JEP325: Switch expressions spec

[Guy Steele]

Sent from my iPhone

> On Apr 23, 2018, at 4:22 PM, Kevin Bourrillion <kevinb at google.com> wrote:
> 
> On Mon, Apr 23, 2018 at 12:00 PM, Guy Steele <guy.steele at oracle.com> wrote:
>>> On Apr 23, 2018, at 2:58 PM, Kevin Bourrillion <kevinb at google.com> wrote:
>>> 
>>>> On Mon, Apr 23, 2018 at 11:20 AM, Guy Steele <guy.steele at oracle.com> wrote:
>>>> 
>>>> (1) We have moved toward allowing “arrow versus colon” to be a syntax choice that is COMPLETELY orthogonal to other choices about the use of `switch`.  If this rule is to hold universally, then any switch statement or expression should be convertible between the arrow form and the colon form using a simple, uniform rule.
>>>> 
>>>> (2) In switch expressions we want to be able to use the concise notation `case a, b, c -> s;` for a switch clause.
>>>> 
>>>> (3) From (1) and (2) we inexorably conclude that `case a, b, c: break s;` must also be a valid syntax.
>>>> 
>>>> (4) But we could also have written (3) as `case a: case b: case c: break s;` and we certainly expect them to have equivalent behavior.
>>>> 
>>>> (5) From (1) and (4) we conclude that we ought also be to be able to write `case a -> case b -> case c -> s;`.
>>> 
>>> Not necessarily, if one simply views (4) as being an artifact of colonform switch's capacity for fall-through, which we know should not carry over. (Although we technically don't use the term "fall-through" in this no-intervening-code case, it works the same way and many people do think of it that way.)
>> 
>> You could view it that way—but such a view is incorrect, going back to JLS1.
> 
> To be more clear, I wasn't trying to make a statement about what is correct or incorrect by the spec. (On such matters I will always be deferring to the rest of you!)
> 
> My claim is just that it is not hard for a user to make sense of why `case A: case B: x` would work in colonform yet `case A -> case B -> x` might not work in arrowform. These don't necessarily feel contradictory. A user may simply understand that since colonform's design is made to support fall-through, that became an obvious way that it could address multiple labels as well, whereas the same does not apply in arrowform.

Fair enough. 

> Okay, so it's a "folk model". I think that neither makes it automatically good nor automatically bad.
> 
> To the user, I believe that the ability to choose `case A -> case B ->` is an unnecessary choice and feels like the same kind of baggage that I'd hoped to leave behind when moving to arrowform.

Yep. Agreed that you would almost never want to use it. But it would give me a warm, fuzzy feeling to know it’s lurking there for the few times you really need it
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