How about this (which is not unlike one of the ideas you proposed earlier for
pattern declarations):
- For a value type V with fields f1 .. fn, let the user write a constructor as
if it were a regular class.
- The compiler inserts synthetic blank finals f`1..f`n, and translates
accesses to this.fi to accesses to f`i
- The compiler requires that all f`i are DA at all normal completion points,
and inserts { default_value / witfield* } copying from the synthetic f`i locals
Now, a value ctor looks _exactly_ like a ctor in a non-value type with final
fields. No new idioms to learn.
> On May 17, 2018, at 7:40 PM, Maurizio Cimadamore
> <[email protected]> wrote:
>
>
>
> On 17/05/18 23:22, John Rose wrote:
>> A Java constructor in a value class will internally use withfield
>> to translate any assignment of the form "this.x = y", and instead
>> of the blank instance being an incoming reference in L[0], the
>> constructor builds a blank value instances out of thin air using
>> vdefault.
> So, if I understand correctly, a classic Java constructor is a void-returning
> instance method; in the model you propose a value class constructor would be
> more similar to a V-returning static method (where V is the value to be
> constructed).
>
> This is all and well, but I feel that this pushes the problem under the
> (assignment) rug. E.g. I believe that reinterpreting the meaning of 'this.x =
> y' inside a value constructor to mean "get a brand new value and stick y into
> x" would be very confusing, as semantically, there's no assignment taking
> place. And, semantically, it doesn't even make sense to think about a 'this'
> (after all this is more like a static factory?).
>
> Of course you can spin this as reinterpreting the meaning of the word 'this'
> inside a value constructor - e.g. the new meaning being "the opaque value
> being constructed"; but that is likely to clash with other utterances of
> 'this' in the same value class (e.g. in other instance methods - where 'this'
> would simply mean 'this value').
>
> Language-wise (and I repeat, it might well be too soon to dive into this), it
> feels like we're missing a way to express a new kind of a primitive operation
> (the wither). Without that, I'm a bit skeptical on our ability to be able to
> express value type constructors in a good way.
>
> Maurizio
