RFR: 8262889: Compiler implementation for Record Patterns [v2]
Maurizio Cimadamore
mcimadamore at openjdk.java.net
Fri May 6 14:46:58 UTC 2022
On Fri, 6 May 2022 14:09:24 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:
>> 8262889: Compiler implementation for Record Patterns
>>
>> A first version of a patch that introduces record patterns into javac as a preview feature. For the specification, please see:
>> http://cr.openjdk.java.net/~gbierman/jep427+405/jep427+405-20220426/specs/patterns-switch-record-patterns-jls.html
>>
>> There are two notable tricky parts:
>> -in the parser, it was necessary to improve the `analyzePattern` method to handle nested/record patterns, while still keeping error recovery reasonable
>> -in the `TransPatterns`, the desugaring of the record patterns is very straightforward - effectivelly the record patterns are desugared into guards/conditions. This will likely be improved in some future version/preview
>>
>> `MatchException` has been extended to cover additional cases related to record patterns.
>
> Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:
>
> Reflecting review feedback.
Looks good
src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java line 752:
> 750: Iterable<? extends JCCaseLabel> labels) {
> 751: Set<Symbol> coveredSymbols = new HashSet<>();
> 752: Map<Symbol, List<JCRecordPattern>> deconstructionPatternsBySymbol = new HashMap<>();
since you seem to have settled on "recordPattern" for implementation names - you can probably revisit some of these names to say "record" instead of "deconstruction".
src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java line 801:
> 799: //i.e. represent all possible combinations.
> 800: //This is done by categorizing the patterns based on the type covered by the given
> 801: //starting component.
Example needed here. For instance (I discussed this with @biboudis):
record Outer(R r) { };
sealed interface I { };
class A implements I { };
class B implements I { };
sealed interface R { };
record Foo(I i) implements R { }
record Bar(I i) implements R { }
switch (o) {
case Outer(Foo(A), Foo(A)):
case Outer(Foo(B), Foo(B)):
case Outer(Foo(A), Foo(B)):
case Outer(Foo(B), Foo(A)):
case Outer(Bar(A), Bar(A)):
case Outer(Bar(B), Bar(B)):
case Outer(Bar(A), Bar(B)):
case Outer(Bar(B), Bar(A)):
}
Which generates two sets:
case Outer(Foo(A), Foo(A)):
case Outer(Foo(B), Foo(B)):
case Outer(Foo(A), Foo(B)):
case Outer(Foo(B), Foo(A)):
And
case Outer(Bar(A), Bar(A)):
case Outer(Bar(B), Bar(B)):
case Outer(Bar(A), Bar(B)):
case Outer(Bar(B), Bar(A)):
Sorry for being pedantic - this code is tricky and I'm worried we'll all forget exactly how it works in 2 months :-)
src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java line 1014:
> 1012: pattern = parsePattern(patternPos, mods, type, false);
> 1013: } else if (token.kind == LPAREN) {
> 1014: pattern = parsePattern(patternPos, mods, type, false);
Nice!
-------------
Marked as reviewed by mcimadamore (Reviewer).
PR: https://git.openjdk.java.net/jdk/pull/8516
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