Hacker News Reader

How does structural subtyping help you implement automatic resource management?

I can define a resource management method which accepts any object which belongs to a class that provides a close method irrespective of whether that class implements close as part of an an interface or not.

Rough example below, partially taken from a book I have on Scala.

//define it like this: def using[T <: {def close(): Unit }, S](obj: T) (operation: T => S) = { val result = operation(obj) obj.close() result }

//use it like this: val writer = new FunkyWriter using(writer) { writer.write("outputting some data") }

ARM was therefore possible in Scala before the Closable interface was even added to the Java libs.

(I had another comment here but I figured out the answer.)

lombok's @Cleanup will close anything, interface or not.

I took a look at lombok and it's pretty obvious from the way cleanup method names can be passed as strings that's it's using reflection. It's a bit of a nasty hack, but it works.

It fiddles with the AST during compilation. No reflection.

Sounds like it's trying to add structural subtyping via the back door.

Reply on news.ycombinator.com