[pattern-switch] Opting into totality

Remi Forax forax at univ-mlv.fr
Thu Sep 3 17:04:32 UTC 2020

I just want to say that the is yet another option, 
say that (statement and expression) arrow switches are always total. 

We have introduced the arrow notation to avoid fallthrough but we have forgotten one important case of fallthrough, in a statement switch when you skip the entire switch, you fallthrough the entire switch. 

So we keep supporting the traditional partial switch with no modification but requires if a user wants a partial arrow switch, to add a "default -> {}". 

This is an incompatible change with the codes written since Java 14 so it's a limited incompatible change. 
Perhaps the main blocker is admitting that we were wrong. 


> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Lundi 31 Août 2020 15:25:13
> Objet: Re: [pattern-switch] Opting into totality

> I think this is the main open question at this point.

> We now have a deeper understanding of what this means, and the shape of the
> remainder. Totality means not only “spot check me that I’m right”, but also “I
> know there might be some remainder, please deal with it.” So totality is not
> merely about type checking, but about affirmative handling of the remainder.

> Expression switches automatically get this treatment, and opting _out_ of that
> makes no sense for expression switches (expressions must be total), but
> statement switches make sense both ways (just like unbalanced and balanced
> if-else.) Unfortunately the default has to be partial, so the main question is,
> how do we indicate the desire for totality in a way that is properly evocative
> for the user?

> We’ve talked about modifying switch (sealed switch), a hyphenated keyword
> (total-switch), a trailing modifier (switch case), and synthetic cases
> (“default: unreachable”). Of course at this point it’s “just syntax”, but I
> think our goal should be picking something that makes it obvious to users that
> what’s going on is not merely an assertion of totality, but also a desire to
> handle the remainder.

>> - How does a switch opt into totality, other than by being an expression switch?
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