JEP325: Switch expressions spec

[Ali Ebrahimi]

Hi,

On Mon, Apr 23, 2018 at 10:50 PM, Guy Steele <guy.steele at oracle.com> wrote:

>
> > On Apr 23, 2018, at 2:02 PM, Kevin Bourrillion <kevinb at google.com>
> wrote:
> >
> > On Fri, Apr 20, 2018 at 11:55 AM, Guy Steele <guy.steele at oracle.com
> <mailto:guy.steele at oracle.com>> wrote:
> >
> > You know, if s2 is short (say, less than 30 or 40 characters), there are
> worse things than writing
> >
> >          case A -> s1;
> >          case null -> s2;
> >          default   -> s2;
> >
> > especially if you use spaces (as I just did) to line up the two
> occurrences of s2 to make it easy to see they are identical.
> >
> > And if s2 is long, there are worse things than making a little sub
> method to handle it:
> >
> >          case A -> s1;
> >          case null -> frobboz(a, b);
> >          default   -> frobboz(a, b);
> >
> >     int frobboz(int a, String b) { … }
> >
> > And if even THAT is not satisfactory, well, there are worse things than
> giving up on the arrows and just using colons (and break, if needed).
> >
> > I think neither of these goes down well. Having to repeat yourself at
> all, while normal cases get to use comma, will feel very wrong. Having to
> abandon arrowform over this would be even worse.
> >
> >
> > BUT, on the other hand, if we wanted to: instead of, or in addition to,
> >
> >         case pat1, pat2, pat3 -> s;
> >
> > we could allow the form
> >
> >         case pat1 -> case pat2 -> case pat3 -> s;
> >
> > This seems like a step backward to me (whether it is instead or in
> addition).
> >
> > fwiw, I think `default, case null ->` is superior to all of these
> options.
>
> As an ad-hoc patch that solves this one problem, I agree.
>
> But let me make a revised pitch for allowing it “in addition to” (which,
> now that I have pondered it over the weekend, I think is clearly the
> correct approach).
>
> (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;`.
>
I think you have missed one rule arrow arms don't support fall-through
(assume they have an explicit break).
So
*case a -> case b -> case c -> s;  *
Would be equivalent to
if in statement switch
*case a : break; case b : break; case c : S; break ; *
But in expression switch
*case a : break; case b : break; case c : break S ;*
Is an compile error, since some case labels breaks without value for
expression switch.

So we can say that
    *case a, b, c: s;*
Can be refactored to
    *case a: case b: case c: s;*
But

*case a, b, c-> s;*
Can't be refactored to
*:*

* case a -> case b -> case c -> s;  *
 Rational: as we saw above they are not actually equivalent.
-- 
Best Regards,
Ali Ebrahimi