Mercurial > vim
annotate runtime/doc/vim9class.txt @ 33682:f126ffc85f7c v9.0.2078
patch 9.0.2078: several problems with type aliases
Commit: https://github.com/vim/vim/commit/feaccd239573a6265d39d3a917862ee40742eab4
Author: Yegappan Lakshmanan <yegappan@yahoo.com>
Date: Sat Oct 28 15:53:55 2023 +0200
patch 9.0.2078: several problems with type aliases
Problem: several problems with type aliases
Solution: Check for more error conditions, add tests,
fix issues
Check for more error conditions and add additional tests
fixes #13434
fixes #13437
fixes #13438
closes #13441
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
author | Christian Brabandt <cb@256bit.org> |
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date | Sat, 28 Oct 2023 16:00:06 +0200 |
parents | 7d9d2404a3d4 |
children | 643db54ed3e7 |
rev | line source |
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1 *vim9class.txt* For Vim version 9.0. Last change: 2023 Sep 18 |
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2 |
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3 |
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4 VIM REFERENCE MANUAL by Bram Moolenaar |
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5 |
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6 |
31885 | 7 NOTE - This is not finished yet, anything can still change! - NOTE |
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8 |
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9 |
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10 Vim9 classes, objects, interfaces, types and enums. |
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11 |
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12 1. Overview |Vim9-class-overview| |
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13 2. A simple class |Vim9-simple-class| |
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14 3. Class variables and methods |Vim9-class-member| |
31885 | 15 4. Using an abstract class |Vim9-abstract-class| |
16 5. Using an interface |Vim9-using-interface| | |
17 6. More class details |Vim9-class| | |
18 7. Type definition |Vim9-type| | |
19 8. Enum |Vim9-enum| | |
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20 |
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21 9. Rationale |
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22 10. To be done later |
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23 |
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24 ============================================================================== |
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25 |
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26 1. Overview *Vim9-class-overview* |
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27 |
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28 The fancy term is "object-oriented programming". You can find lots of study |
31885 | 29 material on this subject. Here we document what |Vim9| script provides, |
30 assuming you know the basics already. Added are helpful hints about how to | |
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31 use this functionality effectively. Vim9 classes and objects cannot be used |
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32 in legacy Vim scripts and legacy functions. |
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33 |
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34 The basic item is an object: |
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35 - An object stores state. It contains one or more variables that can each |
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36 have a value. |
31885 | 37 - An object provides functions that use and manipulate its state. These |
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38 functions are invoked "on the object", which is what sets it apart from the |
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39 traditional separation of data and code that manipulates the data. |
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40 - An object has a well defined interface, with typed member variables and |
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41 methods. |
31885 | 42 - Objects are created from a class and all objects have the same interface. |
43 This does not change at runtime, it is not dynamic. | |
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44 |
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45 An object can only be created by a class. A class provides: |
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46 - A new() method, the constructor, which returns an object for the class. |
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47 This method is invoked on the class name: MyClass.new(). |
31885 | 48 - State shared by all objects of the class: class variables (class members). |
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49 - A hierarchy of classes, with super-classes and sub-classes, inheritance. |
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50 |
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51 An interface is used to specify properties of an object: |
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52 - An object can declare several interfaces that it implements. |
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53 - Different objects implementing the same interface can be used the same way. |
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54 |
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55 The class hierarchy allows for single inheritance. Otherwise interfaces are |
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56 to be used where needed. |
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57 |
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58 |
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59 Class modeling ~ |
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60 |
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61 You can model classes any way you like. Keep in mind what you are building, |
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62 don't try to model the real world. This can be confusing, especially because |
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63 teachers use real-world objects to explain class relations and you might think |
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64 your model should therefore reflect the real world. It doesn't! The model |
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65 should match your purpose. |
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66 |
31885 | 67 Keep in mind that composition (an object contains other objects) is often |
68 better than inheritance (an object extends another object). Don't waste time | |
69 trying to find the optimal class model. Or waste time discussing whether a | |
70 square is a rectangle or that a rectangle is a square. It doesn't matter. | |
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71 |
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72 |
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73 ============================================================================== |
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74 |
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75 2. A simple class *Vim9-simple-class* |
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76 |
31885 | 77 Let's start with a simple example: a class that stores a text position (see |
78 below for how to do this more efficiently): > | |
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79 |
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80 class TextPosition |
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81 this.lnum: number |
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82 this.col: number |
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83 |
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84 def new(lnum: number, col: number) |
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85 this.lnum = lnum |
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86 this.col = col |
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87 enddef |
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88 |
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89 def SetLnum(lnum: number) |
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90 this.lnum = lnum |
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91 enddef |
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92 |
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93 def SetCol(col: number) |
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94 this.col = col |
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95 enddef |
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96 |
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97 def SetPosition(lnum: number, col: number) |
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98 this.lnum = lnum |
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99 this.col = col |
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100 enddef |
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101 endclass |
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103 You can create an object from this class with the new() method: > |
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104 |
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105 var pos = TextPosition.new(1, 1) |
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106 |
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107 The object variables "lnum" and "col" can be accessed directly: > |
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108 |
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109 echo $'The text position is ({pos.lnum}, {pos.col})' |
31430 | 110 < *E1317* *E1327* |
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111 If you have been using other object-oriented languages you will notice that |
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112 in Vim the object members are consistently referred to with the "this." |
31885 | 113 prefix. This is different from languages like Java and TypeScript. The |
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114 naming convention makes the object members easy to spot. Also, when a |
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115 variable does not have the "this." prefix you know it is not an object |
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116 variable. |
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117 |
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118 |
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119 Object variable write access ~ |
