Classes and objects
A tutorial about creating and using classes in Perl 6
Perl 6 has a rich built-in syntax for defining and using classes.
A default constructor allows the setting of attributes for the created object:
# Create a new Rectangle from two Pointsmy = Rectangle.new(lower => Point.new(x => 0, y => 0),upper => Point.new(x => 10, y => 10));say .area(); # OUTPUT: «100»
In the two classes, the default constructor is being used. This constructor will use named parameters in its invocation: Point.new(x => 0, y => 0)
.
You can also provide your own constructor and BUILD
implementation. You need to do it for cases in which there are private attributes that might need to be populated during object construction, as in the example below:
# Example taken from# https://medium.freecodecamp.org/a-short-overview-of-object-oriented-software-design-c7aa0a622c83my = Hero.new(:name('Þor'),:inventory(['Mjölnir','Chariot','Bilskirnir']));say .act;
In this case, we encapsulate the private attribute @!inventory
; but private instance variables cannot be set by the default constructor, which is why we add a BUILD
submethod that takes care of that.
The following, more elaborate example, shows how a dependency handler might look in Perl 6. It showcases custom constructors, private and public attributes, Submethods, methods, and various aspects of signatures. It's not a lot of code, and yet the result is interesting and useful.
my =Task.new(,Task.new(,Task.new(,Task.new(),Task.new()),Task.new()));.perform();
In this case, BUILD
is needed since we have overridden the default new
constructor. bless
is eventually invoking it with the two named arguments that correspond to the two properties without a default value. With its signature, BUILD
converts the two positionals to the two attributes, &!callback
and @!dependencies
, and returns the object (or turns it in to the next phase, TWEAK
, if available).
Declaring new
as a method
and not as a multi method
prevents us from using the default constructor; this implicit constructor uses the attributes as named parameters. This is one of the reasons why using new
as a method's name is discouraged. If you need to declare it anyway, use multi method new
if you do not want to disable the default constructor.
TWEAK
is the last submethod to be called, and it has the advantage of having the object properties available without needing to use the metaobject protocol. It can be used, for instance, to assign values to instance variables based on the values of other attributes or instance variables:
is Strsay Str-with-ID.new(string => 'First').ID; # OUTPUT: «0»say Str-with-ID.new(string => 'Second').ID; # OUTPUT: «1»
In this case, we need to compute $.ID
from the value of a counter that is a class variable, $.counter
, thus we simply assign a value to it and increment the counter at the same time. Please check also this section on TWEAK
in the Object Orientation (OO) document for the mechanics of object construction.
DESTROY
is the submethod that gets called when an object is garbage collected. That is going to happen only if the runtime needs the memory, so we can't rely on when it's going to happen. In particular, it could happen in the middle of some running code in a thread, so we must take special care to not assume any context during that event. We can use it to close any kind of handles or supplies or delete temporary files that are no longer going to be used.
my = 0;my ;for 1 .. 6000say "DESTROY called $in_destructor times";
This might print something like DESTROY called 5701 times
, but it only kicks in after we have stomped over former instances of Foo
6000 times. We can't rely, however, on the order of destruction, for instance.
Starting with class
Perl 6, like many other languages, uses the class
keyword to define a class. The block that follows may contain arbitrary code, just as with any other block, but classes commonly contain state and behavior declarations. The example code includes attributes (state), introduced through the has
keyword, and behaviors, introduced through the method
keyword.
Declaring a class creates a new type object which, by default, is installed into the current package (just like a variable declared with our
scope). This type object is an "empty instance" of the class. For example, types such as Int
and Str
refer to the type object of one of the Perl 6 built-in classes. The example above uses the class name Task
so that other code can refer to it later, such as to create class instances by calling the new
method.
You can use the .DEFINITE
method to find out if what you have is an instance or a type object:
say Int.DEFINITE; # OUTPUT: «False» (type object)say 426.DEFINITE; # OUTPUT: «True» (instance);say Foo.DEFINITE; # OUTPUT: «False» (type object)say Foo.new.DEFINITE; # OUTPUT: «True» (instance)
You can also use type "smileys" to only accept instances or type objects:
multi foo (Int)multi foo (Int)say foo Int; # OUTPUT: «It's a type object!»say foo 42; # OUTPUT: «It's an instance!»
State
In the Task
class, the first three lines inside the block all declare attributes (called fields or instance storage in other languages). Just as a my
variable cannot be accessed from outside its declared scope, attributes are not accessible outside of the class. This encapsulation is one of the key principles of object oriented design.
The first declaration specifies instance storage for a callback (i.e., a bit of code to invoke in order to perform the task that an object represents):
has ;
The &
sigil indicates that this attribute represents something invocable. The !
character is a twigil, or secondary sigil. A twigil forms part of the name of the variable. In this case, the !
twigil emphasizes that this attribute is private to the class.
