[External] : Re: case null and type pattern

[Brian Goetz]

Sure, but again, I think users would find that restriction even more 
annoying; the vast majority of guards in languages with guarded patterns 
use the bindings to refine the match.  I'm not sure users would thank us 
for moving the restriction in this way.

A more interesting flavor of the same question, though, is what happens 
if we allow _ to be used as a binding name.  Then, the restriction on 
fall-through could be relaxed if we wanted; it would be safe to do:

     case Foo _, Bar _: blah

whereas today we don't allow mixing of these patterns.

On 7/19/2021 9:09 AM, Remi Forax wrote:
>
>
> ------------------------------------------------------------------------
>
>     *From: *"Brian Goetz" <brian.goetz at oracle.com>
>     *To: *"Gavin Bierman" <gavin.bierman at oracle.com>, "Manoj Palat"
>     <manoj.palat at in.ibm.com>
>     *Cc: *"amber-spec-experts" <amber-spec-experts at openjdk.java.net>
>     *Sent: *Lundi 19 Juillet 2021 15:00:16
>     *Subject: *Re: case null and type pattern
>
>     Right.  In theory, we could allow guarded patterns here, but they
>     will be hard to use, since users will likely want to use the
>     binding in the guard, and would have to check for nullity every time. 
>
>
> As you said, the error is to be able to use the binding not to be able 
> to express that pattern.
>
> Rémi
>
>
>
>     On 7/19/2021 6:16 AM, Gavin Bierman wrote:
>
>         Hi Manoj,
>
>         This is certainly something we can discuss for Preview 2, but…it is intentional for now. The `case null, T t` is really a special case of extending a type pattern to be null friendly. Remember that the pattern variable `t` is initialised with null if this case label applies.
>
>         The problem with what you are suggesting is that we’re going to end up with code like:
>
>         switch(o) {
>              …
>              case null, String s && s != null -> …
>         }
>
>         Now the developer has a guard saying that `s` is not null, and is probably going to assume this in the body of the switch rule, but has erroneously extended the case label with null.
>
>         So, it’s just an attempt to remove potential puzzlers by design. But we can take a look at whether we can capture a more generous definition. An alternative would be to define a new null-friendly type pattern. Let’s talk about it!
>
>         Thanks,
>         Gavin
>
>
>
>             On 14 Jul 2021, at 23:43, Manoj Palat<manoj.palat at in.ibm.com>  wrote:
>
>             Hi Gavin, All,
>
>             Inhttp://cr.openjdk.java.net/~gbierman/jep406/jep406-20210608/specs/patterns-switch-jls.html#jls-14.30.1, Section 14.11, I see "If a switch label has a null case label element then if the switch label also has any pattern case element labels, they must be type patterns (14.30.1)." implying that the following code:
>
>             private static void foo(Object o) {
>             switch (o) {
>             case null, Integer i && i > 10 -> System.out.println(0); // flag error?
>             default -> System.out.println(o);
>             }
>             should have an error flagged for case label with null since the pattern case label element is not a type pattern but a guarded pattern.
>             Any reason for excluding guarded patterns here? Or does this requires a modification in spec to include guarded patterns as well?
>
>             Regards,
>             Manoj
>
>
>

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