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vim9class.txt*	For Vim version 9.1.  Last change: 2024 Apr 13



		  VIM REFERENCE MANUAL	  by Bram Moolenaar


Vim9 classes, objects, interfaces, types and enums.		*vim9-class*

1.  Overview			|Vim9-class-overview|
2.  A simple class		|Vim9-simple-class|
3.  Class variables and methods	|Vim9-class-member|
4.  Using an abstract class	|Vim9-abstract-class|
5.  Using an interface		|Vim9-using-interface|
6.  More class details		|Vim9-class|
7.  Type definition		|Vim9-type|
8.  Enum			|Vim9-enum|

9.  Rationale
10. To be done later

==============================================================================

1. Overview					*Vim9-class-overview*

The fancy term is "object-oriented programming".  You can find lots of study
material on this subject.  Here we document what |Vim9| script provides,
assuming you know the basics already.  Added are helpful hints about how to
use this functionality effectively.  Vim9 classes and objects cannot be used
in legacy Vim scripts and legacy functions.

The basic item is an object:
- An object stores state.  It contains one or more variables that can each
  have a value.
- An object provides functions that use and manipulate its state.  These
  functions are invoked "on the object", which is what sets it apart from the
  traditional separation of data and code that manipulates the data.
- An object has a well defined interface, with typed member variables and
  methods.
- Objects are created from a class and all objects have the same interface.
  This does not change at runtime, it is not dynamic.

An object can only be created by a class.  A class provides:
- A new() method, the constructor, which returns an object for the class.
  This method is invoked on the class name: MyClass.new().
- State shared by all objects of the class: class variables (class members).
- A hierarchy of classes, with super-classes and sub-classes, inheritance.

An interface is used to specify properties of an object:
- An object can declare several interfaces that it implements.
- Different objects implementing the same interface can be used the same way.

The class hierarchy allows for single inheritance.  Otherwise interfaces are
to be used where needed.


Class modeling ~

You can model classes any way you like.  Keep in mind what you are building,
don't try to model the real world.  This can be confusing, especially because
teachers use real-world objects to explain class relations and you might think
your model should therefore reflect the real world.  It doesn't!  The model
should match your purpose.

Keep in mind that composition (an object contains other objects) is often
better than inheritance (an object extends another object).  Don't waste time
trying to find the optimal class model.  Or waste time discussing whether a
square is a rectangle or that a rectangle is a square.  It doesn't matter.


==============================================================================

2.  A simple class				*Vim9-simple-class*

Let's start with a simple example: a class that stores a text position (see
below for how to do this more efficiently): >

	class TextPosition
	   var lnum: number
	   var col: number

	   def new(lnum: number, col: number)
	      this.lnum = lnum
	      this.col = col
	   enddef

	   def SetLnum(lnum: number)
	      this.lnum = lnum
	   enddef

	   def SetCol(col: number)
	      this.col = col
	   enddef

	   def SetPosition(lnum: number, col: number)
	      this.lnum = lnum
	      this.col = col
	   enddef
	 endclass
<							*object* *Object*
You can create an object from this class with the new() method: >

	var pos = TextPosition.new(1, 1)
<
The object variables "lnum" and "col" can be accessed directly: >

	echo 


The text position is ({pos.lnum}, {pos.col})'
<						    *E1317* *E1327* *:this*
If you have been using other object-oriented languages you will notice that in
Vim, within a class definition, the declared object members are consistently
referred to with the "this." prefix.  This is different from languages like
Java and TypeScript.  The naming convention makes the object members easy to
spot.  Also, when a variable does not have the "this." prefix you know it is
not an object variable.
								*E1411*
From outside the class definition, access an object's methods and variables by
using the object name followed by a dot following by the member: >

	pos.lnum
	pos.SetCol(10)
<
							*E1405* *E1406*
A class name cannot be used as an expression.  A class name cannot be used in
the left-hand-side of an assignment.


Object variable write access ~
						    *read-only-variable*
Now try to change an object variable directly: >

	pos.lnum = 9
<							*E1335*
This will give you an error!  That is because by default object variables can
be read but not set.  That's why the TextPosition class provides a method for
it: >

	pos.SetLnum(9)

Allowing to read but not set an object variable is the most common and safest
way.  Most often there is no problem using a value, while setting a value may
have side effects that need to be taken care of.  In this case, the SetLnum()
method could check if the line number is valid and either give an error or use
the closest valid value.
					*:public* *public-variable* *E1331*
If you don't care about side effects and want to allow the object variable to
be changed at any time, you can make it public: >

	public var lnum: number
	public var col: number

Now you don't need the SetLnum(), SetCol() and SetPosition() methods, setting
"pos.lnum" directly above will no longer give an error.
							*E1326*
If you try to set an object variable that doesn't exist you get an error: >
	pos.other = 9
<	E1326: Member not found on object "TextPosition": other ~

							*E1376*
A object variable cannot be accessed using the class name.