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120 |
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121 Now try to change an object variable directly: > |
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122 |
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123 pos.lnum = 9 |
31885 | 124 < *E1335* |
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125 This will give you an error! That is because by default object variables can |
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126 be read but not set. That's why the TextPosition class provides a method for |
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127 it: > |
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128 |
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129 pos.SetLnum(9) |
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130 |
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131 Allowing to read but not set an object variable is the most common and safest |
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132 way. Most often there is no problem using a value, while setting a value may |
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133 have side effects that need to be taken care of. In this case, the SetLnum() |
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134 method could check if the line number is valid and either give an error or use |
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135 the closest valid value. |
31885 | 136 *:public* *E1331* |
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137 If you don't care about side effects and want to allow the object variable to |
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138 be changed at any time, you can make it public: > |
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139 |
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140 public this.lnum: number |
31885 | 141 public this.col: number |
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142 |
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143 Now you don't need the SetLnum(), SetCol() and SetPosition() methods, setting |
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144 "pos.lnum" directly above will no longer give an error. |
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145 *E1326* |
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146 If you try to set an object variable that doesn't exist you get an error: > |
31579 | 147 pos.other = 9 |
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148 < E1326: Member not found on object "TextPosition": other ~ |
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149 |
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150 *E1376* |
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151 A object variable cannot be accessed using the class name. |
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152 |
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153 Private variables ~ |
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154 *private-variable* *E1332* *E1333* |
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155 On the other hand, if you do not want the object variables to be read directly, |
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156 you can make them private. This is done by prefixing an underscore to the |
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157 name: > |
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158 |
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159 this._lnum: number |
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160 this._col number |
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161 |
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162 Now you need to provide methods to get the value of the private variables. |
31885 | 163 These are commonly called getters. We recommend using a name that starts with |
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164 "Get": > |
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165 |
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166 def GetLnum(): number |
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167 return this._lnum |
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168 enddef |
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169 |
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170 def GetCol() number |
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171 return this._col |
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172 enddef |
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173 |
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174 This example isn't very useful, the variables might as well have been public. |
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175 It does become useful if you check the value. For example, restrict the line |
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176 number to the total number of lines: > |
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177 |
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178 def GetLnum(): number |
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179 if this._lnum > this._lineCount |
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180 return this._lineCount |
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181 endif |
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182 return this._lnum |
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183 enddef |
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184 < |
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185 Private methods ~ |
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186 *private-method* *E1366* |
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187 If you want object methods to be accessible only from other methods of the |
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188 same class and not used from outside the class, then you can make them |
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189 private. This is done by prefixing the method name with an underscore: > |
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190 |
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191 class SomeClass |
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192 def _Foo(): number |
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193 return 10 |
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194 enddef |
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195 def Bar(): number |
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196 return this._Foo() |
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197 enddef |
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198 endclass |
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199 < |
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200 Accessing a private method outside the class will result in an error (using |
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201 the above class): > |
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202 |
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203 var a = SomeClass.new() |
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204 a._Foo() |
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205 < |
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206 Simplifying the new() method ~ |
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207 *new()* *constructor* |
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208 Many constructors take values for the object variables. Thus you very often |
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209 see this pattern: > |
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210 |
31885 | 211 class SomeClass |
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212 this.lnum: number |
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213 this.col: number |
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214 |
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215 def new(lnum: number, col: number) |
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216 this.lnum = lnum |
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217 this.col = col |
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218 enddef |
31885 | 219 endclass |
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220 < |
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221 *E1390* |
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222 Not only is this text you need to write, it also has the type of each |
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223 variables twice. Since this is so common a shorter way to write new() is |
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224 provided: > |
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225 |
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226 def new(this.lnum, this.col) |
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227 enddef |
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228 |
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229 The semantics are easy to understand: Providing the object variable name, |
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230 including "this.", as the argument to new() means the value provided in the |
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231 new() call is assigned to that object variable. This mechanism comes from the |
31885 | 232 Dart language. |
233 | |
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234 Putting together this way of using new() and making the variables public |
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235 results in a much shorter class definition than what we started with: > |
31885 | 236 |
237 class TextPosition | |
238 public this.lnum: number | |
239 public this.col: number | |
240 | |
241 def new(this.lnum, this.col) | |
242 enddef | |
243 | |
244 def SetPosition(lnum: number, col: number) | |
245 this.lnum = lnum | |
246 this.col = col | |
247 enddef | |
248 endclass | |
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249 |
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250 The sequence of constructing a new object is: |
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251 1. Memory is allocated and cleared. All values are zero/false/empty. |
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252 2. For each declared object variable that has an initializer, the expression |
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253 is evaluated and assigned to the variable. This happens in the sequence |
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254 the variables are declared in the class. |
