[pattern-switch] Opting into totality

Brian Goetz brian.goetz at oracle.com
Thu Sep 3 18:16:10 UTC 2020

That came up in the expression switch exploration.  The thinking then, 
which I think is still valid, that it is easier to understand the 
difference when default-totality is attached to the expression versions, 
because expressions _must_ be total and statements totally make sense to 
be partial.

I think this is still coming from a place of retrospective snitch-envy; 
you want to carve out a corner that has the "right" semantics, even if 
its relation to the other corners is totally ad-hoc and random.  The 
upgrade to switch was driven by orthogonality; totality derives from 
whether the context of the switch (statement vs expression) requires 
totality or embraces partiality.  And the kinds of labels are strictly 
about the treatment of what is on the RHS -- either a single { 
expression/statement } vs complex control flow.  Which is orthogonal to 

So, I think we got it right then; we just have some holes to patch.

On 9/3/2020 1:04 PM, Remi Forax wrote:
> 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.
> Rémi
> ------------------------------------------------------------------------
>     *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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