Protected variables ~
					*protected-variable* *E1332* *E1333*
On the other hand, if you do not want the object variables to be read directly
from outside the class or its sub-classes, you can make them protected.  This
is done by prefixing an underscore to the name: >

	var _lnum: number
	var _col: number

Now you need to provide methods to get the value of the protected variables.
These are commonly called getters.  We recommend using a name that starts with
"Get": >

	def GetLnum(): number
	   return this._lnum
	enddef

	def GetCol(): number
	   return this._col
	enddef

This example isn't very useful, the variables might as well have been public.
It does become useful if you check the value.  For example, restrict the line
number to the total number of lines: >

	def GetLnum(): number
	   if this._lnum > this._lineCount
	      return this._lineCount
	   endif
	   return this._lnum
	enddef
<
Protected methods ~
						*protected-method* *E1366*
If you want object methods to be accessible only from other methods of the
same class and not used from outside the class, then you can make them
protected.  This is done by prefixing the method name with an underscore: >

    class SomeClass
	def _Foo(): number
	  return 10
	enddef
	def Bar(): number
	  return this._Foo()
	enddef
    endclass
<
Accessing a protected method outside the class will result in an error (using
the above class): >

    var a = SomeClass.new()
    a._Foo()
<
Simplifying the new() method ~
						*new()* *constructor*
See also |default-constructor| and |multiple-constructors|.

Many constructors take values for the object variables.  Thus you very often
see this pattern: >

	 class SomeClass
	   var lnum: number
	   var col: number

	   def new(lnum: number, col: number)
	      this.lnum = lnum
	      this.col = col
	   enddef
	 endclass
<
							*E1390*
Not only is this text you need to write, it also has the type of each
variable twice.  Since this is so common a shorter way to write new() is
provided: >

	   def new(this.lnum, this.col)
	   enddef

The semantics are easy to understand: Providing the object variable name,
including "this.", as the argument to new() means the value provided in the
new() call is assigned to that object variable.  This mechanism comes from the
Dart language.

Putting together this way of using new() and making the variables public
results in a much shorter class definition than what we started with: >

	class TextPosition
	   public var lnum: number
	   public var col: number

	   def new(this.lnum, this.col)
	   enddef

	   def SetPosition(lnum: number, col: number)
	      this.lnum = lnum
	      this.col = col
	   enddef
	 endclass

The sequence of constructing a new object is:
1. Memory is allocated and cleared.  All values are zero/false/empty.
2. For each declared object variable that has an initializer, the expression
   is evaluated and assigned to the variable.  This happens in the sequence
   the variables are declared in the class.
3. Arguments in the new() method in the "this.name" form are assigned.
4. The body of the new() method is executed.

If the class extends a parent class, the same thing happens.  In the second
step the object variables of the parent class are initialized first.  There is
no need to call "super()" or "new()" on the parent.

							*E1365*
When defining the new() method the return type should not be specified.  It
always returns an object of the class.

							*E1386*
When invoking an object method, the method name should be preceded by the
object variable name.  An object method cannot be invoked using the class
name.

==============================================================================

3.  Class Variables and Methods			*Vim9-class-member*

					    *:static* *E1337* *E1338* *E1368*
Class members are declared with "static".  They are used by the name without a
prefix in the class where they are defined: >

	class OtherThing
	   var size: number
	   static var totalSize: number

	   def new(this.size)
	      totalSize += this.size
	   enddef
	endclass
<							*E1340* *E1341*
Since the name is used as-is, shadowing the name by a method argument name
or local variable name is not allowed.

					    *E1374* *E1375* *E1384* *E1385*
To access a class member outside of the class where it is defined, the class
name prefix must be used.  A class member cannot be accessed using an object.

Just like object members the access can be made protected by using an
underscore as the first character in the name, and it can be made public by
prefixing "public": >

    class OtherThing
	static var total: number	  # anybody can read, only class can write
	static var _sum: number	          # only class can read and write
	public static var result: number  # anybody can read and write
    endclass
<
							*class-method*
Class methods are also declared with "static".  They can use the class
variables but they have no access to the object variables, they cannot use the
"this" keyword:
>
	class OtherThing
	   var size: number
	   static var totalSize: number

	   # Clear the total size and return the value it had before.
	   static def ClearTotalSize(): number
	      var prev = totalSize
	      totalSize = 0
	      return prev
	   enddef
	endclass

Inside the class, the class method can be called by name directly, outside the
class, the class name must be prefixed: `OtherThing.ClearTotalSize()`.  Also,
the name prefix must be used for public class methods in the special contexts
of class variable initializers and of lambda expressions and nested functions:
>
    class OtherThing
	static var name: string = OtherThing.GiveName()

	static def GiveName(): string
	    def DoGiveName(): string
		return OtherThing.NameAny()
	    enddef

	    return DoGiveName()
	enddef

	static def NameAny(): string
	    return "any"
	enddef
    endclass
<

Just like object methods the access can be made protected by using an
underscore as the first character in the method name: >

    class OtherThing
	static def _Foo()
	    echo "Foo"
	enddef
	def Bar()
	    _Foo()
	enddef
    endclass
<
							*E1370*
Note that constructors cannot be declared as "static". They are called like a
static but execute as an object method; they have access to "this".

