https://dlang.org/library/std/sumtype.html

seems right now the match!(...) template only generate a delegate, e.g. suppose the following (silly) code:

bool isF(Temperature t) {
  while (true) {
    t.match!(
      (Fahrenheit f) {return true;},
      (_) {return false;}  // I want to return from the func isF, not the delegate!
    );
  }
}

enforce( isF(t1));

currently, this code will do infinite loop.

I'm wondering if the following are possible:

bool isF(Temperature t) {
  while (true) {
    switch (t) {
      case Fahrenheit f: /* do multiple statements of `f` */ return true;
      default: return false;
    }
  }
}

Or how can I get the tag and storage myself?

https://github.com/dlang/phobos/blob/a3f22129dd2a134338ca02b79ff0de242d7f016e/std/sumtype.d#L310

Thanks.

On Saturday, 7 October 2023 at 19:25:51 UTC, mw wrote:

If I add this line to the above func isF:

  writeln(t.tag);

it won't compile:

sum_type.d(79): Error: no property tag for t of type std.sumtype.SumType!(Fahrenheit, Celsius, Kelvin)

On Saturday, 7 October 2023 at 19:30:23 UTC, mw wrote:

  writeln(t.tag);

Shouldn't the compiler error message be: t.tag is private? rather than "no property tag for t"?

The tag is stored there, why the programmer cannot inspect it, and get the payload?