T parameter question

1

Hej !

I can’t really understand why this example fails, if someone can explain me please ? And advice me how to solve that please ? Try Haxe !

class Session {
  public function new(){}
  public function get(){
		return this;
  }
}
class Test<T:Session> {
  var session	: T;
  public function new( ?s : T ){
    //session	= s;	// OK
	session	= s.get();	// Session should be Test.T
  }
  static function main() {
    var sess = new Session();
    var inst = new Test( sess );
  }
}

Thanks,

2

The problem is that in your program there is no guarantee that Session is assignable to Test.T, because T can be a subtype of Session.

For example consider this:

class Session2 extends Session {}

function main() {
  var inst2 = new Test(new Session2());
}

In this case the field Test.session is of type Session2 but the method Session.get still returns a value of type Session, that is not assignable to Session2.

3

Thx for your answer but this works fine : Try Haxe !

class Session {
  public function new(){}
  public function get(){
		return this;
  }
}
class MySession extends Session{}
class Test<T:Session> {
  var session	: T;
  public function new( ?s : T ){
    session	= s;				// OK
		//session	= s.get();	// Session should be Test.T
  }
  static function main() {
    var sess = new MySession();
    var inst	= new Test( sess );
  }
}
4

You will need a “Self” type but currently it is not supported in Haxe.

See: Polymorphic `this` types by matulkum · Pull Request #36 · HaxeFoundation/haxe-evolution · GitHub

1 Like
5

Thanks for your answer Kevin.
Ok so the only quick workaround I see is to use cast keyword…

6

Your last example works because Test.session and s are of the same type, namely T.
On the other hand the type of Test.session and the return type of Session.get are T and Session, where T is a subtype of Session. So you need a downcast.

7

In my last example MySession is of type extended from Session, not the same type. I would understand to do something in that case but not in the other where I return exact the same type which is expected…

8

Your get() function’s return type is always Session, so it won’t work when T = MySession

9

I know Kevin but I think here we misunderstood, with the 2nd exampe which is not what I would to do first.
Your first reply “satisfy” my curiosity.
The 2nd example is something else.