To access the class methods and class variables of a super class in an
extended class, the class name prefix should be used just as from anywhere
outside of the defining class: >

    vim9script
    class Vehicle
	static var nextID: number = 1000
	static def GetID(): number
	    nextID += 1
	    return nextID
	enddef
    endclass
    class Car extends Vehicle
	var myID: number
	def new()
	    this.myID = Vehicle.GetID()
	enddef
    endclass
<
Class variables and methods are not inherited by a child class.  A child class
can declare a static variable or a method with the same name as the one in the
super class.  Depending on the class where the member is used the
corresponding class member will be used.  The type of the class member in a
child class can be different from that in the super class.

The double underscore (__) prefix for a class or object method name is
reserved for future use.

					*object-final-variable* *E1409*
The |:final| keyword can be used to make a class or object variable a
constant.  Examples: >

    class A
	final v1 = [1, 2]		# final object variable
	public final v2 = {x: 1}	# final object variable
	static final v3 = 'abc'		# final class variable
	public static final v4 = 0z10	# final class variable
    endclass
<
A final variable can be changed only from a constructor function.  Example: >

    class A
	final v1: list<number>
	def new()
	    this.v1 = [1, 2]
	enddef
    endclass
    var a = A.new()
    echo a.v1
<
Note that the value of a final variable can be changed.  Example: >

    class A
	public final v1 = [1, 2]
    endclass
    var a = A.new()
    a.v1[0] = 6			# OK
    a.v1->add(3)		# OK
    a.v1 = [3, 4]		# Error
<
							*E1408*
Final variables are not supported in an interface.  A class or object method
cannot be final.

					*object-const-variable*
The |:const| keyword can be used to make a class or object variable and the
value a constant.  Examples: >

    class A
	const v1 = [1, 2]		# const object variable
	public const v2 = {x: 1}	# const object variable
	static const v3 = 'abc'		# const class variable
	public static const v4 = 0z10	# const class variable
    endclass
<
A const variable can be changed only from a constructor function. Example: >

    class A
	const v1: list<number>
	def new()
	    this.v1 = [1, 2]
	enddef
    endclass
    var a = A.new()
    echo a.v1
<
A const variable and its value cannot be changed.  Example: >

    class A
	public const v1 = [1, 2]
    endclass
    var a = A.new()
    a.v1[0] = 6			# Error
    a.v1->add(3)		# Error
    a.v1 = [3, 4]		# Error
<
							 *E1410*
Const variables are not supported in an interface.  A class or object method
cannot be a const.

==============================================================================

4.  Using an abstract class			*Vim9-abstract-class*

An abstract class forms the base for at least one sub-class.  In the class
model one often finds that a few classes have the same properties that can be
shared, but a class with these properties does not have enough state to create
an object from.  A sub-class must extend the abstract class and add the
missing state and/or methods before it can be used to create objects for.

For example, a Shape class could store a color and thickness.  You cannot
create a Shape object, it is missing the information about what kind of shape
it is.  The Shape class functions as the base for a Square and a Triangle
class, for which objects can be created.  Example: >

	abstract class Shape
	   var color = Color.Black
	   var thickness = 10
	endclass

	class Square extends Shape
	   var size: number

	   def new(this.size)
	   enddef
	endclass

	class Triangle extends Shape
	   var base: number
	   var height: number

	   def new(this.base, this.height)
	   enddef
	endclass
<
An abstract class is defined the same way as a normal class, except that it
does not have any new() method. *E1359*

					    *abstract-method* *E1371* *E1372*
An abstract method can be defined in an abstract class by using the "abstract"
prefix when defining the method: >

	abstract class Shape
	   abstract def Draw()
	endclass
<
A static method in an abstract class cannot be an abstract method.

						*E1373*
A non-abstract class extending the abstract class must implement all the
abstract methods.  The signature (arguments, argument types and return type)
must be exactly the same.  If the return type of a method is a class, then
that class or one of its subclasses can be used in the extended method.

