New syntax for getters & setters inspired by the C# property syntax

proposals/0000-new-getset-syntax.md

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+# New getter & setter syntax
+
+* Proposal: [HXP-NNNN](NNNN-filename.md)
+* Author: [GasInfinity](https://github.com/GasInfinity)
+
+## Introduction
+
+Provide new syntax to make properties
+
+```haxe
+private var _testField:T;
+public var testProperty:T { get -> _testField; set -> _testField = value; }
+```
+
+## Motivation
+
+Every time you want to make properties in Haxe, you need to create the functions for these getters and setters. It's quite verbose. I propose a new way to make properties inspired in the C# syntax:
+
+```haxe
+// Creates a backing field and works like a field, can be overriden if a class inherits this property
+public var propertyWithField:T { get; set; }
+
+// Creates a backing field, but this property can only be assigned once in the constructor
+public var readonlyProperty:T { get; }
+
+// Raw property, doesn't create a backing field
+public var property:T { get -> {} set -> {} }
+```
+  vs
+```haxe
+// Creates a field and works like a field, can be overriden if a class inherits this property
+private var _propertyWithField;
+public var propertyWithField(get, set):T;
+
+function get_propertyWithField():T return _propertyWithField;
+function set_propertyWithField(value:T):T return _propertyWithField = value;
+
+// Creates a field, but this property can only be assigned once in the constructor
+private final var _readonlyProperty;
+public var readonlyProperty(get, never):T;
+
+function get_readonlyProperty():T return _readonlyProperty;
+
+// Raw property, doesn't create a backing field
+public var property(get, set):T;
+
+function get_property():T {}
+function set_property(value:T):T {}
+```
+
+It could achieve the same results with less code while maintaining readability.
+
+## Detailed design
+
+### Syntax
+
+The following syntax is proposed for properties:
+
+* Raw Property
+```haxe
+public var name:Type { get -> expr set -> expr }
+
+// The equivalent of this code is:
+
+public var name(get, set):Type;
+
+function get_name():Type expr
+function set_name(value:Type):Type expr
+```
+
+* Property with only a getter:
+```haxe
+public var name:Type { get; }
+
+// This doesn't have a 1 to 1 equivalent, but this is somewhat equivalent:
+
+private var _name:Type;
+public var name(get, never):Type;
+
+function get_name() return _name;
+```
+
+* Autoproperty:
+```haxe
+public var name:Type { get; set; }
+
+// The equivalent of this code is:
+
+private var _name:Type;
+public var name(get, set):Type;
+
+function get_name():Type return _name;
+function set_name(value:Type) return _name = value;
+```
+
+### Behaviour
+
+* A property will always create internally functions for getters and setters because every setter and getter can be overridden.
+    ```haxe
+    class A
+    {
+        public var property:Int { get; } // An instance of the class A can only get the value
+    }
+
+    class B extends A
+    {
+        public var property:Int { get; set; } // An instance of the class B can get and set the value
+    }
+
+    class C extends A
+    {
+        public var property:Int { override get -> 1; }
+    }
+
+    function main()
+    {
+        var b:B = new B();
+        var a:A = b;
+        // a.property = 20; // Error, property doesn't have a setter
+        b.property = 20; // No errors
+
+        var c = new C();
+        // c.property = 20; // Error, property doesn't have a setter
+
+        trace(c.property); // Output: 1
+
+    }
+    ```
+* A property can have access modifiers
+    ```haxe
+    class A
+    {
+        public var property:Int { inline get; private set; }
+        // private var otherProperty:Int { public get; set; } // Error, the visibility of a getter or setter must be lower than the visibility of the property itself
+
+        public function modifyProperty():Void
+        {
+            property = 20; // No errors
+        }
+    }
+
+    function main()
+    {
+        var a = new A();
+        // a.property = 20; // Error, can't set property, it's private
+        a.modifyProperty();
+
+        trace(a.property); // This gets inlined
+    }
+    ```
+* A property can be initialized
+    ```haxe
+    class A
+    {
+        public var property:Int { get; } = 40;
+    }
+
+    function main()
+    {
+        var a = new A();
+
+        trace(a.property); // Output: 40
+    }
+    ```
+* A property with only a getter can be assigned only once in the constructor
+    ```haxe
+    class A
+    {
+        public var property:Int { get; }
+
+        public function new(propertyValue:Int)
+        {
+            this.property = propertyValue;
+        }
+    }
+
+    function main()
+    {
+        var a = new A(600);
+
+        trace(a.property); // Output: 600
+    }
+    ```
+* The first argument passed to the setter function has the implicit argument name of `value`
+    ```haxe
+    class A
+    {
+        private var field:Int;
+        public var property:Int {
+            get -> {
+                return field;
+            }
+            set -> {
+                return field = value;
+            }
+        } // This could also be written in a short form { get -> field; set -> field = value; }
+
+    }
+
+    function main()
+    {
+        var a = new A();
+        a.property = 10;
+
+        trace(a.property); // Output: 10
+    }
+    ```
+* A property can be static
+    ```haxe
+    class A
+    {
+        public static var property:Int { get; set; }
+    }
+
+    function main()
+    {
+        A.property = 20;
+
+        trace(A.property); // Output: 20
+    }
+    ```
+* A getter or setter without a function implementation and with the `final` modifier in a property should behave like a normal field access.