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255 3. Arguments in the new() method in the "this.name" form are assigned. |
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256 4. The body of the new() method is executed. |
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257 |
31885 | 258 If the class extends a parent class, the same thing happens. In the second |
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259 step the object variables of the parent class are initialized first. There is |
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260 no need to call "super()" or "new()" on the parent. |
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261 |
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262 *E1365* |
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263 When defining the new() method the return type should not be specified. It |
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264 always returns an object of the class. |
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265 |
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266 *E1386* |
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267 When invoking an object method, the method name should be preceded by the |
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268 object variable name. A object method cannot be invoked using the class |
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269 name. |
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270 |
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271 ============================================================================== |
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272 |
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273 3. Class Variables and Methods *Vim9-class-member* |
31885 | 274 |
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275 *:static* *E1337* *E1338* *E1368* |
31885 | 276 Class members are declared with "static". They are used by the name without a |
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277 prefix in the class where they are defined: > |
31885 | 278 |
279 class OtherThing | |
280 this.size: number | |
281 static totalSize: number | |
282 | |
283 def new(this.size) | |
284 totalSize += this.size | |
285 enddef | |
286 endclass | |
287 < *E1340* *E1341* | |
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288 Since the name is used as-is, shadowing the name by a method argument name |
31885 | 289 or local variable name is not allowed. |
290 | |
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291 *E1374* *E1375* *E1384* *E1385* |
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292 To access a class member outside of the class where it is defined, the class |
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293 name prefix must be used. A class member cannot be accessed using an object. |
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294 |
31885 | 295 Just like object members the access can be made private by using an underscore |
296 as the first character in the name, and it can be made public by prefixing | |
297 "public": > | |
298 | |
299 class OtherThing | |
300 static total: number # anybody can read, only class can write | |
301 static _sum: number # only class can read and write | |
302 public static result: number # anybody can read and write | |
303 endclass | |
304 < | |
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305 *class-method* |
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306 Class methods are also declared with "static". They can use the class |
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307 variables but they have no access to the object variables, they cannot use the |
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308 "this" keyword: |
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309 > |
31885 | 310 class OtherThing |
311 this.size: number | |
312 static totalSize: number | |
313 | |
314 # Clear the total size and return the value it had before. | |
315 static def ClearTotalSize(): number | |
316 var prev = totalSize | |
317 totalSize = 0 | |
318 return prev | |
319 enddef | |
320 endclass | |
321 | |
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322 Inside the class the class method can be called by name directly, outside the |
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323 class the class name must be prefixed: `OtherThing.ClearTotalSize()`. To use |
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324 a super class method in a child class, the class name must be prefixed. |
31885 | 325 |
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326 Just like object methods the access can be made private by using an underscore |
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327 as the first character in the method name: > |
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328 |
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329 class OtherThing |
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330 static def _Foo() |
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331 echo "Foo" |
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332 enddef |
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333 def Bar() |
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334 _Foo() |
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335 enddef |
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336 endclass |
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337 < |
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338 *E1370* |
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339 Note that constructors cannot be declared as "static", because they always |
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340 are. |
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341 |
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342 To access the class methods and class variables of a super class in an |
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343 extended class, the class name prefix should be used just as from anywhere |
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344 outside of the defining class: > |
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345 |
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346 vim9script |
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347 class Vehicle |
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348 static nextID: number = 1000 |
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349 static def GetID(): number |
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350 nextID += 1 |
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351 return nextID |
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352 enddef |
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353 endclass |
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354 class Car extends Vehicle |
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355 this.myID: number |
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356 def new() |
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357 this.myID = Vehicle.GetID() |
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358 enddef |
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359 endclass |
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360 < |
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361 Class variables and methods are not inherited by a child class. A child class |
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362 can declare a static variable or a method with the same name as the one in the |
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363 super class. Depending on the class where the member is used the |
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364 corresponding class member will be used. The type of the class member in a |
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365 child class can be different from that in the super class. |
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366 |
31885 | 367 ============================================================================== |
368 | |
369 4. Using an abstract class *Vim9-abstract-class* | |
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370 |
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371 An abstract class forms the base for at least one sub-class. In the class |
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372 model one often finds that a few classes have the same properties that can be |
31885 | 373 shared, but a class with these properties does not have enough state to create |
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374 an object from. A sub-class must extend the abstract class and add the |
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375 missing state and/or methods before it can be used to create objects for. |
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376 |
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377 For example, a Shape class could store a color and thickness. You cannot |
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378 create a Shape object, it is missing the information about what kind of shape |
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379 it is. The Shape class functions as the base for a Square and a Triangle |
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380 class, for which objects can be created. Example: > |
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381 |
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382 abstract class Shape |
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383 this.color = Color.Black |
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384 this.thickness = 10 |
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385 endclass |
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386 |
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387 class Square extends Shape |
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388 this.size: number |
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389 |
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390 def new(this.size) |
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391 enddef |
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392 endclass |
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393 |
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394 class Triangle extends Shape |
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395 this.base: number |
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396 this.height: number |
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397 |
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398 def new(this.base, this.height) |