==============================================================================

5.  Using an interface				*Vim9-using-interface*

The example above with Shape, Square and Triangle can be made more useful if
we add a method to compute the surface of the object.  For that we create the
interface called HasSurface, which specifies one method Surface() that returns
a number.  This example extends the one above: >

	abstract class Shape
	   var color = Color.Black
	   var thickness = 10
	endclass

	interface HasSurface
	   def Surface(): number
	endinterface

	class Square extends Shape implements HasSurface
	   var size: number

	   def new(this.size)
	   enddef

	   def Surface(): number
	      return this.size * this.size
	   enddef
	endclass

	class Triangle extends Shape implements HasSurface
	   var base: number
	   var height: number

	   def new(this.base, this.height)
	   enddef

	   def Surface(): number
	      return this.base * this.height / 2
	   enddef
	endclass
<
					*E1348* *E1349* *E1367* *E1382* *E1383*
If a class declares to implement an interface, all the items specified in the
interface must appear in the class, with the same types.

The interface name can be used as a type: >

	var shapes: list<HasSurface> = [
				Square.new(12),
				Triangle.new(8, 15),
				]
	for shape in shapes
	   echo 


the surface is {shape.Surface()}'
	endfor
<
					*E1378* *E1379* *E1380* *E1387*
An interface can contain only object methods and read-only object variables.
An interface cannot contain read-write or protected object variables,
protected object methods, class variables and class methods.

An interface can extend another interface using "extends".  The sub-interface
inherits all the instance variables and methods from the super interface.

==============================================================================

6.  More class details				*Vim9-class* *Class* *class*

Defining a class ~
					*:class* *:endclass* *:abstract*
A class is defined between `:class` and `:endclass`.  The whole class is
defined in one script file.  It is not possible to add to a class later.

A class can only be defined in a |Vim9| script file.  *E1316*
A class cannot be defined inside a function.  *E1429*

It is possible to define more than one class in a script file.  Although it
usually is better to export only one main class.  It can be useful to define
types, enums and helper classes though.

The `:abstract` keyword may be prefixed and `:export` may be used.  That gives
these variants: >

	class ClassName
	endclass

	export class ClassName
	endclass

	abstract class ClassName
	endclass

	export abstract class ClassName
	endclass
<
							*E1314*
The class name should be CamelCased.  It must start with an uppercase letter.
That avoids clashing with builtin types.
							*E1315*
After the class name these optional items can be used.  Each can appear only
once.  They can appear in any order, although this order is recommended: >
	extends ClassName
	implements InterfaceName, OtherInterface
	specifies SomeInterface
<							*E1355* *E1369*
Each variable and method name can be used only once.  It is not possible to
define a method with the same name and different type of arguments.  It is not
possible to use a public and protected member variable with the same name.  An
object variable name used in a super class cannot be reused in a child class.


Object Variable Initialization ~

If the type of a variable is not explicitly specified in a class, then it is
set to "any" during class definition.  When an object is instantiated from the
class, then the type of the variable is set.

The following reserved keyword names cannot be used as an object or class
variable name: "super", "this", "true", "false", "null", "null_blob",
"null_dict", "null_function", "null_list", "null_partial", "null_string",
"null_channel" and "null_job".

Extending a class ~
							*extends*
A class can extend one other class. *E1352* *E1353* *E1354*
The basic idea is to build on top of an existing class, add properties to it.

The extended class is called the "base class" or "super class".  The new class
is called the "child class".

Object variables from the base class are all taken over by the child class.  It
is not possible to override them (unlike some other languages).

						*E1356* *E1357* *E1358*
Object methods of the base class can be overruled.  The signature (arguments,
argument types and return type) must be exactly the same.  If the return type
of a method is a class, then that class or one of its subclasses can be used
in the extended method.  The method of the base class can be called by
prefixing "super.".

						*E1377*
The access level of a method (public or protected) in a child class should be
the same as the super class.

Other object methods of the base class are taken over by the child class.

Class methods, including methods starting with "new", can be overruled, like
with object methods.  The method on the base class can be called by prefixing
the name of the class (for class methods) or "super.".

Unlike other languages, the constructor of the base class does not need to be
invoked.  In fact, it cannot be invoked.  If some initialization from the base
class also needs to be done in a child class, put it in an object method and
call that method from every constructor().

If the base class did not specify a new() method then one was automatically
created.  This method will not be taken over by the child class.  The child
class can define its own new() method, or, if there isn't one, a new() method
will be added automatically.


A class implementing an interface ~
					*implements* *E1346* *E1347* *E1389*
A class can implement one or more interfaces.  The "implements" keyword can
only appear once *E1350* .  Multiple interfaces can be specified, separated by
commas.  Each interface name can appear only once. *E1351*


A class defining an interface ~
							*specifies*
A class can declare its interface, the object variables and methods, with a
named interface.  This avoids the need for separately specifying the
interface, which is often done in many languages, especially Java.


