List files purely

ListFiles.scala

import java.nio.file.DirectoryStream
import java.nio.file.Files
import java.nio.file.Path
import java.nio.file.Paths

import scalaz.concurrent.Task
import scalaz.std.anyVal._
import scalaz.stream.Cause
import scalaz.stream.Process
import scalaz.stream.io

object Main extends App {
  val proc = files("D:/temp").scan1Map(_ => 1).chunk(10).map("Files found: " + _.last.toString).to(io.stdOutLines)
  
  def files(dir: String): Process[Task, Path] = files(Files.newDirectoryStream(Paths.get(dir)))
  
  def files(dir: => DirectoryStream[Path]): Process[Task, Path] = io.resource(Task.delay((dir, dir.iterator)))
    { case (dir, _) => Task.delay(dir.close) }
    { case (_, it) => Task.delay(if (it.hasNext) it.next else throw Cause.Terminated(Cause.End)) }
  
  proc.run.run // <- Side effects are happening here
  
  // The output is:
  // Files found: 10
  // Files found: 20
  // Files found: 30
  // ...
}