A proposal: Sumtypes

[cc]

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
```