Items in a class ~
						*E1318* *E1325* *E1388*
Inside a class, in between `:class` and `:endclass`, these items can appear:
- An object variable declaration: >
	var _protectedVariableName: memberType
	var readonlyVariableName: memberType
	public var readwriteVariableName: memberType
- A class variable declaration: >
	static var _protectedClassVariableName: memberType
	static var readonlyClassVariableName: memberType
	public static var readwriteClassVariableName: memberType
- A constructor method: >
	def new(arguments)
	def newName(arguments)
- A class method: >
	static def SomeMethod(arguments)
	static def _ProtectedMethod(arguments)
- An object method: >
	def SomeMethod(arguments)
	def _ProtectedMethod(arguments)

For the object variable the type must be specified.  The best way is to do
this explicitly with ": {type}".  For simple types you can also use an
initializer, such as "= 123", and Vim will see that the type is a number.
Avoid doing this for more complex types and when the type will be incomplete.
For example: >
	var nameList = []
This specifies a list, but the item type is unknown.  Better use: >
	var nameList: list<string>
The initialization isn't needed, the list is empty by default.
							*E1330*
Some types cannot be used, such as "void", "null" and "v:none".

Builtin Object Methods ~
                                                       *builtin-object-methods*
Some of the builtin functions like |empty()|, |len()| and |string()| can be
used with an object.  An object can implement a method with the same name as
these builtin functions to return an object-specific value.

                                                       *E1412*
The following builtin methods are supported:
                                                       *object-empty()*
    empty()  Invoked by the |empty()| function to check whether an object is
	     empty.  If this method is missing, then true is returned.  This
	     method should not accept any arguments and must return a boolean.
                                                       *object-len()*
    len()    Invoked by the |len()| function to return the length of an
	     object.  If this method is missing in the class, then an error is
	     given and zero is returned.  This method should not accept any
	     arguments and must return a number.
                                                       *object-string()*
    string() Invoked by the |string()| function to get a textual
	     representation of an object.  Also used by the |:echo| command
	     for an object.  If this method is missing in the class, then a
	     built-in default textual representation is used.  This method
	     should not accept any arguments and must return a string.

                                                       *E1413*
A class method cannot be used as a builtin method.

Defining an interface ~
					*Interface* *:interface* *:endinterface*
An interface is defined between `:interface` and `:endinterface`.  It may be
prefixed with `:export`: >

	interface InterfaceName
	endinterface

	export interface InterfaceName
	endinterface
<							*E1344*
An interface can declare object variables, just like in a class but without
any initializer.
							*E1345*
An interface can declare methods with `:def`, including the arguments and
return type, but without the body and without `:enddef`.  Example: >

	interface HasSurface
	   var size: number
	   def Surface(): number
	endinterface

An interface name must start with an uppercase letter. *E1343*
The "Has" prefix can be used to make it easier to guess this is an interface
name, with a hint about what it provides.
An interface can only be defined in a |Vim9| script file.  *E1342*
An interface cannot "implement" another interface but it can "extend" another
interface. *E1381*


null object ~

When a variable is declared to have the type of an object, but it is not
initialized, the value is null.  When trying to use this null object Vim often
does not know what class was supposed to be used.  Vim then cannot check if
a variable name is correct and you will get a "Using a null object" error,
even when the variable name is invalid. *E1360* *E1362*


Default constructor ~
							*default-constructor*
In case you define a class without a new() method, one will be automatically
defined.  This default constructor will have arguments for all the object
variables, in the order they were specified.  Thus if your class looks like: >

	class AutoNew
	   var name: string
	   var age: number
	   var gender: Gender
	endclass

Then the default constructor will be: >

	def new(this.name = v:none, this.age = v:none, this.gender = v:none)
	enddef

The "= v:none" default values make the arguments optional.  Thus you can also
call `new()` without any arguments.  No assignment will happen and the default
value for the object variables will be used.  This is a more useful example,
with default values: >

	class TextPosition
	   var lnum: number = 1
	   var col: number = 1
	endclass

If you want the constructor to have mandatory arguments, you need to write it
yourself.  For example, if for the AutoNew class above you insist on getting
the name, you can define the constructor like this: >

	def new(this.name, this.age = v:none, this.gender = v:none)
	enddef
<
When using the default new() method, if the order of the object variables in
the class is changed later, then all the callers of the default new() method
need to change.  To avoid this, the new() method can be explicitly defined
without any arguments.

							*E1328*
Note that you cannot use another default value than "v:none" here.  If you
want to initialize the object variables, do it where they are declared.  This
way you only need to look in one place for the default values.

All object variables will be used in the default constructor, including
protected access ones.

If the class extends another one, the object variables of that class will come
first.


