Nullable types and inference

[Brian Goetz]

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.  
> 
>