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diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt new file mode 100644 index 0000000..3c7722c --- /dev/null +++ b/runtime/doc/vim9class.txt @@ -0,0 +1,900 @@ +*vim9class.txt* For Vim version 9.0. Last change: 2023 Feb 26 + + + VIM REFERENCE MANUAL by Bram Moolenaar + + +NOTE - This is not finished yet, anything can still change! - NOTE + + +Vim9 classes, objects, interfaces, types and enums. + +1. Overview |Vim9-class-overview| +2. A simple class |Vim9-simple-class| +3. Class members and functions |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. + +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 + member functions. +- 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 + this.lnum: number + this.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 members "lnum" and "col" can be accessed directly: > + + echo $'The text position is ({pos.lnum}, {pos.col})' +< *E1317* *E1327* +If you have been using other object-oriented languages you will notice that +in Vim the 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 member. + + +Member write access ~ + +Now try to change an object member directly: > + + pos.lnum = 9 +< *E1335* +This will give you an error! That is because by default object members 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 member 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* *E1331* +If you don't care about side effects and want to allow the object member to be +changed at any time, you can make it public: > + + public this.lnum: number + public this.col: number + +Now you don't need the SetLnum(), SetCol() and SetPosition() methods, setting +"pos.lnum" directly above will no longer give an error. + *E1334* +If you try to set an object member that doesn't exist you get an error: > + pos.other = 9 +< E1334: Object member not found: other ~ + + +Private members ~ + *E1332* *E1333* +On the other hand, if you do not want the object members to be read directly, +you can make them private. This is done by prefixing an underscore to the +name: > + + this._lnum: number + this._col number + +Now you need to provide methods to get the value of the private members. +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 members 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 + + +Simplifying the new() method ~ + +Many constructors take values for the object members. Thus you very often see +this pattern: > + + class SomeClass + this.lnum: number + this.col: number + + def new(lnum: number, col: number) + this.lnum = lnum + this.col = col + enddef + endclass + +Not only is this text you need to write, it also has the type of each member +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 member name, +including "this.", as the argument to new() means the value provided in the +new() call is assigned to that object member. This mechanism comes from the +Dart language. + +Putting together this way of using new() and making the members public results +in a much shorter class definition as what we started with: > + + class TextPosition + public this.lnum: number + public this.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 member that has an initializer, the expression is + evaluated and assigned to the member. This happens in the sequence the + members 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 members of the parent class are done first. There is no need to call +"super()" or "new()" on the parent. + +============================================================================== + +3. class members and functions *Vim9-class-member* + + *:static* *E1337* *E1338* +Class members are declared with "static". They are used by the name without a +prefix: > + + class OtherThing + this.size: number + static 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 function argument name +or local variable name is not allowed. + +Just like object members the access can be made private by using an underscore +as the first character in the name, and it can be made public by prefixing +"public": > + + class OtherThing + static total: number # anybody can read, only class can write + static _sum: number # only class can read and write + public static result: number # anybody can read and write + endclass +< + *class-function* +Class functions are also declared with "static". They have no access to +object members, they cannot use the "this" keyword. > + + class OtherThing + this.size: number + static 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 function can be called by name directly, outside the +class the class name must be prefixed: `OtherThing.ClearTotalSize()`. + +============================================================================== + +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 + this.color = Color.Black + this.thickness = 10 + endclass + + class Square extends Shape + this.size: number + + def new(this.size) + enddef + endclass + + class Triangle extends Shape + this.base: number + this.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* + + +============================================================================== + +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 + this.color = Color.Black + this.thickness = 10 + endclass + + interface HasSurface + def Surface(): number + endinterface + + class Square extends Shape implements HasSurface + this.size: number + + def new(this.size) + enddef + + def Surface(): number + return this.size * this.size + enddef + endclass + + class Triangle extends Shape implements HasSurface + this.base: number + this.height: number + + def new(this.base, this.height) + enddef + + def Surface(): number + return this.base * this.height / 2 + enddef + endclass + +If a class declares to implement an interface, all the items specified in the +interface must appear in the class, with the same types. *E1348* *E1349* + +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 + + +============================================================================== + +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* +Each member and function name can be used only once. It is not possible to +define a function with the same name