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399 enddef |
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400 endclass |
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401 < |
31885 | 402 An abstract class is defined the same way as a normal class, except that it |
403 does not have any new() method. *E1359* | |
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404 |
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405 *abstract-method* *E1371* *E1372* |
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406 An abstract method can be defined in an abstract class by using the "abstract" |
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407 prefix when defining the method: > |
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408 |
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409 abstract class Shape |
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410 abstract def Draw() |
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411 abstract static def SetColor() |
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412 endclass |
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413 < |
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414 *E1373* |
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415 A class extending the abstract class must implement all the abstract methods. |
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416 The signature (arguments, argument types and return type) must be exactly the |
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417 same. If the return type of a method is a class, then that class or one of |
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418 its subclasses can be used in the extended method. Class methods in an |
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419 abstract class can also be abstract methods. |
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420 |
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421 ============================================================================== |
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422 |
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424 |
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425 The example above with Shape, Square and Triangle can be made more useful if |
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426 we add a method to compute the surface of the object. For that we create the |
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427 interface called HasSurface, which specifies one method Surface() that returns |
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428 a number. This example extends the one above: > |
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429 |
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430 abstract class Shape |
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431 this.color = Color.Black |
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432 this.thickness = 10 |
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433 endclass |
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434 |
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435 interface HasSurface |
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436 def Surface(): number |
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437 endinterface |
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438 |
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439 class Square extends Shape implements HasSurface |
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440 this.size: number |
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441 |
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442 def new(this.size) |
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443 enddef |
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444 |
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445 def Surface(): number |
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446 return this.size * this.size |
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447 enddef |
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448 endclass |
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449 |
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450 class Triangle extends Shape implements HasSurface |
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451 this.base: number |
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452 this.height: number |
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453 |
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454 def new(this.base, this.height) |
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455 enddef |
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456 |
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457 def Surface(): number |
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458 return this.base * this.height / 2 |
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459 enddef |
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460 endclass |
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461 < |
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462 *E1348* *E1349* *E1367* *E1382* *E1383* |
31671 | 463 If a class declares to implement an interface, all the items specified in the |
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464 interface must appear in the class, with the same types. |
31671 | 465 |
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466 The interface name can be used as a type: > |
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467 |
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468 var shapes: list<HasSurface> = [ |
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469 Square.new(12), |
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470 Triangle.new(8, 15), |
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471 ] |
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472 for shape in shapes |
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473 echo $'the surface is {shape.Surface()}' |
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474 endfor |
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475 < |
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476 *E1378* *E1379* *E1380* *E1387* |
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477 An interface can contain only object methods and read-only object variables. |
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478 An interface cannot contain read-write and private object variables, private |
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479 object methods, class variables and class methods. |
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480 |
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481 An interface can extend another interface using "extends". The sub-interface |
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482 inherits all the instance variables and methods from the super interface. |
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483 |
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484 ============================================================================== |
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485 |
31885 | 486 6. More class details *Vim9-class* *Class* *class* |
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487 |
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488 Defining a class ~ |
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489 *:class* *:endclass* *:abstract* |
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490 A class is defined between `:class` and `:endclass`. The whole class is |
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491 defined in one script file. It is not possible to add to a class later. |
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492 |
31430 | 493 A class can only be defined in a |Vim9| script file. *E1316* |
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494 A class cannot be defined inside a function. |
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495 |
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496 It is possible to define more than one class in a script file. Although it |
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497 usually is better to export only one main class. It can be useful to define |
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498 types, enums and helper classes though. |
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499 |
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500 The `:abstract` keyword may be prefixed and `:export` may be used. That gives |
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501 these variants: > |
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502 |
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503 class ClassName |
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504 endclass |
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505 |
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506 export class ClassName |
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507 endclass |
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508 |
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509 abstract class ClassName |
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510 endclass |
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511 |
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512 export abstract class ClassName |
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513 endclass |
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514 < |
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515 *E1314* |
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516 The class name should be CamelCased. It must start with an uppercase letter. |
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517 That avoids clashing with builtin types. |
31430 | 518 *E1315* |
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519 After the class name these optional items can be used. Each can appear only |
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520 once. They can appear in any order, although this order is recommended: > |
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521 extends ClassName |
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522 implements InterfaceName, OtherInterface |
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523 specifies SomeInterface |
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524 < *E1355* *E1369* |
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525 Each variable and method name can be used only once. It is not possible to |
