Knocking off two more vestiges of legacy switch

[Dan Heidinga]

On Tue, Sep 13, 2022 at 10:08 AM <forax at univ-mlv.fr> wrote:

>
>
> ------------------------------
>
> *From: *"John Rose" <john.r.rose at oracle.com>
> *To: *"Remi Forax" <forax at univ-mlv.fr>
> *Cc: *"Brian Goetz" <brian.goetz at oracle.com>, "amber-spec-experts" <
> amber-spec-experts at openjdk.java.net>
> *Sent: *Tuesday, September 13, 2022 12:58:47 AM
> *Subject: *Re: Knocking off two more vestiges of legacy switch
>
> It’s too harsh to say your example shows the semantics are just wrong.
>
>
> yes, it's more than there is inconsistencies
>
> I think they are right, but possibly incomplete. The exclusion of case 200
> is the job of dead code detection logic in the language, the same kind of
> logic that also reports an error on "foo" instanceof List.
>
> Then there are the old murky rules that allow an integral constant like
> 100 to assign to byte only because 100 fits in the byte range while 200
> does not. The duals of those rules will surely speak to the restriction of case
> 200: matching a byte.
>
>
> The problem with that approach is that the semantics of constant patterns
> and the semantics of primitive type patterns will be not aligned,
> so if you have both pattern in a switch, users will spot the inconsistency.
>
> something like
>   byte b = ...
>   switch(b) {
>     case 200 ->  ... // does not compile, incompatible types between byte
> and int
>     case int i -> ... // ok, compiles
>   }
>

I've been following along on this discussion and I'm not sure what the
inconsistency here is.  Remi, can you clarify?

As a developer, the semantics here are intuitive - I can't have a (signed)
byte that matches 200 so as John said earlier, it's clearly dead code.  On
the other hand, bytes can always be converted to an int so it makes sense
that the `case int i` both compiles and matches to the byte.  Can you
expand on why users would find that confusing?

--Dan


>
> So i agree that we should have primitive type patterns but instead of
> using the casting rules as model, the actual rules complemented with
> boolean, long, float and double seems a better fit.
>
> Compared to what Brian proposed, it means all primitive patterns are
> unconditional apart unboxing if the pattern is not total (the same way
> reference type pattern works with null).
>
> Rémi
>
> On 12 Sep 2022, at 15:29, Remi Forax wrote:
>
> No new rules; just appeal to type patterns.
>
> It shows that the semantics you propose for the primitive type pattern is
> not the right one.
>
> Currently, a code like this does not compile
> byte b = ...
> switch(b) {
> case 200 -> ....
> }
>
> because 200 is not a short which is great because otherwise at runtime it
> will never be reached.
>
> But if we apply the rules above + your definition of the primitive
> pattern, the code above will happily compile because it is equivalent to
>
> byte b = ...
> switch(b) {
> case short s when s == 200 -> ....
> }
>
>
>
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