending to be.
method type_check§
method type_check($obj, $checkee)
If $checkee
is the same object as $obj
or is of any of the types $obj
is pretending to be, returns 1
, otherwise returns 0
.