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526 define a method with the same name and different type of arguments. It is not |
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527 possible to use a public and private member variable with the same name. A |
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528 object variable name used in a super class cannot be reused in a child class. |
31885 | 529 |
530 | |
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531 Object Variable Initialization ~ |
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532 If the type of a variable is not explicitly specified in a class, then it is |
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533 set to "any" during class definition. When an object is instantiated from the |
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534 class, then the type of the variable is set. |
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535 |
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536 The following reserved keyword names cannot be used as an object or class |
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537 variable name: "super", "this", "true", "false", "null", "null_blob", |
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538 "null_dict", "null_function", "null_list", "null_partial", "null_string", |
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539 "null_channel" and "null_job". |
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540 |
31885 | 541 Extending a class ~ |
542 *extends* | |
31671 | 543 A class can extend one other class. *E1352* *E1353* *E1354* |
31885 | 544 The basic idea is to build on top of an existing class, add properties to it. |
545 | |
546 The extended class is called the "base class" or "super class". The new class | |
547 is called the "child class". | |
548 | |
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549 Object variables from the base class are all taken over by the child class. It |
31885 | 550 is not possible to override them (unlike some other languages). |
551 | |
552 *E1356* *E1357* *E1358* | |
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553 Object methods of the base class can be overruled. The signature (arguments, |
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554 argument types and return type) must be exactly the same. If the return type |
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555 of a method is a class, then that class or one of its subclasses can be used |
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556 in the extended method. The method of the base class can be called by |
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557 prefixing "super.". |
31885 | 558 |
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559 *E1377* |
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560 The access level of a method (public or private) in a child class should be |
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561 the same as the super class. |
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562 |
31885 | 563 Other object methods of the base class are taken over by the child class. |
564 | |
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565 Class methods, including methods starting with "new", can be overruled, like |
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566 with object methods. The method on the base class can be called by prefixing |
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567 the name of the class (for class methods) or "super.". |
31885 | 568 |
569 Unlike other languages, the constructor of the base class does not need to be | |
570 invoked. In fact, it cannot be invoked. If some initialization from the base | |
571 class also needs to be done in a child class, put it in an object method and | |
572 call that method from every constructor(). | |
573 | |
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574 If the base class did not specify a new() method then one was automatically |
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575 created. This method will not be taken over by the child class. The child |
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576 class can define its own new() method, or, if there isn't one, a new() method |
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577 will be added automatically. |
31885 | 578 |
579 | |
580 A class implementing an interface ~ | |
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581 *implements* *E1346* *E1347* *E1389* |
31671 | 582 A class can implement one or more interfaces. The "implements" keyword can |
583 only appear once *E1350* . Multiple interfaces can be specified, separated by | |
584 commas. Each interface name can appear only once. *E1351* | |
31885 | 585 |
586 | |
587 A class defining an interface ~ | |
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588 *specifies* |
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589 A class can declare its interface, the object variables and methods, with a |
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590 named interface. This avoids the need for separately specifying the |
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591 interface, which is often done in many languages, especially Java. |
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592 |
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593 |
31430 | 594 Items in a class ~ |
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595 *E1318* *E1325* *E1388* |
32061 | 596 Inside a class, in between `:class` and `:endclass`, these items can appear: |
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597 - An object variable declaration: > |
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598 this._privateVariableName: memberType |
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599 this.readonlyVariableName: memberType |
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600 public this.readwriteVariableName: memberType |
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601 - A class variable declaration: > |
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602 static _privateClassVariableName: memberType |
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603 static readonlyClassVariableName: memberType |
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604 static public readwriteClassVariableName: memberType |
31430 | 605 - A constructor method: > |
32004 | 606 def new(arguments) |
607 def newName(arguments) | |
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608 - A class method: > |
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609 static def SomeMethod(arguments) |
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610 static def _PrivateMethod(arguments) |
31430 | 611 - An object method: > |
32004 | 612 def SomeMethod(arguments) |
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613 def _PrivateMethod(arguments) |
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614 |
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615 For the object variable the type must be specified. The best way is to do |
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616 this explicitly with ": {type}". For simple types you can also use an |
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617 initializer, such as "= 123", and Vim will see that the type is a number. |
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618 Avoid doing this for more complex types and when the type will be incomplete. |
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619 For example: > |
31579 | 620 this.nameList = [] |
621 This specifies a list, but the item type is unknown. Better use: > | |
622 this.nameList: list<string> | |
623 The initialization isn't needed, the list is empty by default. | |
624 *E1330* | |
625 Some types cannot be used, such as "void", "null" and "v:none". | |
31430 | 626 |
627 | |
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628 Defining an interface ~ |
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629 *:interface* *:endinterface* |
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630 An interface is defined between `:interface` and `:endinterface`. It may be |
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631 prefixed with `:export`: > |
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632 |
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633 interface InterfaceName |
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634 endinterface |
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635 |
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636 export interface InterfaceName |
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637 endinterface |
31671 | 638 < *E1344* |
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639 An interface can declare object variables, just like in a class but without |
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640 any initializer. |
31671 | 641 *E1345* |
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642 An interface can declare methods with `:def`, including the arguments and |
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643 return type, but without the body and without `:enddef`. Example: > |
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644 |
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645 interface HasSurface |
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646 this.size: number |
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647 def Surface(): number |
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648 endinterface |
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649 |
31671 | 650 An interface name must start with an uppercase letter. *E1343* |
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651 The "Has" prefix can be used to make it easier to guess this is an interface |
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652 name, with a hint about what it provides. |
31671 | 653 An interface can only be defined in a |Vim9| script file. *E1342* |
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654 An interface cannot "implement" another interface but it can "extend" another |
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655 interface. *E1381* |
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656 |
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657 |
32004 | 658 null object ~ |
659 | |
32061 | 660 When a variable is declared to have the type of an object, but it is not |
32004 | 661 initialized, the value is null. When trying to use this null object Vim often |
662 does not know what class was supposed to be used. Vim then cannot check if | |
