<init> and factories

[John Rose]

On Oct 17, 2019, at 1:38 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> 
>> Fine points in the VM prototype:
>> 
>> Would there be any restrictions on the contents of a constructor/factory method <new>?  (I hope not.)
> 
> I'd be sad if it were possible for a invocation of a `<new>` method to leave a `null` on the stack.

Yes.  And should a factory contract sometimes include a guarantee of an exact type for the non-null return value?
(Maybe yes, sometimes no.  Probably null is always wrong; don’t call that a factory.)

So this leads to one or two use cases for type operators:

1. Non-null decoration on descriptor.  Could be a template specialization NonNull<*C> where
C is the return value and the thing with * is reified.  All factories should return this.

(Could be LC//NonNull; or LNonNull//C; or LC[NonNull]; or LNonNull[C]; as a decoration
syntax for descriptors.  Various other considerations would determine the actual bike shed color.)

2. Exact-type decoration on descriptor.  Could be another template specialization Exact<*C>.
Exact types are sometimes nice to have, although they make API points very rigid. Sometimes
that’s the goal.

— John