First Draft: Static Single Assignment

[Peter C]

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)
}