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663 a variable name is correct and you will get an "Using a null object" error, |
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664 even when the variable name is invalid. *E1360* *E1362* |
32004 | 665 |
666 | |
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667 Default constructor ~ |
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668 *default-constructor* |
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669 In case you define a class without a new() method, one will be automatically |
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670 defined. This default constructor will have arguments for all the object |
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671 variables, in the order they were specified. Thus if your class looks like: > |
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672 |
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673 class AutoNew |
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674 this.name: string |
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675 this.age: number |
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676 this.gender: Gender |
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677 endclass |
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678 |
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679 Then the default constructor will be: > |
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680 |
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681 def new(this.name = v:none, this.age = v:none, this.gender = v:none) |
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682 enddef |
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683 |
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684 The "= v:none" default values make the arguments optional. Thus you can also |
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685 call `new()` without any arguments. No assignment will happen and the default |
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686 value for the object variables will be used. This is a more useful example, |
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687 with default values: > |
31430 | 688 |
689 class TextPosition | |
690 this.lnum: number = 1 | |
691 this.col: number = 1 | |
692 endclass | |
693 | |
694 If you want the constructor to have mandatory arguments, you need to write it | |
695 yourself. For example, if for the AutoNew class above you insist on getting | |
696 the name, you can define the constructor like this: > | |
697 | |
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698 def new(this.name, this.age = v:none, this.gender = v:none) |
31430 | 699 enddef |
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700 < *E1328* |
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701 Note that you cannot use another default value than "v:none" here. If you |
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702 want to initialize the object variables, do it where they are declared. This |
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703 way you only need to look in one place for the default values. |
31430 | 704 |
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705 All object variables will be used in the default constructor, also private |
31671 | 706 access ones. |
707 | |
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708 If the class extends another one, the object variables of that class will come |
31671 | 709 first. |
710 | |
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711 |
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712 Multiple constructors ~ |
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713 |
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714 Normally a class has just one new() constructor. In case you find that the |
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715 constructor is often called with the same arguments you may want to simplify |
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716 your code by putting those arguments into a second constructor method. For |
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717 example, if you tend to use the color black a lot: > |
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718 |
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719 def new(this.garment, this.color, this.size) |
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720 enddef |
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721 ... |
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722 var pants = new(Garment.pants, Color.black, "XL") |
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723 var shirt = new(Garment.shirt, Color.black, "XL") |
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724 var shoes = new(Garment.shoes, Color.black, "45") |
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725 |
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726 Instead of repeating the color every time you can add a constructor that |
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727 includes it: > |
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728 |
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729 def newBlack(this.garment, this.size) |
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730 this.color = Color.black |
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731 enddef |
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732 ... |
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733 var pants = newBlack(Garment.pants, "XL") |
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734 var shirt = newBlack(Garment.shirt, "XL") |
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735 var shoes = newBlack(Garment.shoes, "9.5") |
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736 |
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737 Note that the method name must start with "new". If there is no method called |
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738 "new()" then the default constructor is added, even though there are other |
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739 constructor methods. |
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740 |
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741 |
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742 ============================================================================== |
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743 |
31885 | 744 7. Type definition *Vim9-type* *:type* |
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745 |
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746 A type definition is giving a name to a type specification. This also known |
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747 type alias. For Example: > |
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748 |
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749 :type ListOfStrings = list<string> |
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750 |
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751 The type alias can be used wherever a built-in type can be used. The type |
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752 alias name must start with an upper case character. A type alias can be |
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753 created only at the script level and not inside a function. A type alias can |
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754 be exported and used across scripts. |
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755 |
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756 ============================================================================== |
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757 |
31885 | 758 8. Enum *Vim9-enum* *:enum* *:endenum* |
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759 |
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760 {not implemented yet} |
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761 |
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762 An enum is a type that can have one of a list of values. Example: > |
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763 |
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764 :enum Color |
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765 White |
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766 Red |
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767 Green |
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768 Blue |
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769 Black |
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|
770 :endenum |
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771 |
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|
772 |
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|
773 ============================================================================== |
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|
774 |
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775 9. Rationale |
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776 |
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777 Most of the choices for |Vim9| classes come from popular and recently |
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778 developed languages, such as Java, TypeScript and Dart. The syntax has been |
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779 made to fit with the way Vim script works, such as using `endclass` instead of |
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780 using curly braces around the whole class. |
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781 |
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782 Some common constructs of object-oriented languages were chosen very long ago |
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783 when this kind of programming was still new, and later found to be |
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784 sub-optimal. By this time those constructs were widely used and changing them |
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785 was not an option. In Vim we do have the freedom to make different choices, |
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786 since classes are completely new. We can make the syntax simpler and more |
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787 consistent than what "old" languages use. Without diverting too much, it |
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788 should still mostly look like what you know from existing languages. |
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789 |
