Switch labels (null again), some tweaking

[Brian Goetz]

On 4/28/2021 5:34 PM, Maurizio Cimadamore wrote:
>
> Thanks for this.
>
> It is especially helpful to see all the rules in a single place.
>
> So, armed with these rules - back to the first example I brought up:
>
> switch (lunch) {
>         case Box(Soup s) {
>              if (s == null) {
>                   System.err.println("Box of null");
>              } else {
>                   System.err.println("Box of soup");
>              }
>         }
>
>         case Bag(Soup s): {
>              if (s == null) {
>                   System.err.println("Bag of null");
>              } else {
>                   System.err.println("Bag of soup");
>              }
>
>         }
> }
>
> If I read the rules correctly, Box(Soup) + Bag(Soup) "cover" 
> Container<Lunch>, with the exception of the { null, Box(null), 
> Bag(null) }. So the above will throw when `lunch` is null, and will 
> also throw with Box(null) or Bag(null). Correct?
>

Correct (under the "we make switches total" plan.)

> So the right way to write that would be to add a couple of { case 
> Box(null), case Bag(null) } - these will reduce the remainder of the 
> switch blanket to just { null } - which means the switch will just 
> throw on a null input value, as usual.
>

Correct.  Any of the following would work to capture the remainder 
explicitly (not all of these are valid syntax, though):

     case null, Box(null), Bag(null):   // explicit remainder

     case null, Box, Bag: // basically the same

     case null, default:   // default misses null, so we add it in

     case null, Container(null): // another way to say the same thing

     case Container c:   // total pattern, catches everything