Draft JLS spec for JEP 305: Pattern matching for instanceof

Tagir Valeev amaembo at gmail.com
Fri Sep 20 02:31:54 UTC 2019

Hello, Gavin!

In general, the spec draft looks good. I have a question about
annotations. Currently section @Target says:

9. Local variable declarations (including loop variables of for
statements and resource variables of try-with-resources statements)
(§14.4, §14.14.1, §14.14.2, §14.20.3)
Corresponds to java.lang.annotation.ElementType.LOCAL_VARIABLE

Should not it be extended to include pattern variables as well?

With best regards,
Tagir Valeev.

On Thu, Sep 19, 2019 at 4:30 PM Gavin Bierman <gavin.bierman at oracle.com> wrote:
> A draft language spec for JEP 305 (Pattern Matching for instanceof) is available at:
> http://cr.openjdk.java.net/~gbierman/jep305/jep305-20190918/specs/patterns-instanceof-jls.html <http://cr.openjdk.java.net/~gbierman/jep305/jep305-20190918/specs/patterns-instanceof-jls.html>
> Comments are welcomed on all aspects, but I draw your attention to a couple of things that we’d like your feedback on:
> 1. The instanceof operator restricts the type to be a reifiable reference type. The spec currently keeps that restriction for type test patterns too. But should we go further, i.e. will people expect to be able to say the following (given that this *declares* a pattern variable l)?
> if (o instanceof List<Integer> l) {
>> }
> 2. We’d like to keep the possibility open for merging of multiple pattern declarations, where it makes sense. For example:
> if (a instanceof Foo f || b instanceof Foo f) {
> … // Like to be able to use f here
> }
> The current spec explicitly calls out cases like these as compile-time errors, to allow for forwards compatibility if we add this feature. But what do you think of this feature? (We have textually multiple declarations of a pattern variable, but they are “merged”, so they are really the same thing…)
> 3. [Only for spec nerds] I am proposing to add a new Chapter 16 to discuss patterns (at the moment it’s short, but we’re planning for it to grow). The existing Chapters 16-19 will be renumbered to 17-20. Will this renumbering cause problems for anyone?
> Thanks,
> Gavin

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