Documentation for declarator
class assembled from the following types:
From Object orientation
Classes are declared using the
class keyword, typically followed by a name.
This declaration results in a type object being created and installed in the current package and current lexical scope under the name
Journey. You can also declare classes lexically:
This restricts their visibility to the current lexical scope, which can be useful if the class is an implementation detail nested inside a module or another class.
Attributes are variables that exist per instance of a class; when instantiated to a value, the association between the variable and its value is called a property. They are where the state of an object is stored. In Perl 6, all attributes are private, that means they can be accessed directly only by the class instance itself. They are typically declared using the
has declarator and the
While there is no such thing as a public (or even protected) attribute, there is a way to have accessor methods generated automatically: replace the
! twigil with the
. twigil (the
. should remind you of a method call).
This defaults to providing a read-only accessor. In order to allow changes to the attribute, add the is rw trait:
Now, after a
Journey object is created, its
.notes will all be accessible from outside the class, but only
.notes can be modified.
If an object is instantiated without certain attributes, such as origin or destination, we may not get the desired result. To prevent this, provide default values or make sure that an attribute is set on object creation by marking an attribute with an is required trait.
Since classes inherit a default constructor from
Mu and we have requested that some accessor methods are generated for us, our class is already somewhat functional.
# Create a new instance of the class.my = Journey.new(origin => 'Sweden',destination => 'Switzerland',notes => 'Pack hiking gear!');# Use an accessor; this outputs Sweden.say .origin;# Use an rw accessor to change the value..notes = 'Pack hiking gear and sunglasses!';
Note that, although the default constructor can initialize read-only attributes, it will only set attributes that have an accessor method. That is, even if you pass
travelers => ["Alex", "Betty"] to the default constructor, the attribute
@!travelers is not initialized.
Methods are declared with the
method keyword inside a class body.
A method can have a signature, just like a subroutine. Attributes can be used in methods and can always be used with the
! twigil, even if they are declared with the
. twigil. This is because the
. twigil declares a
! twigil and generates an accessor method.
Looking at the code above, there is a subtle but important difference between using
$.origin in the method
$!origin is an inexpensive and obvious lookup of the attribute.
$.origin is a method call and thus may be overridden in a subclass. Only use
$.origin if you want to allow overriding.
Unlike subroutines, additional named arguments will not produce compile time or runtime errors. That allows chaining of methods via Re-dispatching.
You may write your own accessors to override any or all of the autogenerated ones.
my = " " xx 4;my = Journey.new( :origin<Here>, :destination<There>,travelers => <þor Freya> );.notes("First steps");notes : "Almost there";print ;# OUTPUT:#⤷ Here# First steps# Almost there##There ⤶
The declared multi method
notes overrides the auto-generated methods implicit in the declaration of
$.notes, using a different signature for reading and writing.
Please note that in
notes $trip: "Almost there" we are using indirect invocant syntax, which puts first the method name, then the object, and then, separated by a colon, the arguments:
method invocant: arguments. We can use this syntax whenever it feels more natural than the classical period-and-parentheses one. It works exactly in the same way.
Method names can be resolved at runtime with the
;my = 'b';A.new."$name"().say;# OUTPUT: «(Any)␤»
The syntax used to update
$.notes changed in this section with respect to the previous Attributes section. Instead of an assignment:
.notes = 'Pack hiking gear and sunglasses!';
we now do a method call:
Overriding the default auto-generated accessor means it is no longer available to provide a mutable container on return for an assignment. A method call is the preferred approach to adding computation and logic to the update of an attribute. Many modern languages can update an attribute by overloading assignment with a “setter” method. While Perl 6 can overload the assignment operator for this purpose with a
Proxy object, overloading assignment to set attributes with complex logic is currently discouraged as weaker object oriented design.
A method's signature can have an invocant as its first parameter followed by a colon, which allows for the method to refer to the object it was called on.
Foo.new.greet("Bob"); # OUTPUT: «Hi, I am Foo, nice to meet you, Bob␤»
Providing an invocant in the method signature also allows for defining the method as either as a class method, or as an object method, through the use of type constraints. The
::?CLASS variable can be used to provide the class name at compile time, combined with either
:U (for class methods) or
:D (for instance methods).
my = Pizza.from-ingredients: <cheese pepperoni vegetables>;say .ingredients; # OUTPUT: «[cheese pepperoni vegetables]␤»say .get-radius; # OUTPUT: «42␤»say Pizza.get-radius; # This will fail.CATCH ;# OUTPUT: «X::Parameter::InvalidConcreteness:␤# Invocant of method 'get-radius' must be# an object instance of type 'Pizza',# not a type object of type 'Pizza'.# Did you forget a '.new'?»
A method can be both a class and object method by using the multi declarator:
C.f; # OUTPUT: «class method␤»C.new.f; # OUTPUT: «object method␤»
Inside a method, the term
self is available, which is bound to the invocant.
self can be used to call further methods on the invocant, including constructors:
self can be used in class or instance methods as well, though beware of trying to invoke one type of method from the other:
C.f; # OUTPUT: «42␤»C.new.d; # This will fail.CATCH ;# OUTPUT: «X::Parameter::InvalidConcreteness:␤# Invocant of method 'f' must be a type object of type 'C',# not an object instance of type 'C'. Did you forget a 'multi'?»