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790 Some recently developed languages add all kinds of fancy features that we |
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791 don't need for Vim. But some have nice ideas that we do want to use. |
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792 Thus we end up with a base of what is common in popular languages, dropping |
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793 what looks like a bad idea, and adding some nice features that are easy to |
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794 understand. |
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|
795 |
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|
796 The main rules we use to make decisions: |
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797 - Keep it simple. |
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798 - No surprises, mostly do what other languages are doing. |
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799 - Avoid mistakes from the past. |
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800 - Avoid the need for the script writer to consult the help to understand how |
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801 things work, most things should be obvious. |
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802 - Keep it consistent. |
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803 - Aim at an average size plugin, not at a huge project. |
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804 |
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|
805 |
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|
806 Using new() for the constructor ~ |
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|
807 |
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|
808 Many languages use the class name for the constructor method. A disadvantage |
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809 is that quite often this is a long name. And when changing the class name all |
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810 constructor methods need to be renamed. Not a big deal, but still a |
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811 disadvantage. |
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812 |
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813 Other languages, such as TypeScript, use a specific name, such as |
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814 "constructor()". That seems better. However, using "new" or "new()" to |
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815 create a new object has no obvious relation with "constructor()". |
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816 |
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817 For |Vim9| script using the same method name for all constructors seemed like |
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818 the right choice, and by calling it new() the relation between the caller and |
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819 the method being called is obvious. |
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820 |
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821 |
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822 No overloading of the constructor ~ |
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823 |
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824 In Vim script, both legacy and |Vim9| script, there is no overloading of |
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825 methods. That means it is not possible to use the same method name with |
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826 different types of arguments. Therefore there also is only one new() |
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827 constructor. |
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828 |
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829 With |Vim9| script it would be possible to support overloading, since |
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830 arguments are typed. However, this gets complicated very quickly. Looking at |
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831 a new() call one has to inspect the types of the arguments to know which of |
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832 several new() methods is actually being called. And that can require |
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833 inspecting quite a bit of code. For example, if one of the arguments is the |
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834 return value of a method, you need to find that method to see what type it is |
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835 returning. |
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836 |
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837 Instead, every constructor has to have a different name, starting with "new". |
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838 That way multiple constructors with different arguments are possible, while it |
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839 is very easy to see which constructor is being used. And the type of |
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840 arguments can be properly checked. |
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841 |
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842 |
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843 No overloading of methods ~ |
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844 |
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845 Same reasoning as for the constructor: It is often not obvious what type |
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846 arguments have, which would make it difficult to figure out what method is |
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847 actually being called. Better just give the methods a different name, then |
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848 type checking will make sure it works as you intended. This rules out |
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849 polymorphism, which we don't really need anyway. |
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850 |
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851 |
31671 | 852 Single inheritance and interfaces ~ |
853 | |
854 Some languages support multiple inheritance. Although that can be useful in | |
855 some cases, it makes the rules of how a class works quite complicated. | |
856 Instead, using interfaces to declare what is supported is much simpler. The | |
857 very popular Java language does it this way, and it should be good enough for | |
31885 | 858 Vim. The "keep it simple" rule applies here. |
31671 | 859 |
860 Explicitly declaring that a class supports an interface makes it easy to see | |
861 what a class is intended for. It also makes it possible to do proper type | |
862 checking. When an interface is changed any class that declares to implement | |
863 it will be checked if that change was also changed. The mechanism to assume a | |
864 class implements an interface just because the methods happen to match is | |
865 brittle and leads to obscure problems, let's not do that. | |
866 | |
867 | |
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868 Using "this.variable" everywhere ~ |
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869 |
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870 The object variables in various programming languages can often be accessed in |
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871 different ways, depending on the location. Sometimes "this." has to be |
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872 prepended to avoid ambiguity. They are usually declared without "this.". |
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873 That is quite inconsistent and sometimes confusing. |
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874 |
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875 A very common issue is that in the constructor the arguments use the same name |
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876 as the object variable. Then for these variables "this." needs to be prefixed |
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877 in the body, while for other variables this is not needed and often omitted. |
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878 This leads to a mix of variables with and without "this.", which is |
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879 inconsistent. |
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880 |
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881 For |Vim9| classes the "this." prefix is always used. Also for declaring the |
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882 variables. Simple and consistent. When looking at the code inside a class |
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883 it's also directly clear which variable references are object variables and |
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884 which aren't. |
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885 |
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886 |
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887 Using class variables ~ |
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888 |
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889 Using "static variable" to declare a class variable is very common, nothing |
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890 new here. In |Vim9| script these can be accessed directly by their name. |
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891 Very much like how a script-local variable can be used in a method. Since |
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892 object variables are always accessed with "this." prepended, it's also quickly |
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893 clear what kind of variable it is. |
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894 |
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895 TypeScript prepends the class name before the class variable name, also inside |
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896 the class. This has two problems: The class name can be rather long, taking |
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897 up quite a bit of space, and when the class is renamed all these places need |
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898 to be changed too. |
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899 |
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900 |
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901 Declaring object and class variables ~ |
