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)
      }
    }

### Signature

Unlike most other dynamically-typed languages, in Wren you can have multiple
methods in a class with the same name, as long as they have a different
parameter *signature*. 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 = Unicorn.new()
    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.

Signature is a bit more than just arity. It also lets you distinguish between a
method that takes an *empty* argument list (`()`) and no argument list at all:

    :::dart
    class Confusing {
      method { "no argument list" }
      method() { "empty argument list" }
    }

    var confusing = Confusing.new()
    confusing.method // "no argument list".
    confusing.method() // "empty argument list".

Like the example says, having two methods that differ just by an empty set of
parentheses is pretty confusing. That's not what this is for. It's mainly so
you can define methods that don't take any arguments but look "method-like".

Methods that don't need arguments and don't modify the underlying object tend
to omit the parentheses. These are "getters" and usually access a property of
an object, or produce a new object from it:

    :::dart
    "string".count
    (1..3).min
    0.123.sin

Other methods do change the object, and it's helpful to draw attention to that:

    :::dart
    list.clear()

Since the parentheses are part of the method's signature, the callsite and
definition have to agree. These don't work:

    :::dart
    "string".count()
    list.clear

### 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, call a *constructor method* on its class.
By default, if you don't define any constructors yourself, you get a free one
named `new()`:

    :::dart
    Unicorn.new()

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

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

The `construct` before the method name makes it a constructor. The `new` isn't
special. Constructors can have any name you like, which lets you clarify how it
creates the instance:

    :::dart
    class Unicorn {
      construct brown(name) {
        IO.print("My name is " + name + " and I am brown.")
      }
    }

Constructors can obviously have arguments, and can be overloaded by
[arity](#signature). A constructor *must* be a named method with a (possibly
empty) argument list. Operators, getters, and setters cannot be constructors.

A constructor is actually a pair of methods. You get a method on the class:

    :::dart
    Unicorn.brown("Fred")

That creates the new instance, then it invokes the *initializer* on that
instance. This is where the constructor body you defined gets run.

This distinction is important because it means inside the body of the
constructor, you can access `this`, assign [fields](#fields), call superclass
constructors, etc.

## 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 *private* in Wren&mdash;an object's fields can only be directly
accessed from within methods defined on the object's class. 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.

    :::dart
    class Foo {
      // Set the static field.
      static set(a) {
        __a = a
      }

      setFromInstance(a) {
        __a = a
      }

      // Can use __a in both static methods...
      static bar { __a }

      // ...and instance ones.
      baz { __a }
    }

Just like instance fields, static fields are initially `null`:

    :::dart
    IO.print(Foo.bar) // null.

They can be used from static methods:

    :::dart
    Foo.set("foo")
    IO.print(Foo.bar) // foo.

And also instance methods. When you do so, there is still only one static field
shared among all instances of the class:

    :::dart
    var foo1 = Foo.new()
    var foo2 = Foo.new()

    foo1.setFromInstance("updated")
    IO.print(foo2.baz) // updated.

## 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`.

Note that you should not create classes that inherit from the built-in types (Bool, Num, String, Range, List). The built-in types expect their internal bit representation to be very specific and get horribly confused when you invoke one of the inherited built-in methods on the derived type.

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.

This also means constructors are not inherited:

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

    class Pegasus is Unicorn {}

    Pegasus.new("Fred") // Error!

Each class gets to control how it may be constructed independently of its base
classes. However, constructor *initializers* are inherited since those are
instance methods on the new object.

This means you can do `super` calls inside a constructor:

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

    class Pegasus is Unicorn {
      this new(name) {
        super(name)
      }
    }

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