Is V? what we need ?
john.r.rose at oracle.com
Wed May 1 22:47:06 UTC 2019
On May 1, 2019, at 6:28 AM, Brian Goetz <brian.goetz at oracle.com> wrote:
> Also, we’re conflating two separate questions here.
> 1. What does V? mean. In the proposed world, it is the union type of V and Null, nothing more (alternately: V adjoined with null.).
> 2. What is the relationship between V and V? — is it sub typing, or conversion?
> I think Remi is more interested in (2), but you keep saying “it should be a box”,w which suggests we’re not on the same page with respect to (1) either, yet.
As you folks saw, I wrote a separate note that tries to
explain the difference between legacy L-types and
the new requirements of value (and template) types.
Our best way so far to "shake the box" of language
features is to have native int-like values of class V
be declared with the plain syntax "V". These values
need a new contract which is incompatible with the
old contract for the JVM's references, so to access
the old reference behavior we need a different syntax,
and (so far) that seems to be V?.
But let's not let ourselves be blinded by syntax. The
term V?, if it is to mean "a reference under the old
contract", cannot mean only "the values of V plus
null". It also has to mean "extra indirections" and
"no loading side effects when used" and probably
also "can participate in circular definitions", as well
"unconditionally accepts null".
A language feature which only adjoins null doesn't
pull its weight. We are adding V? because it also
pulls the weight of the old contract, allowing backward
compatibility with the JVM's references. But since
those references are a permanent part of the JVM's
*future*, it is also *future* compatibility with code
which selects the old contract instead of the new.
I suggest the following thought experiment:
In our discussions of "what does V? mean", temporarily
replace "V?" with "V*", read according to the norms
of the C language. It will more accurately reflect
the underlying "physics" of the JVM.
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