Multiple constructors ~
						*multiple-constructors*
Normally a class has just one new() constructor.  In case you find that the
constructor is often called with the same arguments you may want to simplify
your code by putting those arguments into a second constructor method.  For
example, if you tend to use the color black a lot: >

	def new(this.garment, this.color, this.size)
	enddef
	...
	var pants = new(Garment.pants, Color.black, "XL")
	var shirt = new(Garment.shirt, Color.black, "XL")
	var shoes = new(Garment.shoes, Color.black, "45")

Instead of repeating the color every time you can add a constructor that
includes it: >

	def newBlack(this.garment, this.size)
	   this.color = Color.black
	enddef
	...
	var pants = newBlack(Garment.pants, "XL")
	var shirt = newBlack(Garment.shirt, "XL")
	var shoes = newBlack(Garment.shoes, "9.5")

Note that the method name must start with "new".  If there is no method called
"new()" then the default constructor is added, even though there are other
constructor methods.

Compiling methods in a Class ~
							*class-compile*
The |:defcompile| command can be used to compile all the class and object
methods defined in a class: >

	defcompile MyClass	# Compile class "MyClass"
	defcompile		# Compile the classes in the current script
<
==============================================================================

7.  Type definition				*typealias* *Vim9-type* *:type*

					*E1393* *E1395* *E1396* *E1397* *E1398*
A type definition is giving a name to a type specification.  This is also
known as a "type alias".  The type alias can be used wherever a built-in type
can be used.  Example: >

    type ListOfStrings = list<string>
    var s: ListOfStrings = ['a', 'b']

    def ProcessStr(str: ListOfStrings): ListOfStrings
	return str
    enddef
    echo ProcessStr(s)
<
							*E1394*
A type alias name must start with an upper case character.  Only existing
types can be aliased.

							*E1399*
A type alias can be created only at the script level and not inside a
function.  A type alias can be exported and used across scripts.

					*E1400* *E1401* *E1402* *E1403* *E1407*
A type alias cannot be used as an expression.  A type alias cannot be used in
the left-hand-side of an assignment.

For a type alias name, the |typename()| function returns the type that is
aliased: >

    type ListOfStudents = list<dict<any>>
    echo typename(ListOfStudents)
    typealias<list<dict<any>>>
<
==============================================================================

8.  Enum					*Vim9-enum* *:enum* *:endenum*

						*enum* *E1418* *E1419* *E1420*
An enum is a type that can have one of a list of values.  Example: >

    :enum Color
	White,
	Red,
	Green, Blue, Black
    :endenum
<
						*enumvalue* *E1422*
The enum values are separated by commas.  More than one enum value can be
listed in a single line.  The final enum value should not be followed by a
comma.

An enum value is accessed using the enum name followed by the value name: >

    var a: Color = Color.Blue
<
Enums are treated as classes, where each enum value is essentially an instance
of that class.  Unlike typical object instantiation with the |new()| method,
enum instances cannot be created this way.

An enum can only be defined in a |Vim9| script file.	*E1414*
An enum cannot be defined inside a function.

							*E1415*
An enum name must start with an uppercase letter.  The name of an enum value
in an enum can start with an upper or lowercase letter.

							*E1416*
An enum can implement an interface but cannot extend a class: >

    enum MyEnum implements MyIntf
	Value1,
	Value2

	def SomeMethod()
	enddef
    endenum
<
							*enum-constructor*
The enum value objects in an enum are constructed like any other objects using
the |new()| method.  Arguments can be passed to the enum constructor by
specifying them after the enum value name, just like calling a function.  The
default constructor doesn't have any arguments.

							*E1417*
An enum can contain class variables, class methods, object variables and
object methods.  The methods in an enum cannot be |:abstract| methods.

The following example shows an enum with object variables and methods: >

    vim9script
    enum Planet
	Earth(1, false),
	Jupiter(95, true),
	Saturn(146, true)

	var moons: number
	var has_rings: bool
	def GetMoons(): number
	    return this.moons
	enddef
    endenum
    echo Planet.Jupiter.GetMoons()
    echo Planet.Earth.has_rings
<
						*E1421* *E1423* *E1424* *E1425*
Enums and their values are immutable.  They cannot be utilized as numerical or
string types.  Enum values can declare mutable instance variables.

						*enum-name*
Each enum value object has a "name" instance variable which contains the name
of the enum value.  This is a readonly variable.

						*enum-ordinal* *E1426*
Each enum value has an associated ordinal number starting with 0.  The ordinal
number of an enum value can be accessed using the "ordinal" instance variable.
This is a readonly variable.  Note that if the ordering of the enum values in
an enum is changed, then their ordinal values will also change.

						*enum-values*
All the values in an enum can be accessed using the "values" class variable
which is a List of the enum objects.  This is a readonly variable.

