Nullable switch

[Brian Goetz]

This is exactly why I asked you to provide a *complete* proposal.  

Sent from my iPad

> On Aug 8, 2020, at 5:04 PM, forax at univ-mlv.fr wrote:
> 
> 
> 
> 
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "Guy Steele" <guy.steele at oracle.com>
> Cc: "Remi Forax" <forax at univ-mlv.fr>, "amber-spec-experts" <amber-spec-experts at openjdk.java.net>, "John Rose" <john.r.rose at oracle.com>
> Envoyé: Vendredi 7 Août 2020 16:48:13
> Objet: Re: Nullable switch
> 
> Okay, so it would seem that we need two keywords (or other syntax) for use in patterns; I will temporarily call them “anything-but-null” and “anything-including-null”.
> 
> Not necessarily; the approach we've been driving towards has no (new) keywords, and no _explicit_ consideration of nullability.  There's just type patterns, but their semantics take into account whether or not the type pattern "covers" the target type.  This is subtle, I grant, and I can see where people would get confused, but it is far more compositional and less ad-hoc.  
> 
> Ignoring the epicyclical* distastefulness of the "any x" idea, I think the the syntax issues are a bit of a red herring -- the issue is structural.  Under Remi's proposal, there is simply _no_ way to write a switch where any number of cases covers "anything including null", because the switch will throw before you get there:
> 
>     switch (x) {
>         case String s: 
>         case Object o: 
>     }
> 
> would throw on NPE (as switches do today) before any cases are considered, whether you say "var" or "any" or "Object." 
> 
> That is not true.
> You're right that the switch above will generate a NPE as the switches do today because under the rules i propose, there is no case that accept null.
> 
> But if you add an any case (or a null case), then the switch will accept null,
> by example, the switch below accept null.
>   switch (x) {
>         case String s: 
>         case any o:
>   }
> 
> As Guy said, i'm proposing to have two different cases, one “anything-but-null” and one “anything-including-null” instead of relying on the non-local property of totality.
> 
> You can re-read my email from the 6th of August for the rules allowing a switch to accept null and more examples.
> 
> regards,
> Rémi
>