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Clean-up resource with Scala

Posted on November 17, 2008, in Programming

Nov 17 15:48:39 <jinok> i hate wasting vertical space declaring vals that i need to initialize outside of a try block so i can clean them up in a finally block Nov 17 15:48:53 <jinok> so i’ve started doing the initialization using lazy vals.

sealed trait Resource[+T] {
  val value: T
  def close: Unit

  def use[X](f: T => X) = try { f(value) } finally { close }
}

import java.io.InputStream

object Resource {
  def resource[T](t: T, c: => Unit) = new Resource[T] {
    val value = t
    def close = c
  }

  implicit def InputStreamResource(t: InputStream) = resource(t, t.close)
}

object Example {
  import Resource._

  def main(args: Array[String]) {
    val in = new java.io.FileInputStream("/etc/passwd")
    // print the first character of /etc/passwd
    println(in use (_.read.toChar))
  }
}

Reader’s exercise: Resource is a functor and monad. Implement the map and flatMap methods. Observe trickiness and ask why?