Nullable types and inference

[Remi Forax]

I will rain on your parade.

Here, i'm talking about Java the language, not about the VM support of L/Q-types.

V? is a use site annotation and we all know that use site annotations are the devil in disguise, wildcards is a good example of a use site feature nobody understand/want to understand, so the bar to introduce such kind of annotation is the highest bar that exists to introduce something in Java.

so why do we need V?, we need it
- to represent the L variation of a Q type.
- to interact with generics that are not reified.

The first case is a corner case and for generics, it works until someone try to stuff null into a value type.

So instead introducing nullable value types in the language which make the language far more complex than it should be, i think we should come up with a far simpler proposal, to have a declaration site tagging of the method that doesn't work with value types. 


  // proposed syntax
  interface Map<K, V> {
    "this method doesn't work if V is a value type" public V get(Object o);
  }


Rémi

----- Mail original -----
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
> Envoyé: Dimanche 28 Avril 2019 19:07:26
> Objet: Re: Nullable types and inference

> More on this: we’ve ambled our way into a nice and consistent model for “?”
> types.  Which is, T? is the union type of T and Null.  This works for nullable
> values: (V? where V is a zero-default value type), type variables (T? means T
> union Null), and also for nullable type patterns (the pattern Foo? matches any
> non-null Foo, or null.).
> 
> This is not necessarily what users will immediately think T? means, especially
> if they’ve got experience with type systems where ? is an arity indicator
> meaning “zero or one” (such as X#/C_omega).  But its an explainable and stable
> thing, and the fact that we independently came to this from two directions
> (values and patterns) is an encouraging indicator.
> 
>> On Apr 26, 2019, at 9:44 AM, Brian Goetz <brian.goetz at oracle.com> wrote:
>> 
>> I’ve been looking over Chapter 18 (thanks Dan!) and it seems that we are almost
>> there to defining inference to work properly for values and erased generics.
>> 
>> There is already a _null type_, and we’ve defined it so that for every reference
>> type R, Null <: R (JLS 4.10.). In order to make inference work for nullable
>> values, we need to state that for a zero-default value type V:
>> 
>>    Null <! V
>>    Null <: V?
>> 
>> and that LUB(V, Null) = V?
>> 
>> When we gather constraints in 18.1, in addition to adding the upper bound on
>> alpha, we also add in lower bounds Null <: alpha_i for erased type vars.
>> 
>> We adjust 18.4 to not consider Null to be a proper lower bound for purposes of
>> resolution.
>> 
>> Simple example:
>> 
>>    Point p;
>>    var v = new Box<>(p);
>> 
>> We gather bounds alpha <: Object (from the declaration of Box<T>), Point <:
>> alpha (from the argument), and Null <: alpha (T is an erased type var),
>> yielding
>> 
>>    Null, Point <: alpha <: Object
>> 
>> By 18.4, alpha = LUB(Point, Null) = Point?.
>> 
>> Obviously this is only one example, and there’s a bunch of work to thread this
>> all the way through Ch18 (good luck Dan!), but it seems to me that the
>> underpinnings are here already.
>>