Question about JEP 406

[forax at univ-mlv.fr]

> From: "Tesla Ice Zhang" <ice1000kotlin at foxmail.com>
> To: "Remi Forax" <forax at univ-mlv.fr>, "Tesla Ice Zhang"
> <ice1000kotlin at foxmail.com>
> Cc: "amber-dev" <amber-dev at openjdk.java.net>, "Jim Laskey"
> <james.laskey at oracle.com>
> Sent: Vendredi 20 Août 2021 18:26:35
> Subject: Re: Question about JEP 406

> Hi,

> Since Java 17 release is approaching, how is this "GADT" problem of pattern
> matching going? Is there anybody against the proposed behavior?

> Regards,
> Tesla

Hi, 
we have not forgotten that issue, i think it can be combined with the proposal by Dan's Smith on the way to handle multiple branches [1], 

As Dan said, we first need to collect all possible subtypes (the final types + non sealed types) from the permitted subtypes recursively, and then checks if the patterns cover all cases. 
During the first phase, the collection phase, we should scrub the subtypes that are not subtype of the type switched upon, this will solve the issue your have raised. 

regards, 
Rémi 

[1] https://mail.openjdk.java.net/pipermail/amber-spec-experts/2021-July/003049.html 

> ---Original---
> From: "forax"<forax at univ-mlv.fr>
> Date: Wed, Jun 16, 2021 06:09 AM
> To: "Tesla Ice Zhang"<ice1000kotlin at foxmail.com>;
> Cc: "amber-dev"<amber-dev at openjdk.java.net>;"Jim
> Laskey"<james.laskey at oracle.com>;
> Subject: Re: Question about JEP 406

>> De: "Tesla Ice Zhang" <ice1000kotlin at foxmail.com>
>> À: "Remi Forax" <forax at univ-mlv.fr>, "Tesla Ice Zhang"
>> <ice1000kotlin at foxmail.com>
>> Cc: "amber-dev" <amber-dev at openjdk.java.net>, "Jim Laskey"
>> <james.laskey at oracle.com>
>> Envoyé: Mercredi 16 Juin 2021 03:40:38
>> Objet: Re: Question about JEP 406

>> Hi Rémi,

>> > Yes, we have missed that case.
>> Are you (or other people developing openjdk) going to cover this case? Is it
>> possible to see it in the preview versions of openjdk?

> We have passed the point the spec of Java 17 can be modified some weeks ago, so
> it will be available (if everybody agree) as a preview feature of Java 18.
> This is why a preview feature becomes a real features only after several
> releases, it give us time to gather feedback and act accordingly.

> regards,
> Rémi

>> Regards,
>> Tesla
>> ---Original---
>> From: "Remi Forax"<forax at univ-mlv.fr>
>> Date: Wed, Jun 16, 2021 04:00 AM
>> To: "Tesla Ice Zhang"<ice1000kotlin at foxmail.com>;
>> Cc: "amber-dev"<amber-dev at openjdk.java.net>;"Jim
>> Laskey"<james.laskey at oracle.com>;
>> Subject: Re: Question about JEP 406

>> It can be reduced to

>> sealed interface Exp<T> { }
>> record Add() implements Exp<Integer> {}
>> record Lit<T>(T obj) implements Exp<T> {}

>> Yes, we have missed that case.

>> When a subtype of a sealed type is not a subtype of the type switch upon, it
>> should be discarded when testing exhaustiveness.

>> Thanks,
>> Rémi

>> ----- Mail original -----
>> > De: "Jim Laskey" <james.laskey at oracle.com>
>> > À: "amber-dev" <amber-dev at openjdk.java.net>
>> > Cc: "Tesla Ice Zhang" <ice1000kotlin at foxmail.com>
>> > Envoyé: Mardi 15 Juin 2021 18:59:39
>> > Objet: Re: Question about JEP 406

>> > [HTML messed up in original code]

>> > sealed interface Exp<T> {
>> > record Add(Exp<Integer> lhs, Exp<Integer> rhs) implements Exp<Integer> {}
>> > record Lit<T>(T obj) implements Exp<T> {}
>> > }


>> >> On May 21, 2021, at 2:06 AM, Tesla Ice Zhang <ice1000kotlin at foxmail.com> wrote:

>> >> Hi OpenJDK developers,

>> >> I'm very excited about JEP 406. Sealed classes are taken into account in switch
>> >> expressions, which is great! However, there is one special case not mentioned
>> >> in the JEP, about generic sealed interfaces:


>> >> sealed interface Exp<T> {
>> >>  record Add(Exp<Integer> lhs, Exp<Integer> rhs) implements
>> >> Exp<Integer> {}
>> >>  record Lit<T>(T obj) implements Exp<T> {}
>> >> }



>> >> In the above example, if we do a switch on an expression of type
>> >> Exp<Boolean>, do we need to provide a case for Add?


>> >> Best regards,
>> > > Tesla