wren/doc/site/classes.markdown

^title Classes
^category types

Every value in Wren is an object, and every object is an instance of a class.
Even `true` and `false` are full-featured objects—instances of the `Bool`
class.

Classes contain both *behavior* and *state*. Behavior is defined in *methods*
which are stored in the class. State is defined in *fields*, whose values are
stored in each instance.

## Defining a class

Classes are created using the `class` keyword, unsurprisingly:

    :::dart
    class Unicorn {}

This creates a class named `Unicorn` with no methods or fields.

## Methods

To let our unicorn do stuff, we need to give it methods.

    :::dart
    class Unicorn {
      prance {
        IO.print("The unicorn prances in a fancy manner!")
      }
    }

This defines a `prance` method that takes no arguments. To support parameters,
add a parenthesized parameter list after the method's name:

    :::dart
    class Unicorn {
      prance(where, when) {
        IO.print("The unicorn prances in " + where + " at " + when)
      }
    }

### Arity

Unlike most other dynamically-typed languages, in Wren you can have multiple
methods in a class with the same name, as long as they take a different number
of parameters. In technical terms, you can *overload by arity*. So this class
is fine:

    :::dart
    class Unicorn {
      prance {
        IO.print("The unicorn prances in a fancy manner!")
      }

      prance(where) {
        IO.print("The unicorn prances in " + where)
      }

      prance(where, when) {
        IO.print("The unicorn prances in " + where + " at " + when)
      }
    }

And you can call each of the methods like so:

    :::dart
    var unicorn = new Unicorn
    unicorn.prance
    unicorn.prance("Antwerp")
    unicorn.prance("Brussels", "high noon")

The number of arguments provided at the callsite determines which method is
chosen.

It's often natural to have the same conceptual operation work with different
sets of arguments. In other languages, you'd define a single method for the
operation and have to check for "undefined" or missing arguments. Wren just
treats them as different methods that you can implement separately.

### Operators

Operators are just special syntax for a method call on the left hand operand
(or only operand in the case of unary operators like `!` and `~`). In other
words, you can think of `a + b` as meaning `a.+(b)`.

You can define operators in your class like so:

    :::dart
    class Unicorn {
      // Infix:
      +(other) {
        IO.print("Adding to a unicorn?")
      }

      // Prefix:
      ! {
        IO.print("Negating a unicorn?!")
      }
    }

This can be used to define any of these operators:

    :::dart
    // Infix:
    +  -  *  /  %  <  >  <=  >=  ==  !=  &  |

    // Prefix:
    !  ~  -

Note that `-` can be both a prefix and infix operator. If there's a parameter
list, it's the infix one, otherwise, it's prefix. Since Wren supports
overloading by arity, it's no problem for a class to define both.

### Subscript operators

**TODO**

### Setters

**TODO**

## Constructors

To create a new instance of a class, you use the `new` keyword. We can make a
unicorn like so:

    :::dart
    new Unicorn

You almost always want to define some state or do some other initialization on
a new object. For that, you'll want to define a constructor, like so:

    :::dart
    class Unicorn {
      new {
        IO.print("I am a constructor!")
      }
    }

When you create an instance with `new`, its constructor will be invoked. It's
just a method with a special name. Like methods, you can pass arguments to the
constructor by adding a parenthesized parameter list after `new`:

    :::dart
    class Unicorn {
      new(name, color) {
        IO.print("My name is " + name + " and I am " + color + ".")
      }
    }

Values are passed to the constructor like so:

    :::dart
    new Unicorn("Flicker", "purple")

Like other methods, you can overload constructors by [arity](#arity).

## Fields

All state stored in instances is stored in *fields*. Each field has a named
that starts with an underscore.

    :::dart
    class Rectangle {
      area { _width * _height }

      // Other stuff...
    }

Here, `_width` and `_height` in the `area` [getter](classes.html#methods) refer
to fields on the rectangle instance. You can think of them like `this.width`
and `this.height` in other languages.

When a field name appears, Wren looks for the nearest enclosing class and looks
up the field on the instance of that class. Field names cannot be used outside
of an instance method. They *can* be used inside a [function](functions.html)
in a method. Wren will look outside any nested functions until it finds an
enclosing method.

Unlike [variables](variables.html), fields are implicitly declared by simply
assigning to them. If you access a field before it has been initialized, its
value is `null`.

### Encapsulation

All fields are *protected* in Wren&mdash;an object's fields can only be
directly accessed from within methods defined on the object's class, or its
superclasses. You cannot even access fields on another instance of your own
class, unlike C++ and Java.

If you want to make a property of an object visible, you need to define a
getter to expose it:

    :::dart
    class Rectangle {
      width { _width }
      height { _height }

      // ...
    }

To allow outside code to modify the field, you'll also need to provide setters:

    :::dart
    class Rectangle {
      width=(value) { _width = value }
      height=(value) { _height = value }
    }

One thing we've learned in the past forty years of software engineering is that
encapsulating state tends to make code easier to maintain, so Wren defaults to
keeping your object's state pretty tightly bundled up. Don't feel that you have
to or even should define getters or setters for most of your object's fields.

## Metaclasses and static members

**TODO**

### Static fields

A name that starts with *two* underscores is a *static* field. They work
similar to [fields](#fields) except the data is stored on the class itself, and
not the instance. They can be used in *both* instance and static methods.

**TODO: Example.**


## Inheritance

A class can inherit from a "parent" or *superclass*. When you invoke a method
on an object of some class, if it can't be found, it walks up the chain of
superclasses looking for it there.

By default, any new class inherits from `Object`, which is the superclass from
which all other classes ultimately descend. You can specify a different parent
class using `is` when you declare the class:

    :::dart
    class Pegasus is Unicorn {}

This declares a new class `Pegasus` that inherits from `Unicorn`.

The metaclass hierarchy does *not* parallel the regular class hierarchy. So, if
`Pegasus` inherits from `Unicorn`, `Pegasus`'s metaclass will not inherit from
`Unicorn`'s metaclass. In more prosaic terms, this means that static methods
are not inherited.

    :::dart
    class Unicorn {
      // Unicorns cannot fly. :(
      static canFly { false }
    }

    class Pegasus is Unicorn {}

    Pegasus.canFly // ERROR: Static methods are not inherited.

Constructors, however, initialize the instance *after* it has been created.
They are defined as instance methods on the class and not on the metaclass.
That means that constructors *are* inherited.

    :::dart
    class Unicorn {
      new(name) {
        IO.print("My name is " + name + ".")
      }
    }

    class Pegasus is Unicorn {}

    new Pegasus("Fred") // Prints "My name is Fred.".