Allow case constants as nested patterns?

[Brian Goetz]

To add to this: there are several categories of questions that we wanted 
to settle before considering constant patterns (which would (almost?) 
complete the rehabilitation of switch):

  - Syntactic.  If `0` is both a literal and a pattern, then locutions 
like `f(0)` can either be method invocations or pattern matches.  It was 
not obvious in the early days whether such ambiguities would pose 
roadblocks for using patterns in other contexts.  This is seeming more 
viable now but we didn't want to foreclose this path too early.

  - User perception.  Related to the above, if it is not obvious what 
`f(0)` means, even if the grammar can be parsed unambiguously, it might 
make code harder to read.  So we might hold out for a syntax that is 
somewhat wordier but more obvious.

  - Semantics.  At the beginning, it was very much not clear (to the 
point where our first ideas would have been pretty wrong) what the 
semantics of matching a constant are.  JEP 455 has been working through 
those issues, so we believe we now have a firm foundation for the 
meaning of `0` as a pattern.

  - Alignment with switch.  The "obvious" meaning of constant patterns 
may or may not align with the treatment of constants in switch today; 
this needs to be worked through carefully, otherwise refactoring a 
switch like:

     case Foo(k)
     case Foo(k')
     case Foo(k'')

to a switch like

     case Foo(var x) {
         switch (x) {
             case k
             case k'
             case k''
          }
     }

will have subtle asymmetries.

So to sum this up, constant patterns are an "obvious" addition, but 
which turn out to be very much non-obvious when you consider all the 
connections with other language features and future language 
directions.  So we've been waiting for these to settle.

On 11/7/2023 4:37 AM, Gavin Bierman wrote:
> Hi Sebastian,
>
> Yes, there has been some discussion about adding constant patterns if you look through the archive. There are interesting syntactic issues, but IIRC we have been waiting for the primitive type patterns feature to settle so we have a proper basis to deal with any type conversion questions. (For example, you write switch(new Box(b)) { case Box(0) -> … } where b is a byte and 0 is an integer constant…)
>
> So, it’s on our radar :-)
>
> Gavin
>
>> On 2 Nov 2023, at 11:13, Sebastian Fischer<mail at sebfisch.de>  wrote:
>>
>> Hello.
>>
>> I would like to improve my understanding of the interaction of two JEPs: Pattern matching for switch and Record patterns.
>>
>> I am new to this list so might have missed previous discussion about this but could not find what I was looking for in the archives.
>>
>> JEP 441 shows the following grammar for switch labels.
>>
>> SwitchLabel:
>>    case CaseConstant { , CaseConstant }
>>    case null [, default]
>>    case Pattern [ Guard ]
>>    default
>>
>> JEP 440 shows the following grammar for patterns.
>>
>> Pattern:
>>    TypePattern
>>    RecordPattern
>>
>> TypePattern:
>>    LocalVariableDeclaration
>>
>> RecordPattern:
>>    ReferenceType ( [ PatternList ] )
>>
>> PatternList :
>>    Pattern { , Pattern }
>>
>> As a consequence it is not possible to have case constants in the pattern list of a record pattern. For example consider the following definitions modeling (a simplified version of) boolean expressions.
>>
>> sealed interface BoolExpr {
>>      enum Constant implements BoolExpr { TRUE, FALSE }
>>      record And(BoolExpr left, BoolExpr right) implements BoolExpr {}
>>
>>      default BoolExpr recursively(UnaryOperator<BoolExpr> transform) {
>>          return transform.apply(switch (this) {
>>              case Constant c -> c;
>>              case And(var left, var right) -> new And(
>>                  left.recursively(transform),
>>                  right.recursively(transform)
>>              );
>>          });
>>      }
>>
>>      default BoolExpr shortCircuit() {
>>          return recursively(expr -> switch (expr) {
>>              case And(var left, var unused)
>>                  when left == Constant.FALSE -> Constant.FALSE;
>>              default ->
>>                  expr;
>>          });
>>      }
>> }
>>
>> In the definition of `shortCircuit` (which does partial evaluation) we need to use a guard to check if the left argument of `And` is false.
>>
>> Instead of using an enum, we can model constants as empty records as follows.
>>
>>      sealed interface Constant extends BoolExpr permits True, False {}
>>      record True() implements Constant {}
>>      record False() implements Constant {}
>>
>> Now we can write `shortCircuit` using a nested record pattern.
>>
>>      default BoolExpr shortCircuit() {
>>          return recursively(expr -> switch (expr) {
>>              case And(False(), var unused) -> new False();
>>              default -> expr;
>>          });
>>      }
>>
>> It would be convenient to be able to use a nested pattern instead of a guard with the original definition using an enum for constants. Here is a hypothetical implementation of `shortCircuit` that is currently not supported.
>>
>>      default BoolExpr shortCircuit() {
>>          return recursively(expr -> switch (expr) {
>>              case And(Constant.FALSE, var unused) -> Constant.FALSE;
>>              default -> expr;
>>          });
>>      }
>>
>> What are potential problems with allowing case constants as nested patterns?
>>
>> Kind regards,
>> Sebastian
>>
>>
>>
>>
>>
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