First Draft: Static Single Assignment

[Peter C]

On Wednesday, 3 December 2025 at 10:18:50 UTC, Nick Treleaven 
wrote:
> On Wednesday, 3 December 2025 at 04:05:28 UTC, Peter C wrote:
>> class Bag
>> {
>>     final int[] items; // This is a binding-level guarantee - 
>> also acts an API invariant.
>>     this() { items = []; }
>> }
>>
>> void main()
>> {
>>     auto b = new Bag();
>>     b.items ~= 1; // fine. still mutable.
>>     b.items = []; // was expecting an error here??
>> }
>
> Just to note that final on a dynamic array `a` should mean that 
> `a.ptr` and `a.length` never change, but elements e.g. `a[0]` 
> can change. So if final was allowed on fields, both assignments 
> above would error.

Yes, I forgot how dynamic arrays work.

Dynamic arrays are allocated on the heap and will incur 'a 
reallocation' when their size changes (via ~= for example). So 
appending, removing, subsequently reserving a size, or 
subsequently reassigning explicately, should all be considered as 
a being disallowed.

When you said "So if final was allowed on fields.." are you 
saying final does not work yet on class fields? Is that why I get 
no errors here?