and different type of arguments. + + +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 members 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. The method of the +base class can be called by prefixing "super.". + +Other object methods of the base class are taken over by the child class. + +Class functions, including functions starting with "new", can be overruled, +like with object methods. The function on the base class can be called by +prefixing the name of the class (for class functions) 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() function then one was automatically +created. This function will not be taken over by the child class. The child +class can define its own new() function, or, if there isn't one, a new() +function will be added automatically. + + +A class implementing an interface ~ + *implements* *E1346* *E1347* +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 members 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* *E1326* +Inside a class, in between `:class` and `:endclass`, these items can appear: +- An object member declaration: > + this._memberName: memberType + this.memberName: memberType + public this.memberName: memberType +- A constructor method: > + def new(arguments) + def newName(arguments) +- An object method: > + def SomeMethod(arguments) +< *E1329* +For the object member 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: > + this.nameList = [] +This specifies a list, but the item type is unknown. Better use: > + this.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* *: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 members, 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 + this.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* + + +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 member name is correct and you will get an "Using a null object" error, +even when the member name is invalid. *E1360* *E1362* + + +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 +members, in the order they were specified. Thus if your class looks like: > + + class AutoNew + this.name: string + this.age: number + this.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 members will be used. This is a more useful example, +with default values: > + + class TextPosition + this.lnum: number = 1 + this.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 +< *E1328* +Note that you cannot use another default value than "v:none" here. If you +want to initialize the object members, do it where they are declared. This +way you only need to look in one place for the default values. + +All object members will be used in the default constructor, also private +access ones. + +If the class extends another one, the object members of that class will come +first. + + +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 *Vim9-type* *:type* + +A type definition is giving a name to a type specification. For Example: > + + :type ListOfStrings list<string> + +TODO: more explanation + + +============================================================================== + +8. Enum *Vim9-enum* *:enum* *:endenum* + +An enum is a type that can have one of a list of values. Example: > + + :enum Color + White + Red + Green + Blue + Black + :endenum + +TODO: more explanation + + +============================================================================== + +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 +functions. That means it is not possible to use the same function 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.member" everywhere ~ + +The object members 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 member. Then for these members "this." needs to be prefixed in +the body, while for other members this is not needed and often omitted. This +leads to a mix of members with and without "this.", which is inconsistent. + +For |Vim9| classes the "this." prefix is always used. Also for declaring the +members. Simple and consistent. When looking at the code inside a class it's +also directly clear which variable references are object members and which +aren't. + + +Using class members ~ + +Using "static member" to declare a class member 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 function. Since object +members are always accessed with "this." prepended, it's also quickly clear +what kind of member it is. + +TypeScript prepends the class name before the class member, 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 members ~ + +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 members 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" changes that: > + class Point + var this.x: number + var this.y = 0 + endclass + +We also need to be able to declare class members using the "static" keyword. +There we can also choose to leave out "var": > + class Point + var this.x: number + static count = 0 + endclass + +Or do use it, before "static": > + class Point + var this.x: number + var static count = 0 + endclass + +Or after "static": > + class Point + var this.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, Java and other popular languages do not use it. +2. Less clutter. + + +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. + + +Default read access to object members ~ + +Some users will remark that the access rules for object members 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 members 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 members private with an underscore ~ + +When an object member is private, 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 member 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 member inside the class to make it private, 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 "private" keyword, just like "public" +changes the access in the other direction. Well, that's just to reduce the +number of keywords. + + +No protected object members ~ + +Some languages provide several ways to control access to object members. The +most known is "protected", and the meaning varies from language to language. +Others are "shared", "private" 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 provide 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: |