Draft JEPs: Pattern Matching for switch and Record Patterns

Gavin Bierman gavin.bierman at oracle.com
Mon Oct 3 15:29:40 UTC 2022

Hi Tagir,

So it was a number of issues actually. First, there is a nasty ambiguity problem. Consider:

record R(){}
switch(e) {
    case R() when when (true) -> ...

The label could be parsed as either:

    case (R() when) when (true) ->


    case (R()) when (when(true)) ->

(where `when` is a static boolean method).

There is another issue which is this variable declaration is the only one in the language that can’t be annotated or marked as `final` which feels like a design smell. None of the obvious solutions to this looked good.

In most other languages with pattern matching they keep these two things - a destructing pattern and a naming pattern - separate. In both Haskell and Scala, they write `x at p` to “name” a pattern p as x. So that seems like a possibility. But for now, we noted that in most cases you can rewrite pretty simply, e.g.

    case Point(var x, var y) when p.isVisible() ->

can be rewritten:

    case Point p
        when p.isVisible() && p instanceof Point(var x, var y): …

or if it was in an instanceof:

    if (x instanceof Point p && p.isVisible() && p instanceof Point(var x, var y)) { … }

Neither of these versions read too badly.



On 3 Oct 2022, at 14:40, Tagir Valeev <amaembo at gmail.com<mailto:amaembo at gmail.com>> wrote:


Remove support for named record patterns.

This surprises me. Probably there was a discussion about the rationale
behind this change? Could you please point me? Thanks.

With best regards,
Tagir Valeev

On Mon, Oct 3, 2022 at 2:48 PM Gavin Bierman <gavin.bierman at oracle.com<mailto:gavin.bierman at oracle.com>> wrote:

Dear all,

The draft JEPs for the next preview of the Pattern Matching for switch and Record Patterns features are now available at:

Pattern matching for switch: https://bugs.openjdk.org/browse/JDK-8294285
Record Patterns: https://bugs.openjdk.org/browse/JDK-8294078

Comments welcomed!

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