Raku provides a rich built-in syntax for defining and using classes. It makes writing classes expressive and short for most cases, but also provides mechanisms to cover the rare corner cases.
A quick overview§
Let's start with an example to give an overview:
my = Rectangle.new(length => 2, width => 3);say .area(); # OUTPUT: «6»
We define a new Rectangle class using the class keyword. It has two attributes, $!length and $!width introduced with the has keyword. Both default to 1. Read only accessor methods are automatically generated. (Note the . instead of ! in the declaration, which triggers the generation. Mnemonic: ! resembles a closed door, . an open one.)
The method named area will return the area of the rectangle.
It is rarely necessary to explicitly write a constructor. An automatically inherited default constructor called new will automatically initialize attributes from named parameters passed to the constructor.
The Task example§
As a more elaborate example the following piece of code implements a dependency handler. It showcases custom constructors, private and public attributes, methods, and various aspects of signatures. It's not a lot of code, and yet the result is interesting and useful. It will be used as an example throughout the following sections.
my =Task.new(,Task.new(,Task.new(,Task.new(),Task.new()),Task.new()));.perform();
Class§
Raku, 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.
Attributes§
In the Task class, the first three lines inside the block all declare attributes (called fields or instance storage in other languages) using the has declarator. Just as a my variable cannot be accessed from outside its declared scope, attributes are never directly accessible from outside of the class (this is in contrast to many other languages). This encapsulation is one of the key principles of object oriented design.
Twigil $!§
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 is built;
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 attribute is encapsulated. Private attributes will not be set by the default constructor by default, which is why we add the is built trait to allow just that. Mnemonic: ! looks like a closed door.
The second declaration also uses the private twigil:
has Task is built;
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).
Twigil $.§
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 Raku 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.
is rw trait§
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.
is built trait§
has is built;
By default private attributes are not automatically set by the default constructor. (They are private after all.) In the above example we want to allow the user to provide the initial value but keep the attribute otherwise private. The is built trait allows to do just that.
One can also use it to do the opposite for public attributes, i.e. prevent them to be automatically initialized with a user provided value, but still generate the accessor method:
has is built(False);
Above declaration makes sure one can't construct finished tasks, but still allow users to look if a task is done.
The is built trait was introduced in Rakudo version 2020.01.
is required trait§
Providing a value for an attribute during initialization is optional by default. Which in the task example makes sense for all three, the &!callback, the @!dependencies and the $.done attribute. But lets say we want to add another attribute, $.name, that holds a tasks name and we want to force the user to provide a value on initialization. We can do that as follows:
has is required;
Default values§
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.
Class variables§
A class declaration can also include class variables, declared with my or our, 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. So class variables act similarly to static variables known from other programming languages. They look the same as normal (non class) lexical variables (and in fact they are the same):
is Stris Str-with-IDsay Str-with-ID.new(string => 'First').ID; # OUTPUT: «1»say Str-with-ID.new(string => 'Second').ID; # OUTPUT: «2»say Str-with-ID-and-tag.new( string => 'Third', tag => 'Ordinal' ).ID; # OUTPUT: «3»say ::our-counter; # OUTPUT: «3»
Class variables are shared by all subclasses, in this case Str-with-ID-and-tag. Additionally, when the package scope our declarator is used, the variable is visible via their fully qualified name (FQN), while lexically scoped my variables are "private". This is the exact behavior that my and our also show in non class context.
Class variables act similarly to static variables in many other programming languages.
In this implementation of the Singleton pattern a class variable is used to save the instance.
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. Default values behave as expected and are assigned only once.
Methods§
While attributes give objects state, methods give objects behaviors. Back to our Task example. 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 pushes 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. In the following implementation of a MP3TagData class to extract ID3v1 metadata from an mp3 file, methods parse-data, can-read-format, and trim-nulls are private methods while the remaining ones are public methods:
To call a private method of another class, the caller has to be trusted by the callee. 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. A trust relationship 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.
Construction§
The object construction mechanisms described up to now suffice for most use cases. But if one actually needs to tweak object construction more than said mechanisms allow, it's good to understand how object construction works in more detail.
Raku 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 the Task example does:
method new(, *)
bless§
The biggest difference between constructors in Raku and constructors in languages such as C# and Java is that rather than setting up state on a somehow already magically created object, Raku constructors create the object themselves. They do this 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 nicer 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). One should refrain from putting logic other than reformulating the parameters in the constructor, because constructor methods are not recursively called for parent classes. This is different from e.g. Java.
Declaring new as a method and not as a multi method prevents access to the default constructor. +So if you intend to keep the default constructor available, use multi method new.
TWEAK§
After bless has initialized the classes attributes from the passed values, it will in turn call TWEAK for each class in the inheritance hierarchy. TWEAK gets passed all the arguments passed to bless. This is where custom initialization logic should go.
Remember to always make TWEAK a submethod and not a normal method. If in a class hierarchy a class contains a TWEAK method (declared as a method instead of a submethod) that method is inherited to its subclass and will thus be called twice during construction of the subclass!
