Completing C code with D style

[jfondren]

On Saturday, 6 November 2021 at 13:27:55 UTC, kdevel wrote:
> On Thursday, 4 November 2021 at 00:53:11 UTC, jfondren wrote:
>> On Wednesday, 3 November 2021 at 20:36:08 UTC, russhy wrote:
>>> Keeping things simple helps debugging!
>>
>> I'd still have to run your program to be sure of its simple 
>> logic, though. The real star d feature that would help with 
>> debugging is unittest:
>
> What about the number 0 (zero)?

Not treating it as positive is a bug that I introduced in a 
rewrite.

> If 0 is considered to be excluded from the domain how would one 
> define in D a type which cannot represent 0 in the first place? 
> Follow-up question: Are there language facilities such that one 
> could initialize an array of these non-zero-ints like?:
>
> ```
>    nzint [] numbers = [ -2, -1, 1, 2 ];
> ```

In general, you use a constructor that tests for non-zero and you 
take care to maintain the invariant. D has some tools (like 
invariant) to help with that, but it's not a dependent-typed 
language.

For this specific example, you can sort of do that with 
std.typecons.Nullable

```d
unittest {
     import std.typecons : Nullable, nullable;
     import std.algorithm : map;
     import std.array : array;

     alias NonZero(T) = Nullable!(T, 0);

     NonZero!int[] numbers = [-2, -1, 0, 1, 2].map!(i => 
NonZero!int(i)).array;

     int sum;
     int invalids;
     foreach (n; numbers) {
         if (n.isNull) invalids++;
         else sum += n.get;
     }
     assert(sum == 0);
     assert(invalids == 1);
}
```