+    ```haxe
+    class A
+    {
+        public var property:Int { final get; set; }
+    }
+
+    class B extends A
+    {
+        @:isVar
+        public var property:Int { override set -> property = value; } // Pointless, but we're just showing what can be done or not
+    }
+
+    function main()
+    {
+        var a = new A();
+        var b:B = a;
+
+        a.property = 20;
+
+        trace(a.property); // Normal field access because of the final modifier without function implementation
+        b.property = 30; // This would call the function of the B class
+
+        trace(b.property); // Still normal field access
+    }
+    ```
+* A property can be declared in an `interface`, but it cannot have any implementation, classes that extend that interface must implement the property (As an Autoproperty or as a raw property)
+    ```haxe
+    interface A
+    {
+        var property:Int { get; }
+    }
+
+    interface B extends A
+    {
+        var property:Int { get; set; } // This works, we're only declaring that the property has a getter and a setter
+        // var otherProperty:Int { get -> 1; } // Error, a property in an interface cannot have any implementation
+    }
+
+    class C implements B // Error, missing implementation of the getter & setter of the property 'property'
+    {
+
+    }
+    ```
+* A property in an `abstract class` can have abstract getters/setters
+    ```haxe
+    abstract class A
+    {
+        var property:Int { abstract get; }
+    }
+
+    class B extends A // Error, B must implement the getter of the property 'property'
+    {
+    }
+    ```
+* A property in an `abstract` must have implementations
+    ```haxe
+    abstract A(Int)
+    {
+        // public var badProperty:Int { get; set; } // Error, the getters/setters in an abstract must have a function implementation
+
+        public var goodProperty:Int { inline get -> 1; } // Good, we have a function implementation
+        public var otherGoodProperty:Int { inline get -> this; inline set -> this = value; } // Good, we have only function implementations
+    }
+    ```
+* A property can be overriden in a derived class, but it must be overriden as a raw property. 
+    ```haxe
+    class A
+    {
+        public var property:Int { get; set; }
+    }
+
+    class B extends A 
+    {
+        // Allowed, we're making function implementations for the property
+        public var property: Int { override get -> 1; override set -> -1; }
+
+        // Maybe we could use the @:isVar metadata to be able to access it's backing field from the class
+        // @:isVar
+        // public var property: Int { override get -> property * 2; override set -> property = value / 2; }
+    }
+
+    /* Not allowed, it must have an implementation
+    class C extends A
+    {
+        public var property:Int { override final get; override final set; }
+    }*/
+
+    function main()
+    {
+        var a = new A();
+        var b = new B();
+
+        trace(a.property); // Output: 0 (default value of Int)
+        trace(b.property); // Output: 1
+
+        a = b;
+        trace(a.property = 2); // Output: -1
+        trace(a.property); // Output: 1
+    }
+    ```
+* If the property has the `@:isVar` metadata, it can assign to the backing field generated by the compiler when assigning to itself:
+    ```haxe
+    @:isVar
+    public static var property:Int { get -> property; set -> property = value; }
+    ```
+
+### Final notes
+
+If this gets implemented in any way, we could soft deprecate the old property syntax and move to the new one.
+So, in the next major version, one breaking change might be properties.
+
+## Impact on existing code
+
+This is new syntax so I don't think it will break existing code. 
+*Unless in the next major version we remove the old property syntax. That would be a very **big** breaking change.*
+
+## Drawbacks
+
+No drawbacks. *I think, correct me if I'm wrong, please*
+
+## Alternatives
+
+We could do it in many other ways, like:
+```haxe
+public var property:T { get => expr; set => expr; }
+```
+
+But I think that we should maintain the same syntax used for arrow functions, and, instead of doing:
+```haxe
+public var property:T { () -> expr; (value) -> expr }
+```
+or
+```haxe
+public var property:T { get() -> expr; set(value) -> expr }
+```
+
+We remove the parenthesis and get a syntax similar to the C# property syntax but using the Haxe `->` used for functions
+
+## Unresolved questions
+
+* Should we allow only setters without getters like now? (The current property syntax allows making a property with only a setter)
+    ```haxe
+    class A
+    {
+        public var property:Int { set; } // Error here?
+    }
+
+    function main()
+    {
+        var a = new A();
+        a.property = 20;
+
+        // trace(a.property); // or Error here?
+    }
+    ```
+    ```haxe
+    // The current property syntax allows making only a setter without a getter
+
+    class A
+    {
+        @:isVar
+        public var currentProperty(never, set):Int;
+
+        function set_currentProperty(value:Int):Int return currentProperty = value;
+    }
+
+    function main()
+    {
+        var a = new A();
+        a.currentProperty = 20;
+
+        // trace(a.currentProperty); // Error
+    }
+    ```
+