Draft JEPs for Patterns in switch and Record Patterns

Remi Forax forax at univ-mlv.fr
Tue Feb 28 17:09:04 UTC 2023

----- Original Message -----
> From: "Gavin Bierman" <gavin.bierman at oracle.com>
> To: "amber-dev" <amber-dev at openjdk.org>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.org>
> Sent: Tuesday, February 28, 2023 5:21:42 PM
> Subject: Draft JEPs for Patterns in switch and Record Patterns

> Hello,
> We are planning to finalize the two pattern matching JEPs in JDK 21. Drafts of
> these final JEPs are available here:
> Pattern matching for switch: https://openjdk.org/jeps/8300542
> Record patterns:             https://openjdk.org/jeps/8300541
> We're proposing some small changes from the preview versions about to appear in
> JDK 20. These include:
> - We're dropping parenthesized patterns. They were leftover from a previous
> version of patterns, and they weren't used very much. They complicate the spec
> for not a lot of gain.

And reuse parenthesis as the syntax for tuple patterns (whatever it is exactly).

> - We're going to support case labels that are the qualified name of enum
> constants, and allow switches over non-enum types to have enum case labels
> provided they use the qualified names of the enum constants and these labels
> are assignment compatible with the switch type.

I wonder if we can not extends the unqualified enum value syntax to any sealed hierarchy that contains an enum with that constant.
By example,
  sealed interface HList permits Cons, Nil {}
  enum Nil implements HList { NIL }
  record Cons(int val, HList next) implements HList {}

HList list = ...
switch(list) {
  case NIL -> ...   // the compiler can replace it by Nil.NIL if there is only one enum value named NIL in the hierarchy

Perhaps it's a step too far ?

> - We're dropping the support for record patterns in the header of enhanced for
> statements. These will re-appear in a separate forthcoming JEP.

I hope that enhanced for and destructuring assignment will have the same semantics.

> Please take a look at these new JEPs and give us your feedback (either on this
> list or directly to me).
> Thanks,
> Gavin


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