Tagged unions [Was: What's wrong with std.variant.Variant?]

[Stanislav Blinov]

On Tuesday, 16 June 2020 at 12:44:54 UTC, Simen Kjærås wrote:
> On Tuesday, 16 June 2020 at 12:21:56 UTC, Paul Backus wrote:

>> If it's not too much trouble, can you give an example of the 
>> kind of thing you'd like to be able to do? I suspect it's 
>> possible with SumType, but it may not be obvious how, which 
>> makes it a good candidate for the documentation's "examples" 
>> section.
>
> I'm not Dukc, but here's an example I came up with:
>
> float myFun() {
>     MyTaggedUnion!(float, int, string, int[]) a = fun();
>
>     float value;
>     final switch (a.type) {
>         case a._float:
>             return a.as!float();
>         case a._int:
>             return a.as!int();
>         case a._string:
>             value = a.as!string().process();
>         case a._intArray:
>             value = a.as!(int[]).process();
>     }
>
>     // Lots of code that manipulates 'value'.
>     if (a.type == a._string) {
>        // Do something special for only one of the types,
>        // but after doing lots of common things
>     }
>     // More code that manipulates 'value'.
>
>     return value;
> }
>
> Essentially, there's shortcut returns for a few cases, but the 
> others have some common behavior, even better if this is 
> slightly different in one single place for one single held 
> type, so you can't really factor it out as a separate function.

I wonder how that final switch would look like if the tagged 
union contains a type such as Some!(Other!(immutable(int)[]), 3). 
I.e. how would you spell out the case :)