$.origin works the same as
self.origin, however the colon-syntax for method arguments is only supported for method calls using
self, not the shortcut.
Note that if the relevant methods
CREATE of Mu are not overloaded,
self will point to the type object in those methods.
On the other hand,
BUILDALL and the submethods
TWEAK are called on instances, in different stages of initialization. In the latter two submethods, submethods of the same name from subclasses have not yet run, so you should not rely on potentially virtual method calls inside these methods.
Methods with an exclamation mark
! before the method name are not callable from anywhere outside the defining class; such methods are private in the sense that they are not visible from outside the class that declares them. Private methods are invoked with an exclamation mark instead of a dot:
method !do-something-private()method public()
Private methods are not inherited by subclasses.
Submethods are public methods that are not inherited by subclasses. The name stems from the fact that they are semantically similar to subroutines.
Submethods are useful for object construction and destruction tasks, as well as for tasks that are so specific to a certain type that subtypes must certainly override them.
is Point2Dsay InvertiblePoint2D.new(x => 1, y => 2);# OUTPUT: «Initializing Point2D␤»# OUTPUT: «Initializing InvertiblePoint2D␤»# OUTPUT: «InvertiblePoint2D.new(x => 1, y => 2)␤»
See also: Object Construction.
Classes can have parent classes.
is Parent1 is Parent2
If a method is called on the child class, and the child class does not provide that method, the method of that name in one of the parent classes is invoked instead, if it exists. The order in which parent classes are consulted is called the method resolution order (MRO). Perl 6 uses the C3 method resolution order. You can ask a type for its MRO through a call to its meta class:
say List.^mro; # ((List) (Cool) (Any) (Mu))
All calls to public methods are "virtual" in the C++ sense, which means that the actual type of an object determines which method to call, not the declared type:
is Parentmy Parent ;= Child.new;.frob; # calls the frob method of Child rather than Parent# OUTPUT: «the child's somewhat more fancy frob is called␤»
Objects are generally created through method calls, either on the type object or on another object of the same type.
my = Point.new( x => 5, y => 2);# ^^^ inherited from class Musay "x: ", .x;say "y: ", .y;# OUTPUT: «x: 5␤»# OUTPUT: «y: 2␤»
Mu.new calls method bless on its invocant, passing all the named arguments.
bless creates the new object and then calls method
BUILDALL on it.
BUILDALL walks all subclasses in reverse method resolution order (i.e. from Mu to most derived classes) and in each class checks for the existence of a method named
BUILD. If the method exists, the method is called with all the named arguments from the
new method. If not, the public attributes from this class are initialized from named arguments of the same name. In either case, if neither
BUILD nor the default mechanism has initialized the attribute, default values are applied.
Due to the default behavior of
BUILD submethods, named arguments to the constructor
new derived from
Mu can correspond directly to public attributes of any of the classes in the method resolution order, or to any named parameter of any
This object construction scheme has several implications for customized constructors. First, custom
BUILD methods should always be submethods, otherwise they break attribute initialization in subclasses. Second,
BUILD submethods can be used to run custom code at object construction time. They can also be used for creating aliases for attribute initialization:
my = EncodedBuffer.new( encoding => 'UTF-8', data => [64, 65] );my = EncodedBuffer.new( enc => 'UTF-8', data => [64, 65] );# both enc and encoding are allowed now
Since passing arguments to a routine binds the arguments to the parameters, a separate binding step is unnecessary if the attribute is used as a parameter. Hence the example above could also have been written as:
submethod BUILD(:encoding(:), :)
However, be careful when using this auto-binding of attributes when the attribute may have special type requirements, such as an
:$!id that must be a positive integer. Remember, default values will be assigned unless you specifically take care of this attribute, and that default value will be
Any, which would cause a type error.
The third implication is that if you want a constructor that accepts positional arguments, you must write your own
However this is considered poor practice, because it makes correct initialization of objects from subclasses harder.
Another thing to note is that the name
new is not special in Perl 6. It is merely a common convention. You can call
bless from any method at all, or use
CREATE to fiddle around with low-level workings.
Another pattern of hooking into object construction is by writing your own method
BUILDALL. To make sure that initialization of superclasses works fine, you need to
callsame to invoke the parent classes
TWEAK submethod allows you to check things or modify attributes after object construction:
say RectangleWithCachedArea.new( x2 => 5, x1 => 1, y2 => 1, y1 => 0).area;# OUTPUT: «4␤»
The cloning is done using clone method available on all objects, which shallow-clones both public and private attributes. New values for public attributes can be supplied as named arguments.
my = Foo.new;my = .clone: :bar(5000);say ; # Foo.new(foo => 42, bar => 100)say ; # Foo.new(foo => 42, bar => 5000)
See document for clone for details on how non-scalar attributes get cloned, as well as examples of implementing your own custom clone methods.