The second declaration also uses the private twigil:
has Task ;
However, this attribute represents an array of items, so it requires the @
sigil. These items each specify a task that must be completed before the present one is completed. Furthermore, the type declaration on this attribute indicates that the array may only hold instances of the Task
class (or some subclass of it).
The third attribute represents the state of completion of a task:
has Bool ;
This scalar attribute (with the $
sigil) has a type of Bool
. Instead of the !
twigil, the .
twigil is used. While Perl 6 does enforce encapsulation on attributes, it also saves you from writing accessor methods. Replacing the !
with a .
both declares a private attribute and an accessor method named after the attribute. In this case, both the attribute $!done
and the accessor method done
are declared. It's as if you had written:
has Bool ;method done()
Note that this is not like declaring a public attribute, as some languages allow; you really get both a private attribute and a method, without having to write the method by hand. You are free instead to write your own accessor method, if at some future point you need to do something more complex than returning the value.
Note that using the .
twigil has created a method that will provide read-only access to the attribute. If instead the users of this object should be able to reset a task's completion state (perhaps to perform it again), you can change the attribute declaration:
has Bool is rw;
The is rw
trait causes the generated accessor method to return a container so external code can modify the value of the attribute.
You can also supply default values to attributes (which works equally for those with and without accessors):
has Bool = False;
The assignment is carried out at object build time. The right-hand side is evaluated at that time, and can even reference earlier attributes:
has Task ;has = not ;
Writable attributes are accessible through writable containers:
say (a-class.new.an-attribute = "hey"); # OUTPUT: «hey»
This attribute can also be accessed using the .an-attribute
or .an-attribute()
syntax. See also the is rw
trait on classes for examples on how this works on the whole class.
A class declaration can also include class variables, which are variables whose value is shared by all instances, and can be used for things like counting the number of instantiations or any other shared state. Class variables use the same syntax as the rest of the attributes, but are declared as my
or our
, depending on the scope; our
variables will be shared by subclasses, since they have package scope.
is Stris Str-with-IDsay Str-with-ID.new(string => 'First').ID; # OUTPUT: «0»say Str-with-ID.new(string => 'Second').ID; # OUTPUT: «1»say Str-with-ID-and-tag.new( string => 'Third', tag => 'Ordinal' ).ID;# OUTPUT: «2»say ::hierarchy-counter; # OUTPUT: «4»
In this case, using new
might be the easiest way to initialize the $.ID
field and increment the value of the counter at the same time. new
, through bless
, will invoke the default BUILD
, assigning the values to their properties correctly. You can obtain the same effect using TWEAK
, which is considered a more p6y way of achieving such effect. Please check the section on submethods for an alternative example on how to do this. Since TWEAK
is called in every object instantiation, it's incremented twice when creating objects of class Str-with-ID-and-tag
; this is a class hierarchy variable that is shared by all subclasses of Str-with-ID
. Additionally, class variables declared with package scope are visible via their fully qualified name (FQN), while lexically scoped class variables are "private".
Static fields?
Perl 6 has no static keyword. Nevertheless, any class may declare anything that a module can, so making a scoped variable sounds like a good idea.
Class attributes defined by my or our may also be initialized when being declared, however we are implementing the Singleton pattern here and the object must be created during its first use. It is not 100% possible to predict the moment when attribute initialization will be executed, because it can take place during compilation, runtime or both, especially when importing the class using the use keyword.
Class attributes may also be declared with a secondary sigil – in a similar manner to instance attributes – that will generate read-only accessors if the attribute is to be public.
Methods
While attributes give objects state, methods give objects behaviors. Let's ignore the new
method temporarily; it's a special type of method. Consider the second method, add-dependency
, which adds a new task to a task's dependency list:
method add-dependency(Task )
In many ways, this looks a lot like a sub
declaration. However, there are two important differences. First, declaring this routine as a method adds it to the list of methods for the current class, thus any instance of the Task
class can call it with the .
method call operator. Second, a method places its invocant into the special variable self
.
The method itself takes the passed parameter – which must be an instance of the Task
class – and push
es it onto the invocant's @!dependencies
attribute.
The perform
method contains the main logic of the dependency handler:
method perform()
It takes no parameters, working instead with the object's attributes. First, it checks if the task has already completed by checking the $!done
attribute. If so, there's nothing to do.
Otherwise, the method performs all of the task's dependencies, using the for
construct to iterate over all of the items in the @!dependencies
attribute. This iteration places each item – each a Task
object – into the topic variable, $_
. Using the .
method call operator without specifying an explicit invocant uses the current topic as the invocant. Thus the iteration construct calls the .perform()
method on every Task
object in the @!dependencies
attribute of the current invocant.
