Draft JLS spec for JEP 305: Pattern matching for instanceof

Brian Goetz brian.goetz at oracle.com
Fri Sep 20 14:01:14 UTC 2019

The rules for scoping outlined here

    http://cr.openjdk.java.net/~briangoetz/amber/pattern-semantics.html <http://cr.openjdk.java.net/~briangoetz/amber/pattern-semantics.html>

certainly covered the propagation of binding variables through ternaries.  And those rules appear to be covered by in Gavin’s draft. 

We value being able to freely refactor between the ternary expression and if-statement forms; if you’ve got examples where that can’t be done, or where the semantics and scoping differ, please point those out!  

> On Sep 20, 2019, at 9:57 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> I don't remember if we have discuss this or not but if i read the spec correctly,
> there is no support for the operator ?:
> so a code like this is ok
>   if (o instanceof String s) {
>     return s.length();
>   } else {
>     return 0;
>   }
> while a code like this is not
>   return (o instanceof String s)? s.length(): 0;
> supporting the "if statement" without supporting the "if expression" seems arbitrary given the duality of both constructs.
> Rémi
> De: "Gavin Bierman" <gavin.bierman at oracle.com>
> À: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Jeudi 19 Septembre 2019 11:28:42
> Objet: Draft JLS spec for JEP 305: Pattern matching for instanceof 
> 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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