Object Orientation in Perl 6
Perl 6 provides strong support for object oriented programming. Although Perl 6 allows programmers to program in multiple paradigms, object oriented programming is at the heart of the language.
Perl 6 comes with a wealth of predefined types, which can be classified in two categories: normal and native types.
Native types are used for low-level types (like
uint64). They do not have the same capabilities as objects, though if you call methods on them, they are boxed into normal objects.
Everything that you can store in a variable is either a native value or an object. That includes literals, types (type objects), code and containers.
To call a method on an object, add a dot, followed by the method name:
say "abc".uc;# OUTPUT: «ABC␤»
This calls the
uc method on
"abc", which is an object of type
Str. To supply arguments to the method, add arguments inside parentheses after the method.
my = "Fourscore and seven years ago...".indent(8);say ;# OUTPUT: « Fourscore and seven years ago...␤»
$formatted-text now contains the above text, but indented 8 spaces.
Multiple arguments are separated by commas:
my = "Abe", "Lincoln";.push("said", .comb(/\w+/));say ;# OUTPUT: «[Abe Lincoln said (Fourscore and seven years ago)]␤»
Multiple arguments can be specified by separating the argument list with a colon:
say .join: '--';# OUTPUT: «Abe--Lincoln--said--Fourscore--and--seven--years--ago␤»
Since you have to put a
: after the method if you want to pass arguments without parentheses, a method call without a colon or parentheses is unambiguously a method call without an argument list:
say 4.log: ; # OUTPUT: «1.38629436111989␤» ( natural logarithm of 4 )say 4.log: +2; # OUTPUT: «2␤» ( base-2 logarithm of 4 )say 4.log +2; # OUTPUT: «3.38629436111989␤» ( natural logarithm of 4, plus 2 )
Many operations that don't look like method calls (for example, smart matching, or interpolating an object into a string) result in method calls under the hood.
Methods can return mutable containers, in which case you can assign to the return value of a method call. This is how read-writable attributes to objects are used:
.nl-in = "\r\n";
Here, we call method
nl-in on the
$*IN object, without arguments, and assign to the container it returned with the
All objects support methods from class Mu, which is the type hierarchy root. All objects derive from
Types themselves are objects and you can get the type object by writing its name:
my = Int;
You can ask any object for its type object by calling the
WHAT method (which is actually a macro in method form):
my = 1.WHAT;
sub f(Int )
Although, in most cases, the
.isa method will suffice:
Subtype checking is done by smart-matching:
if ~~ Real
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. They are where the state of an object is stored. In Perl 6, all attributes are private. 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
$.notes, using a different signature for reading and writing.
Method names can be resolved at runtime with the
;my = 'b';A.new."$name"().say;# OUTPUT: «(Any)␤»
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 method 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.
Roles are in some ways similar to classes, in that they are a collection of attributes and methods. They differ in that roles are also meant for describing only parts of an object's behavior and in how roles are applied to classes. Or to phrase it differently, classes are meant for managing objects and roles are meant for managing behavior and code reuse.
use MONKEY-SEE-NO-EVAL;does Serializablemy = Point.new(:x(1), :y(2));my = .serialize;my = Point.deserialize();say .x; # OUTPUT: «1␤»
Roles are immutable as soon as the compiler parses the closing curly brace of the role declaration.
Role application differs significantly from class inheritance. When a role is applied to a class, the methods of that role are copied into the class. If multiple roles are applied to the same class, conflicts (e.g. attributes or non-multi methods of the same name) cause a compile-time error, which can be solved by providing a method of the same name in the class.
This is much safer than multiple inheritance, where conflicts are never detected by the compiler, but are instead resolved to the superclass that appears earlier in the method resolution order, which might not be what the programmer wanted.
For example, if you've discovered an efficient method to ride cows, and are trying to market it as a new form of popular transportation, you might have a class
Bull, for all the bulls you keep around the house, and a class
Automobile, for things that you can drive.
is Bull is Automobilemy = Taurus.new;say .steer;# OUTPUT: «Taurus.new(castrated => Bool::True, direction => Any)␤»
With this setup, your poor customers will find themselves unable to turn their Taurus and you won't be able to make more of your product! In this case, it may have been better to use roles:
does Bull-Like does Steerable
This code will die with something like:
===SORRY!===Method 'steer' must be resolved by because it exists inmultiple roles (Steerable, Bull-Like)
This check will save you a lot of headaches:
does Bull-Like does Steerable
When a role is applied to a second role, the actual application is delayed until the second role is applied to a class, at which point both roles are applied to the class. Thus
does R1does R2
produces the same class
does R1 does R2
When a role contains a stubbed method, a non-stubbed version of a method of the same name must be supplied at the time the role is applied to a class. This allows you to create roles that act as abstract interfaces.