Example: >
    enum Planet
	Mercury,
	Venus,
	Earth
    endenum

    echo Planet.Mercury
    echo Planet.Venus.name
    echo Planet.Venus.ordinal
    for p in Planet.values
	# ...
    endfor
<
An enum is a class with class variables for the enum value objects and object
variables for the enum value name and the enum value ordinal: >

    enum Planet
	Mercury,
	Venus
    endenum
<
The above enum definition is equivalent to the following class definition: >

    class Planet
      public static final Mercury: Planet = Planet.new('Mercury', 0)
      public static final Venus: Planet = Planet.new('Venus', 1)

      public static const values: list<Planet> = [Planet.Mercury, Planet.Venus]

      public const name: string
      public const ordinal: number
    endclass
<
==============================================================================

9.  Rationale

Most of the choices for |Vim9| classes come from popular and recently
developed languages, such as Java, TypeScript and Dart.  The syntax has been
made to fit with the way Vim script works, such as using `endclass` instead of
using curly braces around the whole class.

Some common constructs of object-oriented languages were chosen very long ago
when this kind of programming was still new, and later found to be
sub-optimal.  By this time those constructs were widely used and changing them
was not an option.  In Vim we do have the freedom to make different choices,
since classes are completely new.  We can make the syntax simpler and more
consistent than what "old" languages use.  Without diverting too much, it
should still mostly look like what you know from existing languages.

Some recently developed languages add all kinds of fancy features that we
don't need for Vim.  But some have nice ideas that we do want to use.
Thus we end up with a base of what is common in popular languages, dropping
what looks like a bad idea, and adding some nice features that are easy to
understand.

The main rules we use to make decisions:
- Keep it simple.
- No surprises, mostly do what other languages are doing.
- Avoid mistakes from the past.
- Avoid the need for the script writer to consult the help to understand how
  things work, most things should be obvious.
- Keep it consistent.
- Aim at an average size plugin, not at a huge project.


Using new() for the constructor ~

Many languages use the class name for the constructor method.  A disadvantage
is that quite often this is a long name.  And when changing the class name all
constructor methods need to be renamed.  Not a big deal, but still a
disadvantage.

Other languages, such as TypeScript, use a specific name, such as
"constructor()".  That seems better.  However, using "new" or "new()" to
create a new object has no obvious relation with "constructor()".

For |Vim9| script using the same method name for all constructors seemed like
the right choice, and by calling it new() the relation between the caller and
the method being called is obvious.


No overloading of the constructor ~

In Vim script, both legacy and |Vim9| script, there is no overloading of
methods.  That means it is not possible to use the same method name with
different types of arguments.  Therefore there also is only one new()
constructor.

With |Vim9| script it would be possible to support overloading, since
arguments are typed.  However, this gets complicated very quickly.  Looking at
a new() call one has to inspect the types of the arguments to know which of
several new() methods is actually being called.  And that can require
inspecting quite a bit of code.  For example, if one of the arguments is the
return value of a method, you need to find that method to see what type it is
returning.

Instead, every constructor has to have a different name, starting with "new".
That way multiple constructors with different arguments are possible, while it
is very easy to see which constructor is being used.  And the type of
arguments can be properly checked.


No overloading of methods ~

Same reasoning as for the constructor: It is often not obvious what type
arguments have, which would make it difficult to figure out what method is
actually being called.  Better just give the methods a different name, then
type checking will make sure it works as you intended.  This rules out
polymorphism, which we don't really need anyway.


Single inheritance and interfaces ~

Some languages support multiple inheritance.  Although that can be useful in
some cases, it makes the rules of how a class works quite complicated.
Instead, using interfaces to declare what is supported is much simpler.  The
very popular Java language does it this way, and it should be good enough for
Vim.  The "keep it simple" rule applies here.

Explicitly declaring that a class supports an interface makes it easy to see
what a class is intended for.  It also makes it possible to do proper type
checking.  When an interface is changed any class that declares to implement
it will be checked if that change was also changed.  The mechanism to assume a
class implements an interface just because the methods happen to match is
brittle and leads to obscure problems, let's not do that.


Using "this.variable" everywhere ~

The object variables in various programming languages can often be accessed in
different ways, depending on the location.  Sometimes "this." has to be
prepended to avoid ambiguity.  They are usually declared without "this.".
That is quite inconsistent and sometimes confusing.

A very common issue is that in the constructor the arguments use the same name
as the object variable.  Then for these variables "this." needs to be prefixed
in the body, while for other variables this is not needed and often omitted.
This leads to a mix of variables with and without "this.", which is
inconsistent.

For |Vim9| classes the "this." prefix is always used for declared methods and
variables.  Simple and consistent.  When looking at the code inside a class
it's also directly clear which variable references are object variables and
which aren't.


Using class variables ~

Using "static variable" to declare a class variable is very common, nothing
new here.  In |Vim9| script these can be accessed directly by their name.
Very much like how a script-local variable can be used in a method.  Since
object variables are always accessed with "this." prepended, it's also quickly
clear what kind of variable it is.

