A proposal: Sumtypes

[Richard (Rikki) Andrew Cattermole]

On 08/03/2024 12:59 AM, cc wrote:
> On Thursday, 8 February 2024 at 15:42:25 UTC, Richard (Rikki) Andrew 
> Cattermole wrote:
>> sum types
> 
> In D there is a case where a variable can be declared in a condition, 
> e.g. `if (auto a = someFunc) a.doWhatever;`.  Could similar logic like 
> this work?
> 
> ```d
> sumtype Foo = int | float | string;
> Foo foo;
> foo = 2;
> assert(foo is int);
> assert(foo !is float);
> 
> void barFloat(float) {}
> Foo getPi() => {
>      if (userIsBaker)
>          return "apple";
>      return 3.14f;
> }
> 
> foo = getPi();
> barFloat(foo); // Illegal, use:
> if (foo is float) { // inside this scope, accesses to foo treat it as float
>      barFloat(foo); // ok now!
> } else if (foo is string) {
>      writefln("Sure could go for some %s PIE...", foo.toUpper);
> }
> writefln("handy foo debugger: %s", foo); // template/trait magic would 
> make this ok though and format to library standards. static if 
> (is(typeof(arg) == sumtype)) ... etc
> ```

It could be made to work yes.

However for matching, I'm waiting on Walter before having opinions about it.