RefObject and ValObject

[Brian Goetz]

Please, no.



> On Apr 15, 2019, at 11:03 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> 
> 
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "Maurizio Cimadamore" <maurizio.cimadamore at oracle.com>
> Cc: "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
> Envoyé: Lundi 15 Avril 2019 16:38:58
> Objet: Re: RefObject and ValObject
> Let's model the value vs. reference constraint explicitly - e.g. with 'val' and 'ref' type kinds (and let's not open the can worm as to what the syntax should be, whether it should be a type anno, etc.)
> 
> So:
> 
> val Object ---> accepts all values
> ref Object ---> accepts all references
> any Object ---> accepts all references
> 
> 
> We explored this sort of thing in Q world.  One place where this really was painful was what it did to generic tvar constraints; we had an entire new algebra of constraints: 
> 
>     <ref T extends Bound> void m(T t) { … }
> 
> Brian,
> val, ref and any only applies to Object.
> 
> if you prefer in term of notation, let's rename ref Object to Object?, val Object to Object! and any Object to Object*,
> 
> you can apply ? on any value type, Object included,
> you can apply ! and * only on Object.
> 
> Object? is erased to Ljava/lang/Object; by the generic signature is Ljava/lang/Object/* (so it's Object for the VM and Object! for the compiler),
> Object! is erased to Qjav/lang/Object;
> Object* is erased to Ljava/lang/Object;
> 
> As a bound of a type variable, Object is equivalent to Object? by backward compatibility.
> and
>   <T extends Object!> void m(T t) { … }
> means that T is not nullable, so it's a value type.
> 
> And at runtime, instead of
>    (o instanceof Object!)
> one will write
>   o.getClass().isValue()
> 
> Rémi
> 
> 
> which was ad hoc and composed terribly.  This hell is _exactly_ the thing that pushed us to Ref/ValObject as _types_ in the first place.  (More of the same: what is “ref Object”.class, and how does it differ from “any Object”.class?).  
> 
> 2) reinterpret `Object` as `any Object`
> 
> That is, the semantics of `Object` is being redefined here - code which assumed to work with references might need to opt-in to additional constraints (e.g. add `ref`) in order to make sure it still work as intended.
> 
> Right.  Q-world tried it the other way, and we were in utter migration hell.  There are migration cases in this direction too, but we are convinced they are orders of magnitude fewer.  
> I don't see another way out of this conundrum - other than adding a special rule (z2) which says that `new Object()` is treated specially and always has kind `ref`. But doing so will run afoul in almost every possible way - as soon as you manipulate the result of the `new` in any way (cast, assignment to variable of type `Object`, ...) you go back to `any` and you are back to a place that is incompatible with `ref Object`.
> 
> Yes, this is the cost.  I have to think that given a choice between some weirdness around ’new Object’, and dramatic, awful new kinds of types that complicate type uses, type descriptors, reflection, etc, etc, etc, that making the former work is going to be less painful, both for us and for users.
> 
> Your idea of treating Object as abstract is, I believe, a sound one (which doesn't need any extra rule) - but we might have to figure out some story for anonymous inner classes of the kind `new Object() { ... }`.
> 
> 
> Right.  And, again, this can be treated as a migration issue, and we can start warning users to migrate their source now.