Fwd: Draft JEP: Unnamed local variables and patterns

[Brian Goetz]

Received on the -comments list, on the subject of unnamed locals in TWR.


-------- Forwarded Message --------
Subject: 	Re: Draft JEP: Unnamed local variables and patterns
Date: 	Tue, 18 Oct 2022 06:43:18 +0000
From: 	Till Brychcy <till.brychcy at unite.eu>
To: 	amber-spec-comments at openjdk.org <amber-spec-comments at openjdk.org>



 > 7. a resource specification of a try-with-resources statement
 >
 > Today we get this warning with a TWR-statement if the local is ignored:
 >
 > >
 > javac -Xlint:all MyScopedLock.java
 >
 > MyScopedLock.java:23: warning: [try] auto-closeable resource ignored 
is never referenced in body of corresponding try statement
 > try(MyScopedLock ignored = l.lock()) {
 > ^
 > 1 warning
 >
 > So here we may have a clash of philosophies for the enhanced-for and 
how people
 > use the `AutoCloseable`.


We have a similar use case: a logging framework that uses the close() 
call to measure the runtime of some code block.

It looks like this:

try (__ __ = ThreadLogger.block(SomeClass.class, "someAction")) {
// some code
}

Note we named the helper class and variable already "__" to make clear 
that they are not used - before Java 8 we used to called them "_“ :-)

Big advantages in comparison to e.g. a lambda based solution are that 
this doesn’t interfere with exceptions and variables outside the try 
statement can be assigned.

There are also more cases reported by other people in 
https://bugs.eclipse.org/bugs/show_bug.cgi?id=560733 , which why an 
already implemented warning for such variables by Eclipse was removed again.

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