First Draft: Static Single Assignment

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On Thursday, 27 November 2025 at 22:23:33 UTC, Peter C wrote:
> On Thursday, 27 November 2025 at 19:41:18 UTC, Jonathan M Davis 
> wrote:
>>
>> ...
>> Of course, personally, I don't think that this feature adds 
>> enough value to be worth adding at all, but it does feel very 
>> much like something being welded onto the language to solve a 
>> corner case that doesn't work with const rather than having a 
>> more holistic solution.
>>
>> ...
>> - Jonathan M Davis
>
> Here is a 'simple' example of its value:
>
> class Data { int value = 1; }
>
> void main()
> {
>     // const Data c = new Data();
>     //c.value = 42; // Error: cannot modify `const` expression 
> `c.value`
>     //c = new Data(); // Error: cannot modify `const` 
> expression `c` c = new Data();
>
>     final Data c = new Data();
>     c.value = 42; // fine.
>     c = new Data(); // Error (SAA): Cannot reassign a 'final' 
> variable.
>
>     // .....
>
>     // Pass the fixed reference to the performCheck function
>     performCheck(c);
>
>     // .....
>
>     // We have a guarantee that here, 'c' will still refer to 
> the same object it was initialized with
>     writeln("\nValue: ", c.value);
>
> }
>
> // The compiler guarantees that performCheck cannot
> // introduce a side effect by reassigning the reference.
> //
> void performCheck(final Data obj)
> {
>     obj = new Data();  // Error (SAA): Cannot reassign a 
> 'final' variable.
>     obj.value = 1; // fine. (Mutating the object is allowed)
> }

The compiler can already guarantee that, because the reference to 
c is passed by value. To pass it by reference and introduce such 
a side effect, the function would have to accept a ref Data.