After all of the dependencies have completed, it's time to perform the current Task
's task by invoking the &!callback
attribute directly; this is the purpose of the parentheses. Finally, the method sets the $!done
attribute to True
, so that subsequent invocations of perform
on this object (if this Task
is a dependency of another Task
, for example) will not repeat the task.
Private methods
Just like attributes, methods can also be private. Private methods are declared with a prefixed exclamation mark. They are called with self!
followed by the method's name. To call a private method of another class the calling class has to be trusted by the called class. A trust relationship is declared with trusts
and the class to be trusted must already be declared. Calling a private method of another class requires an instance of that class and the fully qualified name (FQN) of the method. Trust also allows access to private attributes.
C.new.yours-to-use(); # the context of this call is GLOBAL, and not trusted by CB.new.i-am-trusted();
Trust relationships are not subject to inheritance. To trust the global namespace, the pseudo package GLOBAL
can be used.
Constructors
Perl 6 is rather more liberal than many languages in the area of constructors. A constructor is anything that returns an instance of the class. Furthermore, constructors are ordinary methods. You inherit a default constructor named new
from the base class Mu
, but you are free to override new
, as this example does:
method new(, *)
The biggest difference between constructors in Perl 6 and constructors in languages such as C# and Java is that rather than setting up state on a somehow already magically created object, Perl 6 constructors create the object themselves. The easiest way to do this is by calling the bless method, also inherited from Mu. The bless
method expects a set of named parameters to provide the initial values for each attribute.
The example's constructor turns positional arguments into named arguments, so that the class can provide a nice constructor for its users. The first parameter is the callback (the thing which will execute the task). The rest of the parameters are dependent Task
instances. The constructor captures these into the @dependencies
slurpy array and passes them as named parameters to bless
(note that :&callback
uses the name of the variable – minus the sigil – as the name of the parameter).
Private attributes really are private. This means that bless
is not allowed to bind things to &!callback
and @!dependencies
directly. To do this, we override the BUILD
submethod, which is called on the brand new object by bless
:
submethod BUILD(:, :)
Since BUILD
runs in the context of the newly created Task
object, it is allowed to manipulate those private attributes. The trick here is that the private attributes (&!callback
and @!dependencies
) are being used as the bind targets for BUILD
's parameters. Zero-boilerplate initialization! See objects for more information.
The BUILD
method is responsible for initializing all attributes and must also handle default values:
has ;has ;has Bool (, );submethod BUILD(:,:,: = False,: = not ,)
See Object Construction for more options to influence object construction and attribute initialization.
Consuming our class
After creating a class, you can create instances of the class. Declaring a custom constructor provides a simple way of declaring tasks along with their dependencies. To create a single task with no dependencies, write:
my = Task.new();
An earlier section explained that declaring the class Task
installed a type object in the namespace. This type object is a kind of "empty instance" of the class, specifically an instance without any state. You can call methods on that instance, as long as they do not try to access any state; new
is an example, as it creates a new object rather than modifying or accessing an existing object.
Unfortunately, dinner never magically happens. It has dependent tasks:
my =Task.new(,Task.new(,Task.new(,Task.new(),Task.new()),Task.new()));
Notice how the custom constructor and the sensible use of whitespace makes task dependencies clear.
Finally, the perform
method call recursively calls the perform
method on the various other dependencies in order, giving the output:
making some moneygoing to the storebuying foodcleaning kitchenmaking dinnereating dinner. NOM!
Inheritance
Object Oriented Programming provides the concept of inheritance as one of the mechanisms for code reuse. Perl 6 supports the ability for one class to inherit from one or more classes. When a class inherits from another class it informs the method dispatcher to follow the inheritance chain to look for a method to dispatch. This happens both for standard methods defined via the method
keyword and for methods generated through other means, such as attribute accessors.
is Employee
Now, any object of type Programmer can make use of the methods and accessors defined in the Employee class as though they were from the Programmer class.
my = Programmer.new(salary => 100_000,known_languages => <Perl5 Perl6 Erlang C++>,favorite_editor => 'vim');say .code_to_solve('halting problem')," will get \$ ";# OUTPUT: «Solving halting problem using vim in Perl5 will get $100000»
Overriding inherited methods
Of course, classes can override methods and attributes defined by parent classes by defining their own. The example below demonstrates the Baker
class overriding the Cook
's cook
method.
is Employeeis Cookmy = Cook.new(utensils => <spoon ladle knife pan>,cookbooks => 'The Joy of Cooking',salary => 40000);.cook( 'pizza' ); # OUTPUT: «Cooking pizza»say .utensils.perl; # OUTPUT: «["spoon", "ladle", "knife", "pan"]»say .cookbooks.perl; # OUTPUT: «["The Joy of Cooking"]»say .salary; # OUTPUT: «40000»my = Baker.new(utensils => 'self cleaning oven',cookbooks => "The Baker's Apprentice",salary => 50000);.cook('brioche'); # OUTPUT: «Baking a tasty brioche»say .utensils.perl; # OUTPUT: «["self cleaning oven"]»say .cookbooks.perl; # OUTPUT: «["The Baker's Apprentice"]»say .salary; # OUTPUT: «50000»
Because the dispatcher will see the cook
method on Baker
before it moves up to the parent class the Baker
's cook
method will be called.