31671 | 902 |
903 The main choice is whether to use "var" as with variable declarations. | |
904 TypeScript does not use it: > | |
905 class Point { | |
906 x: number; | |
907 y = 0; | |
908 } | |
909 | |
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910 Following that Vim object variables could be declared like this: > |
31671 | 911 class Point |
912 this.x: number | |
913 this.y = 0 | |
914 endclass | |
915 | |
916 Some users pointed out that this looks more like an assignment than a | |
917 declaration. Adding "var" changes that: > | |
918 class Point | |
919 var this.x: number | |
920 var this.y = 0 | |
921 endclass | |
922 | |
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923 We also need to be able to declare class variables using the "static" keyword. |
31671 | 924 There we can also choose to leave out "var": > |
925 class Point | |
926 var this.x: number | |
927 static count = 0 | |
928 endclass | |
929 | |
930 Or do use it, before "static": > | |
931 class Point | |
932 var this.x: number | |
933 var static count = 0 | |
934 endclass | |
935 | |
936 Or after "static": > | |
937 class Point | |
938 var this.x: number | |
939 static var count = 0 | |
940 endclass | |
941 | |
942 This is more in line with "static def Func()". | |
943 | |
944 There is no clear preference whether to use "var" or not. The two main | |
945 reasons to leave it out are: | |
946 1. TypeScript, Java and other popular languages do not use it. | |
947 2. Less clutter. | |
948 | |
949 | |
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950 Using "ClassName.new()" to construct an object ~ |
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951 |
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952 Many languages use the "new" operator to create an object, which is actually |
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953 kind of strange, since the constructor is defined as a method with arguments, |
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954 not a command. TypeScript also has the "new" keyword, but the method is |
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955 called "constructor()", it is hard to see the relation between the two. |
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956 |
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957 In |Vim9| script the constructor method is called new(), and it is invoked as |
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958 new(), simple and straightforward. Other languages use "new ClassName()", |
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959 while there is no ClassName() method, it's a method by another name in the |
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960 class called ClassName. Quite confusing. |
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961 |
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962 |
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963 Default read access to object variables ~ |
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964 |
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965 Some users will remark that the access rules for object variables are |
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966 asymmetric. Well, that is intentional. Changing a value is a very different |
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967 action than reading a value. The read operation has no side effects, it can |
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968 be done any number of times without affecting the object. Changing the value |
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969 can have many side effects, and even have a ripple effect, affecting other |
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970 objects. |
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971 |
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972 When adding object variables one usually doesn't think much about this, just |
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973 get the type right. And normally the values are set in the new() method. |
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974 Therefore defaulting to read access only "just works" in most cases. And when |
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975 directly writing you get an error, which makes you wonder if you actually want |
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976 to allow that. This helps writing code with fewer mistakes. |
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977 |
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978 |
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979 Making object variables private with an underscore ~ |
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980 |
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981 When an object variable is private, it can only be read and changed inside the |
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982 class (and in sub-classes), then it cannot be used outside of the class. |
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983 Prepending an underscore is a simple way to make that visible. Various |
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984 programming languages have this as a recommendation. |
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985 |
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986 In case you change your mind and want to make the object variable accessible |
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987 outside of the class, you will have to remove the underscore everywhere. |
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988 Since the name only appears in the class (and sub-classes) they will be easy |
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989 to find and change. |
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990 |
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991 The other way around is much harder: you can easily prepend an underscore to |
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992 the object variable inside the class to make it private, but any usage |
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993 elsewhere you will have to track down and change. You may have to make it a |
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994 "set" method call. This reflects the real world problem that taking away |
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995 access requires work to be done for all places where that access exists. |
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996 |
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997 An alternative would have been using the "private" keyword, just like "public" |
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998 changes the access in the other direction. Well, that's just to reduce the |
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999 number of keywords. |
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1000 |
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1001 |
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1002 No protected object variables ~ |
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1003 |
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1004 Some languages provide several ways to control access to object variables. |
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1005 The most known is "protected", and the meaning varies from language to |
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1006 language. Others are "shared", "private" and even "friend". |
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1007 |
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1008 These rules make life more difficult. That can be justified in projects where |
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1009 many people work on the same, complex code where it is easy to make mistakes. |
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1010 Especially when refactoring or other changes to the class model. |
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1011 |
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1012 The Vim scripts are expected to be used in a plugin, with just one person or a |
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1013 small team working on it. Complex rules then only make it more complicated, |
32294 | 1014 the extra safety provided by the rules isn't really needed. Let's just keep |
1015 it simple and not specify access details. | |
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1016 |
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1017 |
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1018 ============================================================================== |
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1019 |
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1020 10. To be done later |
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1021 |
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1022 Can a newSomething() constructor invoke another constructor? If yes, what are |
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1023 the restrictions? |
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1024 |
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1025 Thoughts: |
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1026 - Generics for a class: `class <Tkey, Tentry>` |
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1027 - Generics for a function: `def <Tkey> GetLast(key: Tkey)` |
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1028 - Mixins: not sure if that is useful, leave out for simplicity. |
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1029 |
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1030 Some things that look like good additions: |
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1031 - For testing: Mock mechanism |
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1032 |
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1033 An important class to be provided is "Promise". Since Vim is single |
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1034 threaded, connecting asynchronous operations is a natural way of allowing |
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1035 plugins to do their work without blocking the user. It's a uniform way to |
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1036 invoke callbacks and handle timeouts and errors. |
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1037 |
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1038 |
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1039 vim:tw=78:ts=8:noet:ft=help:norl: |