Types
#####

Types describe how values behave and what they are made of. Since everything in
Jam is a value, types are also values and can be used as such.

Nullability
===========

A type may be explicitly marked as nullable, meaning that the ``null`` value may
be assigned to it. A nullable type does not allow member access unless the value
is checked for ``null``, or is cast to the type's non-nullable counterpart.

A type may be marked as nullable either at definition or declaration.

Classes
=======

A class is a user defined type. A class may have instance fields and methods,
which can be accessed as attributes. It may also inherit functionality from
other types, or include functionality from :doc:`traits`.

Metaclasses
===========

Since a core concept of Jam is that everything is an object, and objects have
types, classes must therefore also have a type. The type of a class is called a
metaclass. A metaclass is a class whose instances are classes, as such they
define the attributes and functionality of the class itself.

.. note::
    There is no syntax for metaclasses yet.

Syntax
======

.. productionlist::
    Nullable: `Value` ["?"]
    InstanceVariable: `Variable` [ "=" `Value` ]
    ClassMethod: `Method` | `TypeCastDef` | `OperationDef`
    TypeCastDef: `ImplicitCastDef` | `ExplicitCastDef`
    ImplicitCastDef: "def" "self" ":" `Value` `InstructionSet` "end"
    ExplicitCastDef: "def" "self" "as" `Value` `InstructionSet` "end"
    OperationDef: `BinaryOperationDef` | `UnaryOperationDef`
    BinaryOperationDef: "def" "self" `BinaryOperation` `Argument` [ "->" `Value` ]
    UnaryOperationDef: "def" `UnaryOperation` "self" [ "->" `Value` ]
    ClassInstruction: `ClassConstructor` | `InstanceVariable` | `ClassMethod`
    ClassInstructionSet: ( `ClassInstruction` \n )*
    ClassConstructorPrototype: "(" [ [`Argument` ","]* `Argument` ] ")"
    ClassConstructor: "new" `ClassConstructorPrototype` `InstructionSet` "end"
    ClassPrototype: `Identifier` ["?"] [ "(" `Value` ")" ]
    Class: "class" `ClassPrototype` `ClassInstructionSet` "end"