case null and type pattern

[Remi Forax]

> 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 [ mailto:manoj.palat at in.ibm.com |
>>> <manoj.palat at in.ibm.com> ] wrote:

>>> Hi Gavin, All,

>>> In [
>>> http://cr.openjdk.java.net/~gbierman/jep406/jep406-20210608/specs/patterns-switch-jls.html#jls-14.30.1
>>> |
>>> http://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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