allow variance of property types?

[Raoul Duke]

(pseudocode)
class Base {}
class Sub extends Base {}
interface Foo { var x(get,null):Base; }
class Foo { public var x(get,null):Sub; } // i think i wish this were
not a compilation error.

Level1.hx:8: lines 8-31 : Field scene has different type than in ILevel
Level1.hx:8: lines 8-31 : com.haxepunk.Scene should be Scene1

p.s. i am pretty sure that i always have and always will think that
last error message is backwards.

[Juraj Kirchheim]

Assume this compiles:

  class Base {}
  class Sub extends Base {}

  interface IFoo { var x(get, set):Base; }
  class Foo implements IFoo { public var x(get, set):Sub; } 

Then this can happen:

  var foo = new Foo();

  var ifoo:IFoo = foo;

  ifoo.x = new Base();//ouch!

If anything, this could only work on readonly properties. In fact it does already for anonymous types: http://try.haxe.org/#210ee

Generally, nominal types are not too good with variance in Haxe. I don't quite know why, but I think that several target runtime don't really allow for it to start with. Haxe already does quite a dance to allow for variance on methods.

Best,

Juraj

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[Raoul Duke]

hi,

> If anything, this could only work on readonly properties.

Thanks for the reminder. Probably ever time I hit this, I think I
maybe get re-told that immutability as the default is one way to get
this? I mean in some other language that starts off with immutability
as the main law of the land.

I mean, in my case right now, I'd be happy with it being read only
from the outside; see how my pseudo code had null in it.

software is hard.