TypeScript prepends the class name before the class variable name, also inside
the class.  This has two problems: The class name can be rather long, taking
up quite a bit of space, and when the class is renamed all these places need
to be changed too.


Declaring object and class variables ~

The main choice is whether to use "var" as with variable declarations.
TypeScript does not use it: >
	class Point {
	  x: number;
	  y = 0;
	}

Following that Vim object variables could be declared like this: >
	class Point
	  this.x: number
	  this.y = 0
	endclass

Some users pointed out that this looks more like an assignment than a
declaration.  Adding "var" and omitting "this." changes that: >
	class Point
	  var x: number
	  var y = 0
	endclass

We also need to be able to declare class variables using the "static" keyword.
There we can also choose to leave out "var": >
	class Point
	  var x: number
	  static count = 0
	endclass

Or do use it, before "static": >
	class Point
	  var x: number
	  var static count = 0
	endclass

Or after "static": >
	class Point
	  var x: number
	  static var count = 0
	endclass

This is more in line with "static def Func()".

There is no clear preference whether to use "var" or not.  The two main
reasons to leave it out are:
1. TypeScript and other popular languages do not use it.
2. Less clutter.

However, it is more common for languages to reuse their general variable and
function declaration syntax for class/object variables and methods.  Vim9 also
reuses the general function declaration syntax for methods.  So, for the sake
of consistency, we require "var" in these declarations.


Using "ClassName.new()" to construct an object ~

Many languages use the "new" operator to create an object, which is actually
kind of strange, since the constructor is defined as a method with arguments,
not a command.  TypeScript also has the "new" keyword, but the method is
called "constructor()", it is hard to see the relation between the two.

In |Vim9| script the constructor method is called new(), and it is invoked as
new(), simple and straightforward.  Other languages use "new ClassName()",
while there is no ClassName() method, it's a method by another name in the
class called ClassName.  Quite confusing.


Vim9class access modes ~
						    *vim9-access-modes*
The variable access modes, and their meaning, supported by Vim9class are
	|public-variable|	read and write from anywhere
	|read-only-variable|	read from anywhere, write from inside the
				class and sub-classes
	|protected-variable|	read and write from inside the class and
				sub-classes

The method access modes are similar, but without the read-only mode.


Default read access to object variables ~

Some users will remark that the access rules for object variables are
asymmetric.  Well, that is intentional.  Changing a value is a very different
action than reading a value.  The read operation has no side effects, it can
be done any number of times without affecting the object.  Changing the value
can have many side effects, and even have a ripple effect, affecting other
objects.

When adding object variables one usually doesn't think much about this, just
get the type right.  And normally the values are set in the new() method.
Therefore defaulting to read access only "just works" in most cases.  And when
directly writing you get an error, which makes you wonder if you actually want
to allow that.  This helps writing code with fewer mistakes.


Making object variables protected with an underscore ~

When an object variable is protected, it can only be read and changed inside
the class (and in sub-classes), then it cannot be used outside of the class.
Prepending an underscore is a simple way to make that visible.  Various
programming languages have this as a recommendation.

In case you change your mind and want to make the object variable accessible
outside of the class, you will have to remove the underscore everywhere.
Since the name only appears in the class (and sub-classes) they will be easy
to find and change.

The other way around is much harder: you can easily prepend an underscore to
the object variable inside the class to make it protected, but any usage
elsewhere you will have to track down and change.  You may have to make it a
"set" method call.  This reflects the real world problem that taking away
access requires work to be done for all places where that access exists.

An alternative would have been using the "protected" keyword, just like
"public" changes the access in the other direction.  Well, that's just to
reduce the number of keywords.


No private object variables ~

Some languages provide several ways to control access to object variables.
The most known is "protected", and the meaning varies from language to
language.  Others are "shared", "private", "package" and even "friend".

These rules make life more difficult.  That can be justified in projects where
many people work on the same, complex code where it is easy to make mistakes.
Especially when refactoring or other changes to the class model.

The Vim scripts are expected to be used in a plugin, with just one person or a
small team working on it.  Complex rules then only make it more complicated,
the extra safety provided by the rules isn't really needed.  Let's just keep
it simple and not specify access details.


==============================================================================

10. To be done later

Can a newSomething() constructor invoke another constructor?  If yes, what are
the restrictions?

Thoughts:
- Generics for a class: `class <Tkey, Tentry>`
- Generics for a function: `def <Tkey> GetLast(key: Tkey)`
- Mixins: not sure if that is useful, leave out for simplicity.

Some things that look like good additions:
- For testing: Mock mechanism

An important class to be provided is "Promise".  Since Vim is single
threaded, connecting asynchronous operations is a natural way of allowing
plugins to do their work without blocking the user.  It's a uniform way to
invoke callbacks and handle timeouts and errors.


 vim:tw=78:ts=8:noet:ft=help:norl: