Tuesday, March 12, 2013

Scala: Automatic resourse management

After completing wonderful course by Martin Odesky, I have eventually had a chance to have a little play with Scala and create something more useful than "hello world" app. And even I have had some experience with that language just a few week before, I felt slightly frustrated. I reckon all that is because I become too dull and silly spending too much time with Java :) First surprise was that I realized that this language has a compiler - with Java it almost doesn't exist, you never 'compile' you do 'build', which is very different kind of thing. With Java you always almost curtain that you code is compilable, because modern IDEs (like Intellij) do not give you a chance to leave compilation error in your code. Another surprize is that Scala compiler is deadly slow, I have a good feeling that big project will suffer with it. So, you can say that with Scala it feels like comming back to good old C++ days :)

Ok, that's was introduction, here is some stuff I wrote, and which I almost sure is just another 'bicycle', but was useful for me. After some time with language, I realized that it doesn't have any standard resource-management construction, which probably is good for Scala - language is so flexible that it allows you to build your own without much effort (mostly code is stolen from this post):

  trait Managed[T] {
    def onEnter(): T
    def onExit(t:Throwable = null)
    def attempt(block: => Unit) {
      try { block } finally {}

  def using[T <: Any, R](managed: Managed[T])(block: T => R): R = {
    val resource = managed.onEnter()
    var exception = false
    try {
    } catch  {
      case t:Throwable => {
        exception = true
        throw t
    } finally {
      if (!exception) {

  def using[T <: Any, U <: Any, R] (managed1: Managed[T], managed2: Managed[U]) (block: T => U => R): R = {
    using[T, R](managed1) { r =>
      using[U, R](managed2) { s => block(r)(s) }

  class ManagedClosable[T <: Closeable](closable:T) extends Managed[T] {
    def onEnter(): T = closable
    def onExit(t:Throwable = null) {

  implicit def closable2managed[T <: Closeable](closable:T): Managed[T] = {
    new ManagedClosable(closable)
and the usage looks like this:
  def readLine() {
    using(new BufferedReader(new FileReader("file.txt"))) {
      file => {

1 comment:

Anonymous said...

Welcome to Scala Koby!