[18][guarded pattern] conditional-and query - spec clarification

[Manoj Palat]

Hi Gavin,
Thanks for the reply – yes, agree with you in that precedence doesn’t figure out here – my bad I went into a wrong path in the grammar.

That said,  with further analysis, the code in question:
            case String a && o != null ? true : false -> 1;//ecj flags syntax error here
is still an error because there seems to be no way for the reduction into conditionalAndExpression as mentioned below.

Ie:
case String a && o != null ? true : false -> // Reject

Add a parenthesis and then we get a path – side note/
case String a && (o != null ? true : false) -> // Accept

because,
without parenthesis, ie the code:
      case String a && o != null ? true : false ->

follows the path of:

  AssignmentExpression -> ConditionalExpression
  Expression ::= AssignmentExpression
  ConstantExpression -> Expression

And this cannot be reduced to a ConditionalAndExpression and consequently not further to a GuardedPattern

while the one with parenthesis, can be reduced to GuardedPattern eventually

case String a && (o != null ? true : false) ->
  AssignmentExpression -> ConditionalExpression
  Expression ::= AssignmentExpression
  PrimaryNoNewArray ::= ‘(‘ Expression_NotName ‘)’
ie and so on..
Primary -> PrimaryNoNewArray
PostfixExpression -> Primary
…
InclusiveOrExpression -> ExclusiveOrExpression
ConditionalAndExpression -> InclusiveOrExpression
…
With the combination of “String a &&” eventually leading to
GuardedPattern ::= PrimaryPattern AND_AND ConditionalAndExpression


Regards,
Manoj


From: Gavin Bierman <gavin.bierman at oracle.com>
Date: Friday, 11 March 2022 at 1:51 AM
To: Manoj Palat <manoj.palat at in.ibm.com>
Cc: "amber-spec-experts at openjdk.java.net" <amber-spec-experts at openjdk.java.net>
Subject: [EXTERNAL] Re: [18][guarded pattern] conditional-and query - spec clarification

Hi Manoj, It’s a slightly moot point, given that we are likely to drop guarded patterns in the next preview but I think there has been some confusion here... On 7 Mar 2022, at 07:08, Manoj Palat <manoj.palat at in.ibm.com> wrote: ‍ ‍ ‍ ‍ ‍ ‍ ‍ ZjQcmQRYFpfptBannerStart
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Hi Manoj,

It’s a slightly moot point, given that we are likely to drop guarded patterns in the next preview but I think there has been some confusion here...


On 7 Mar 2022, at 07:08, Manoj Palat <manoj.palat at in.ibm.com<mailto:manoj.palat at in.ibm.com>> wrote:

Hi,

Given,
      public void bar(Object o) {
      int i = switch(o) {
            case String a && o != null ? true : false -> 1;//ecj flags syntax error here
            default -> 1;
      };
    }

ECJ(eclipse compiler for Java) flags a syntax error on the guarded pattern. However, javac accepts.

Ecj translates this into:
case ((String a) && (o != null)) ? true : false
and flags an error instead of
case ((String a) && ((o != null) ? true : false))


The idea of guarded patterns is that we give a secondary role to `&&` to serve as an operator for patterns.

After the `case` we parse a pattern. One of the form of a pattern is a guarded pattern which is:

GuardedPattern:
    PrimaryPattern && ConditionalAndExpression

Given the grammar as per http://cr.openjdk.java.net/~gbierman/jep420/jep420-20211208/specs/patterns-switch-jls.html<http://cr.openjdk.java.net/~gbierman/jep420/jep420-20211208/specs/patterns-switch-jls.html> I think javac is parsing this correctly.

I don’t know quite what ecj is doing here because the translation you give above seems to suggest that it was accepting an expression after the `case` which is not correct. Moreover, the inner expression (String a) && (o != null) is not an expression but a (guarded) pattern.



And I think the ecj is correct in flagging the error due to:

From https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html<https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html> we see that  Conditional-And Operator “&&”  has higher operator precedence than the Conditional Operator “?:” . From https://docs.oracle.com/javase/specs/jls/se17/html/jls-15.html#jls-15.23<https://docs.oracle.com/javase/specs/jls/se17/html/jls-15.html#jls-15.23>, we see that “The conditional-and operator is syntactically left-associative (it groups left-to-right).”


Also, I don't see any mention of the precedence changes in spec 420 [latest at https://cr.openjdk.java.net/~gbierman/jep420/latest<https://cr.openjdk.java.net/~gbierman/jep420/latest>]


I don’t see the connection with precedence - we certainly didn’t make any changes.

Am I understanding your issue correctly?

Thanks,
Gavin


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