Scala _ Placeholders (How does this code function?)

Question

I am learning Scala (coming from a background of mostly Java). I am trying to wrap my head around the following code:

object Main {
  def main(args : Array[String]) {
    for (file <- filesEnding(".txt"))
      println(file.getName)
  }

  private val filesHere = (new java.io.File(".")).listFiles

  def filesMatching(matcher: String => Boolean) =
    for (file <- filesHere; if matcher(file.getName))
        yield file

  def filesEnding(query: String) = filesMatching(_.endsWith(query))
  /* Other matcher functions */
}

In particular I am confused where Scala gets the value for _ in each of the matcher functions. I can see that filesEnding is called with an argument of .txt. That argument is assigned to query. filesEnding then calls filesMatching with an argument consistent with a String => Boolean function. Finally I can see that file.getName is what eventually replaces the _ placeholder.

What I don't get is how Scala knows to put file.getName in place of _. I am having trouble tracing this code in my head and the eclipse debugger isn't much help in this situation. Can somebody walk me through what is happening in this code?

Answer 1

The _ is just a shorthand for making an anonymous function:

_.endsWith(query)

is the same as the anonymous function

fileName => fileName.endsWith(query)

This function is then fed as the argument matcher to filesMatching. Inside that function you can see the call

matcher(file.getName)

This calls the anonymous function with file.getName as the _ argument (which I called fileName in the explicit example).

Answer 2

If you write _.someMethod(someArguments), this desugars to x => x.someMethod(someArguments), so filesMatching(_.endsWith(query)) desugars to filesMatching(x => x.endsWith(query)).

So filesMatching is called with matcher being the function x => x.endsWith(query), i.e. a function which takes one argument x and calls x.endsWith(query) on that argument.