BUILD§
It is possible to disable the automatic attribute initialization and perform the initialization of attributes oneself. To do so one needs to write a custom BUILD submethod. There are several edge cases one needs to be aware of and take into account though. This is detailed in the Object Construction Reference. Because of the difficulty of using BUILD, it is recommended to only make use of it when none of the other approaches described above suffices.
Destruction§
Raku is a garbage collecting language. This means that one usually doesn't need to care about cleaning up objects, because Raku does so automatically. Raku does not give any guarantees as to when it will clean up a given object though. It usually does a cleanup run only if the runtime needs the memory, so we can't rely on when it's going to happen.
To run custom code when an object is cleaned up one can use the DESTROY submethod. It can for example be used to close handles or supplies or delete temporary files that are no longer going to be used. As garbage collection can happen at arbitrary points during the runtime of our program, even in the middle of some totally unrelated piece of code in a different thread, we must make sure to not assume any context in our DESTROY submethod.
my = 0;my ;for 1 .. 6000say "DESTROY called $in_destructor times";
This might print something like DESTROY called 5701 times and possibly only kicks in after we have stomped over former instances of Foo a few thousand times. We also can't rely, on the order of destruction.
Same as TWEAK: Make sure to always declare DESTROY as a submethod.
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 money going to the store buying food cleaning kitchen making dinner eating dinner. NOM!
A word on types§
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 Raku built-in classes. One can call methods on these type objects. So there is nothing special with calling the new method on a type object.
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)
In function signatures one can use so called 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!»
Inheritance§
Object Oriented Programming provides the concept of inheritance as one of the mechanisms for code reuse. Raku 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 => <Raku Perl Erlang C++>,favorite_editor => 'vim');say .code_to_solve('halting problem')," will get \$ ";# OUTPUT: «Solving halting problem using vim in Raku will get $100000»
Overriding inherited methods§
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.raku; # OUTPUT: «["spoon", "ladle", "knife", "pan"]»say .cookbooks.raku; # 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.raku; # OUTPUT: «["self cleaning oven"]»say .cookbooks.raku; # 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. Raku 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 Raku',utensils => ('stainless steel pot', 'knife', 'calibrated oven'),favorite_editor => 'MacVim',known_languages => <Raku>);.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.
also§
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 .raku;say .^methods(:local)».name.join(', ');
The output might look like this:
It's an employee
Not a geeky cook
Programmer
Programmer.new(known_languages => ["Perl", "Python", "Pascal"],
favorite_editor => "gvim", salary => "too small")
code_to_solve, known_languages, favorite_editor
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 call $o.^name tells us the type of $o; in this case Programmer.
$o.raku returns a string that can be executed as Raku 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]
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.
Given an object $mp3 and the MP3TagData class definition from the section Private methods, we can inquire about its public methods with .^methods:
my = MP3TagData.new(filename => 'football-head.mp3');say .^methods(:local);# OUTPUT: (TWEAK filename title artist album year comment genre track version# type Submethod+{is-hidden-from-backtrace}.new)
$mp3.^methods(:local) produces a list of Methods that can be called on $mp3. The :local named argument limits the returned methods to those defined in the MP3TagData class and excludes the inherited methods; MP3TagData inherits from no class, so providing :local makes no difference.
To check if a type object (or an instance object) implements a certain public method, use the .^find-method metamethod, which returns the method object if it exists. Otherwise, it returns Mu.
say .^find_method('name'); # OUTPUT: «(Mu)»say .^find_method('artist'); # OUTPUT: «artist»
Type objects can also be introspected for its private methods. However, public and private methods don't use the same APIs, and thus different metamethods must be used: .^private_methods and .^find_private_method.
say .^private_methods; # OUTPUT: «(parse-data can-read-format trim-nulls)»say .^find_private_method('parse-data'); # OUTPUT: «parse-data»say .^find_private_method('remove-nulls'); # OUTPUT: «(Mu)»
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 .^name, seeing a construction recipe for it with .raku, 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 their own version of gist, which overrides the terse way they are 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.
A practical introspection example§
When one creates a new class, it is sometimes useful to have informative (and safe) introspection accessible more easily as a public method. For example, the following class is used to hold attributes for a record row in a CSV spreadsheet with a header row defining its field (attribute) names.
unit ;has ;has ;#...more fields (attributes)...method fields(--> List)method values(--> List)
We use it with a simple CSV file with contents:
last, first #...more fields... Wall, Larry Conway, Damian
Load the first record and show its contents:
my = CSV-Record.new: :, :;say .fields.raku; # OUTPUT: «["last", "first"]»say .values.raku; # OUTPUT: «["Wall", "Larry"]»
Note that practically we would have designed the class so that it has the fields list as a constant since its values are the same for all class objects:
constant = <last first>;method fields(--> List)
Downsides of using the introspective method for attribute names include slightly more processing time and power and the probable need to remove the sigil and twigil for public presentation.