Unreachable

kristoff · March 22, 2024, 1:10pm

It definitely isn’t wrong at all, as long as it marks an impossible code path.

kristoff-it/ziggy/blob/main/src/ziggy/Parser.zig#L301-L308


      
          pub fn parseBool(self: *Parser, true_or_false: Token) !bool {
              try self.mustAny(true_or_false, &.{ .true, .false });
              return switch (true_or_false.tag) {
                  .true => true,
                  .false => false,
                  else => unreachable,
              };
          }

In this example the Token union has a lot of cases in it, but because I’m using mustAny, which returns an error if the given token is anything other than .true or .false, then I know that the subsequent switch will only have to handle those two cases and any other is impossible.

Using unreachable as a placeholder is something that I’ve done myself in the past but it’s a bit of a dangerous thing to do, because leaving a wrong unreachable in the code is much worse than leaving a wrong @panic as you might not get any immediately visible misbehavior when you hit it in release mode.