Draft JLS Spec about unnamed patterns and variables

Angelos Bimpoudis angelos.bimpoudis at oracle.com
Fri Feb 24 16:23:01 UTC 2023

The main takeaways:

  *   case Number _ can fall out to other patterns since it doesn't introduce any bindings (adjustment in the spec draft is needed)
  *   case Number _ dominates case String _, Integer _ (adjustment is not needed in current draft, but I will double check before I circulate the revised version).

Thanks for all the comments!
Thought experiment: what if we had union type patterns?  Then the case label `case String _, Integer _` would be like matching the the union type pattern `(String|Integer) _`:

    case Number n: ...
    case (String|Integer) _: ...

Would javac then complain that `String|Integer` could be simplified to just `String` on the bsais of flow analysis?  (IntelliJ would, of course.)

I initially thought as Tagir did, but then Gavin turned me around and reminded me that it was not dead code, but unreachable statements that we try to avoid.  So now I am torn...
Would union type patterns imply the existence of union types? If yes, then, the second case could even exist with a binding, correct? In your example the LUB is Object so even the case (String|Integer) x : x.getClass() can work. The difficult scenario would arise with the case (Customer|Human) x : x.getName();

If the first case in your example did not introduce a binding, would both case be equal with Number | (String | Integer)? Union types a la Ceylon support this (http://web.mit.edu/ceylon_v1.3.3/ceylon-1.3.3/doc/en/spec/html_single/#uniontypes). On the other hand in Ceylon, the switch needs to be exhaustive and​ all cases need to be disjoint. So this switch would be invalid. hm..

From: Maurizio Cimadamore <maurizio.cimadamore at oracle.com>
Sent: 23 February 2023 20:27
To: Brian Goetz <brian.goetz at oracle.com>; Angelos Bimpoudis <angelos.bimpoudis at oracle.com>; amber-spec-experts <amber-spec-experts at openjdk.java.net>
Subject: Re: Draft JLS Spec about unnamed patterns and variables

On 23/02/2023 18:46, Brian Goetz wrote:
but we really wanted the case merging.


I just wanted to point out that there are two questions here (one about fall-through and one about domination), and when reading the emails it was not obvious to me that a change in how fall-through was defined was being proposed.

If merging unrelated type tests is a goal, I think there should be an example for it in the JEP under "Motivation".

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