Constructor this.arg syntax

proposals/0000-instance_args.md

@@ -0,0 +1,78 @@
+# Constructor instance arguments
+
+* Proposal: [HXP-NNNN](NNNN-filename.md)
+* Author: [RblSb](https://github.com/RblSb)
+
+## Introduction
+
+Provide `this.arg` syntax for assigning constructor arguments to instance fields.
+
+## Motivation
+
+This operation is very common in object-oriented programming and with this syntax data-classes should be easier to write, read and change.
+
+## Detailed design
+
+Lets see a typical class example, that is used to keep some data:
+```haxe
+class People
+  final name:String;
+  final age:Int;
+  function new(name:String, age:Int) {
+    this.name = name;
+    this.age = age;
+  }
+}
+```
+This is how it would look with the new syntax:
+```haxe
+class People
+  final name:String;
+  final age:Int;
+  function new(this.name, this.age) {}
+}
+```
+With the second example there is less code duplication and no requirement to specify the types, they are derived from respective fields. `this.arguments` are set before all code in the constructor body.
+Access to `name` and `this.name` will be identical and mean access to the field `name`, like there isn't any argument variable in the constructor:
+```haxe
+class People extends Bug {
+  final name:String;
+  final age:Int;
+  function new(this.name, this.age) {
+    // after fields are assigned, constructor code starts execution
+
+    age++; // changes this.age
+
+    super();
+
+    name = 'Cooler $name'; // changes this.name
+  }
+}
+```
+This should mean less variable shadowing mistakes.
+
+`this.` arguments can be used with normal arguments in any order:
+```haxe
+function new(type:Int, this.context, isBlock = false) {}
+```
+
+If `foo` field for `this.foo` argument doesn't exist in a class, there should be a compiler error.
+
+This syntax is disabled outside of constructor functions.
+
+## Impact on existing code
+
+Should be no impact.
+
+## Alternatives
+
+There is a possible macro code generation replacement, but I think this syntax is simple enough to understand, to be in language itself.
+
+## Opening possibilities
+
+We can omit constructor body requirement, if there is already any `this.argument` in `new` args:
+
+`function new(this.name, this.age);`
+
+
+## Unresolved questions