Towards Minimal L World

Maurizio Cimadamore maurizio.cimadamore at
Fri May 18 08:57:23 UTC 2018

On 18/05/18 01:33, Brian Goetz wrote:
> 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 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.
Seems more natural - as we're only doing assignments of locals, not 
assignment of fields (which is, in fact, impossible in the V-world). 
That's a well-spotted connection!

>> On May 17, 2018, at 7:40 PM, Maurizio Cimadamore <maurizio.cimadamore at> 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

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