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*vim9class.txt* For Vim version 9.1. Last change: 2024 Jan 06 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()`. To use a class method from a parent class in a child class, the class name must be prefixed. 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. 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". 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. ============================================================================== 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* {not implemented yet} An enum is a type that can have one of a list of values. Example: > :enum Color White Red Green Blue Black :endenum ============================================================================== 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: