First Draft: Static Single Assignment

[Peter C]

On Monday, 17 November 2025 at 15:05:17 UTC, jmh530 wrote:
>
> Would it make sense to make final/head-const part of the type 
> system? Other than that it makes it more difficult to implement.
>
> I recall there were a lot of discussions in the past about 
> converting something that is head const to tail const, or maybe 
> the reverse. I think the issue was that it was built-in for D's 
> arrays, but you can't easily do that if you have your own 
> aggregate. There seemed to have been some desire to have that 
> functionality in the language.

Wouldn't this change the re-engineering scope of final from 
low-impact (a local compiler rule check - essentially a hack) to 
high-impact (a fundamental redesign of the type system).

It seems improbable [to me at least], that Walter would seriously 
consider fundamentally redesigning the type system.

The most pragmatic option here, is the Minimal Viable Change 
(MVC) approach - i.e one that satisfies the guarantee of SSA 
immutability without incurring massive technical debt.

  - Goal: Guarantee SSA immutability.
  - Method: Use a simple declaration constraint (metadata) on the 
variable identifier.
  - Safety Net: Use the existing, well-understood mechanism of 
synthesizing a const pointer when the address is taken, which is 
the necessary "hack" to uphold the original final promise.