# the following is a compile time error, for example# Method 'serialize' must be implemented by Point because# it's required by a roledoes AbstractSerializable# this works:does AbstractSerializable
The implementation of the stubbed method may also be provided by another role.
Roles cannot inherit from classes, but they may cause any class which does that role to inherit from another class. So if you write:
is Exceptiondoes AX::Ouch.^parents.say # OUTPUT: «((Exception))␤»
X::Ouch will inherit directly from Exception, as we can see above by listing its parents.
A method defined directly in a class will always override definitions from applied roles or from inherited classes. If no such definition exists, methods from roles override methods inherited from classes. This happens both when said class was brought in by a role, and also when said class was inherited directly.
is A does Mis A does MB.new.f; # OUTPUT «I am in class B␤»C.new.f; # OUTPUT «I am in role M␤»
Note that each candidate for a multi-method is its own method. In this case, the above only applies if two such candidates have the same signature. Otherwise, there is no conflict, and the candidate is just added to the multi-method.
Any attempt to directly instantiate a role, as well as many other operations on it, will automatically create an instance of a class with the same name as the role, making it possible to transparently use a role as if it were a class.
say Point.new(x => 6, y => 8).abs; # OUTPUT «10␤»
We call this automatic creation of classes punning, and the generated class a pun.
Roles can be parameterized, by giving them a signature in square brackets:
[::Type]my = BinaryTree[Int].new-from-list(4, 5, 6);.visit-preorder(); # OUTPUT: «5␤4␤6␤».visit-postorder(); # OUTPUT: «4␤6␤5␤»
Here the signature consists only of a type capture, but any signature will do:
<debug info warn error critical>;[ = ]Logging.log(debug, 'here we go'); # OUTPUT: «[DEBUG] here we go␤»
You can have multiple roles of the same name, but with different signatures; the normal rules of multi dispatch apply for choosing multi candidates.
Mixins of Roles
Roles can be mixed into objects. A role's given attributes and methods will be added to the methods and attributes the object already has. Multiple mixins and anonymous roles are supported.
;my = 2 but R;sub f(\bound);f(); # OUTPUT: «hidden!␤»
Note that the object got the role mixed in, not the object's class or the container. Thus, @-sigiled containers will require binding to make the role stick. Some operators will return a new value, which effectively strips the mixin from the result.
Mixins can be used at any point in your object's life.
# A counter for Table of Contentsmy Num = NaN; # don't do math with Not A Numbersay ; # OUTPUT: «NaN␤»does TOC-Counter; # now we mix the role in.inc(1).inc(2).inc(2).inc(1).inc(2).inc(2).inc(3).inc(3);put / 1; # OUTPUT: «NaN␤» (because that's numerical context)put ; # OUTPUT: «2.2.2␤» (put will call TOC-Counter::Str)
Roles can be anonymous.
my of Int is default(0 but role :: );say <not-there>; # OUTPUT: «NULL␤»say <not-there>.defined; # OUTPUT: «True␤» (0 may be False but is well defined)say Int.new(<not-there>); # OUTPUT: «0␤»
Perl 6 has a meta object system, which means that the behavior of objects, classes, roles, grammars, enums, etc. are themselves controlled by other objects; those objects are called meta objects. Meta objects are, like ordinary objects, instances of classes, in this case we call them meta classes.
For each object or class you can get the meta object by calling
.HOW on it. Note that although this looks like a method call, it works more like a macro.
So, what can you do with the meta object? For one you can check if two objects have the same meta class by comparing them for equality:
say 1.HOW === 2.HOW; # OUTPUT: «True␤»say 1.HOW === Int.HOW; # OUTPUT: «True␤»say 1.HOW === Num.HOW; # OUTPUT: «False␤»
Perl 6 uses the word HOW, Higher Order Workings, to refer to the meta object system. Thus it should be no surprise that in Rakudo, the class name of the meta class that controls class behavior is called
Perl6::Metamodel::ClassHOW. For each class there is one instance of
But of course the meta model does much more for you. For example, it allows you to introspect objects and classes. The calling convention for methods on meta objects is to call the method on the meta object and pass in the object of interest as first argument to the object. So to get the name of the class of an object, you could write:
my = 1;my = 1.HOW;say .name(); # OUTPUT: «Int␤»# or shorter:say 1.HOW.name(1); # OUTPUT: «Int␤»
(The motivation is that Perl 6 also wants to allow a more prototype-based object system, where it's not necessary to create a new meta object for every type).
There's a shortcut to keep from using the same object twice:
say 1.^name; # OUTPUT: «Int␤»# same assay 1.HOW.name(1); # OUTPUT: «Int␤»