To access methods in the inheritance chain, use re-dispatch or the MOP.
Multiple inheritance
As mentioned before, a class can inherit from multiple classes. When a class inherits from multiple classes the dispatcher knows to look at both classes when looking up a method to search for. Perl 6 uses the C3 algorithm to linearize multiple inheritance hierarchies, which is better than depth-first search for handling multiple inheritance.
is Programmer is Cookmy = GeekCook.new(books => 'Learning Perl 6',utensils => ('stainless steel pot', 'knife', 'calibrated oven'),favorite_editor => 'MacVim',known_languages => <Perl6>);.cook('pizza');.code_to_solve('P =? NP');
Now all the methods made available to the Programmer and the Cook classes are available from the GeekCook class.
While multiple inheritance is a useful concept to know and occasionally use, it is important to understand that there are more useful OOP concepts. When reaching for multiple inheritance it is good practice to consider whether the design wouldn't be better realized by using roles, which are generally safer because they force the class author to explicitly resolve conflicting method names. For more information on roles, see Roles.
The also
declarator
Classes to be inherited from can be listed in the class declaration body by prefixing the is
trait with also
. This also works for the role composition trait does
.
;;
Introspection
Introspection is the process of gathering information about some objects in your program, not by reading the source code, but by querying the object (or a controlling object) for some properties, such as its type.
Given an object $o
and the class definitions from the previous sections, we can ask it a few questions:
my Programmer .= new;if ~~ Employee ;say ~~ GeekCook ?? "It's a geeky cook" !! "Not a geeky cook";say .^name;say .perl;say .^methods(:local)».name.join(', ');
The output might look like this:
It's an employeeNot a geeky cook(Programmer)Programmer.new(known_languages => ["Perl", "Python", "Pascal"],favorite_editor => "gvim", salary => "too small")code_to_solve, known_languages, favorite_editorProgrammer
The first two tests each smartmatch against a class name. If the object is of that class, or of an inheriting class, it returns true. So the object in question is of class Employee
or one that inherits from it, but not GeekCook
.
The .WHAT
method returns the type object associated with the object $o
, which tells us the exact type of $o
: in this case Programmer
.
$o.perl
returns a string that can be executed as Perl code, and reproduces the original object $o
. While this does not work perfectly in all cases, it is very useful for debugging simple objects. [1] $o.^methods(:local)
produces a list of Methods that can be called on $o
. The :local
named argument limits the returned methods to those defined in the Programmer
class and excludes the inherited methods.
The syntax of calling a method with .^
instead of a single dot means that it is actually a method call on its metaclass, which is a class managing the properties of the Programmer
class – or any other class you are interested in. This metaclass enables other ways of introspection too:
say .^attributes.join(', ');say .^parents.map().join(', ');
Finally $o.^name
calls the name
method on the metaobject, which unsurprisingly returns the class name.
Introspection is very useful for debugging and for learning the language and new libraries. When a function or method returns an object you don't know about, by finding its type with .WHAT
, seeing a construction recipe for it with .perl
, and so on, you'll get a good idea of what its return value is. With .^methods
, you can learn what you can do with the class.
But there are other applications too. For instance, a routine that serializes objects to a bunch of bytes needs to know the attributes of that object, which it can find out via introspection.
Overriding default gist method
Some classes might need its own version of gist
, which overrides the terse way it is printed when called to provide a default representation of the class. For instance, exceptions might want to write just the payload
and not the full object so that it is clearer what to see what's happened. However, this isn't limited to exceptions; you can do that with every class:
my = Cook.new(utensils => <spoon ladle knife pan>,cookbooks => ['Cooking for geeks','The French Chef Cookbook']);say Cook.gist; # OUTPUT: «⚗Cook⚗»say .gist; # OUTPUT: «⚗ Cooks with spoon ‣ ladle ‣ knife ‣ pan using «Cooking for geeks» and «The French Chef Cookbook»»
Usually you will want to define two methods, one for the class and another for class instances; in this case, the class method uses the alembic symbol, and the instance method, defined below it, aggregates the data we have on the cook to show it in a narrative way.