Empty associative array

[jfondren]

On Monday, 9 August 2021 at 13:10:53 UTC, Dukc wrote:
> ```d
> import std;
>
> @safe:
>
> int[int] emptyAA;
>
> void setEmptyAA()
> { alias Key = int, Value = int;
>
>   emptyAA = [Key.init: Value.init];
>   auto remover = emptyAA;
>   remover.remove(Key.init);
> }
>
> void main()
> { setEmptyAA;
>   int[int] a = emptyAA;
>   int[int] b = a;
>
>   b[5] = 3;
>
>   a.writeln; //[5:3]
> }
> ```
>
> ...however, if you initialize `a` with `emptyAA.dup` it will 
> again be `null`!

You get this with dynamic arrays as well.

```d
unittest {
     int[] a;
     assert(a == []);
     assert(a is null);
     a ~= 1;
     a = a[1 .. $];
     assert(a == []);
     assert(a !is null);
     assert(a.length == 0);
     a = a.dup;
     assert(a is null);
}
```

> I believe we can only conclude that empty AA's are not supposed 
> to rely on being non-`null`. Sorry Walter - in my eyes this is 
> a design failure.

They're very convenient, scripting-language-like types, and this 
comes with a few caveats that can be emphasized in tutorials and 
learned. If the convenience isn't worth the caveats for you, 
there's std.container.array and perhaps 
https://code.dlang.org/packages/bcaa