Non-atomicity field questions

[Brian Goetz]

On 5/24/2023 1:52 PM, - wrote:
> Hello, quick question:
> If there's a nullable reference field in a null-restricted non-atomic 
> value class, is this possible:

We don't null-restrict classes, we null-restrict variables.  So I think 
the above pencils out to:

     __non-atomic value class X {
         public String? s;

         public implicit X();
         public X(String! s) { this.s = s; }
     }

     X! x;

right?

> 1. Declare a reference field, like a String, without heap pollution 
> concerns;
> 2. Limit the values of fields (of course, permit zero-default values) 
> for all instances of this class, such as nonnegative long field, null 
> or String reference containing no tabs, etc.

If I understand your question, you want to impose representational 
invariants, with the proviso that the zero-default representation is 
always valid.  To do so, you can declare an implicit constructor (which 
produces the zero-default value) and an explicit constructor which does 
the plain vanilla argument validation / mapping to representation that 
constructors always do.

> This requires constructors to be executed for non-default instance 
> creation, but individual field validation is independent, and valid 
> object state is still the cartesian product of valid individual field 
> states.

The role of a constructor is to establish an object's state consistent 
with its invariants.  But some invariants are per-field, and some are 
cross-field:

     class NonNegativeInt {
         // per-field invariants
         NonNegativeInt(int i) {
             if (i < 0) throw ...
             this.i = i;
         }
     }

     class Range {
         // cross-field invariant
         Range(int lo, int hi) {
              if (lo > hi) throw ...
              ...
         }
     }

The choice to accept non-atomicity should include "no cross-field 
invariants", since a non-atomic value could tear under race.  Plenty of 
classes, though, are more like NonNegativeInt than Range.

Does this answer your question?