deprecation warning when compiling: eta expansion of zero argument method - scala

Question

When compiling this snippet, the scala compiler issues the following warning:
Eta-expansion of zero-argument method values is deprecated. Did you
intend to write Main.this.porFiles5()? [warn] timerFunc(porFiles5)
It occurs when I pass a function to another one for a simple timing. The timer function takes a parameterless function returning unit, at this line: timerFunc(porFiles5). Is this warning necessary? What would be the idiomatic way to avoid it?
package example
import java.nio.file._
import scala.collection.JavaConverters._
import java.time._
import scala.collection.immutable._
object Main extends App {
val dir = FileSystems.getDefault.getPath("C:\\tmp\\testExtract")
def timerFunc (func:()=>Unit ) = {
val start = System.currentTimeMillis()
timeNow()
func()
val finish = System.currentTimeMillis()
timeNow()
println((finish - start) / 1000.0 + " secs.")
println("==================")
}
def porFiles5(): Unit = {
val porFiles5 = Files.walk(dir).count()
println(s"You have $porFiles5 por5 files.")
}
def timeNow(): Unit = {
println(LocalTime.now)
}
timeNow()
timerFunc(porFiles5)
timeNow()
}

Answer 1

porFiles5 is not a function. It is a method, which is something completely different in Scala.
If you have a method, but you need a function, you can use η-expansion to lift the method into a function, like this:
someList.foreach(println _)
Scala will, in some cases, also perform η-expansion automatically, if it is absolutely clear from context what you mean, e.g.:
someList.foreach(println)
However, there is an ambiguity for parameterless methods, because Scala allows you to call parameterless methods without an argument list, i.e. a method defined with an empty parameter list can be called without any argument list at all:
def foo() = ???
foo // normally, you would have to say foo()
Now, in your case, there is an ambiguity: do you mean to call porFiles5 or do you mean to η-expand it? At the moment, Scala arbitrarily disambiguates this situation and performs η-expansion, but in future versions, this will be an error, and you will have to explicitly perform η-expansion.
So, to get rid of the warning, simply use explicit η-expansion instead of implicit η-expansion:
timerFunc(porFiles5 _)