Question

Suppose, I would like to catch an exception, fix the problem caused the exception and return to the same execution point where the exception occurred to continue.

How can I implement it with continuations in Scala? Does it make any sense?

Answer 1

Here is one of the possible ways of implementing resumable error handling:

import java.io.File
import java.lang.IllegalStateException
import scala.util.continuations._

// how it works

ctry {
  println("start")

  val operationResult = someOperation(new File("c:\\ttttest"))

  println("end " + operationResult)
} ccatch {
  case (DirNotExists(dir), resume) =>
    println("Handling error")
    dir.mkdirs()
    resume()
}

def someOperation(dir: File) = {
  cthrow(DirNotExists(dir))
  println(dir.getAbsolutePath + " " + dir.exists)
  "Operation finished"
}

// exceptions

trait CException
case class DirNotExists(file: File) extends CException

// ctry/ccatch classes and methods

sealed trait CTryResult[T] {
  def get: T
  def ccatch(fn: PartialFunction[(CException, () => T), T]): T
}
case class COk[T](value: T) extends CTryResult[T] {
  def ccatch(fn: PartialFunction[(CException, () => T), T]) = value
  def get = value
}
case class CProblem[T](e: CException, k: Any => Any) extends CTryResult[T] {
  def ccatch(fn: PartialFunction[(CException, () => T), T]) = 
          fn((e, () => k(Unit).asInstanceOf[T]))
  def get = throw new IllegalStateException("Exception was not processed: " + e)
}

def ctry[T](body: => T @cps[Any]) = reset (body) match {
  case (e: CException, k: (Any => Any)) => CProblem[T](e, k)
  case value => COk(value)
}

def cthrow(e: CException): Any @cps[Any] = shift((k: Any => Any) => (e, k))

This code produces following output:

start 
Handling error
c:\ttttest true
end Operation finished