Unwrapping Variants

Okay now let's move on to variant unwrapping. In the last chapter you have learned how to check if a variant holds a given type and much more about comparing variants. But, in those if branches, when we know the variant holds the given type we actually want to get that value from the variant somehow. If you think back about optionals we had the ! operator for forceful unwrapping and the ? operator for optional chaining. For variants, this actually is quite similar. To unwrap a variant we use the !(T) syntax. In between the parenthesis we write which type we want to force-unwrap from the variant. Similar to how the force-unwrap operator ! worked for optionals, this will print a small error message and crash the program if the variant does not hold the requested type. Let's actually look at a modified version of one of the last chapter's examples:

use Core.print

variant MyVar:
	i32, f32, bool;

def main():
	MyVar var = 3.14;

	if var == i32:
		print($"holds i32 value: {var!(i32)}\n");
	else if var == f32:
		print($"holds f32 value: {var!(f32)}\n");
	else if var == bool:
		print($"holds bool value: {var!(bool)}\n");

This program will print this line to the console:

holds f32 value: 3.14

It actually is semantically quite similar to the force-unwrap operator for optionals, and this is intended. Whenever you see the ! symbol you should know that "Okay the programmer asserted that this will always have a value (of that type).". This forms confidence over time. The more you see the same symbols used in the same context and same semantics the more you familiarize with them, just like how the _ default expression works.

The above example is not really useful, though, as in this case the switch statement would be a much better fit. But the variant unwrapping operator is a very powerful tool for handling variants.

Also, we can of course provide a tag instead of the type T for variant unwrapping too, as always. So, if i32 had a tag Int we must write var!(MyVar.Int) as usual, but that should be clear by now.