Scala continuation and exception handling

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

Answer 2

I once did something like that in ruby. It was just a test to see whether I could implement common lisp's "resumable exceptions" in ruby. You should be able to do the same thing in Scala, but I haven't tried it. Is your question about the general concept or about implementation details?

Anyway, here is the code (without warranty ;) )

#!/usr/bin/env ruby

require 'continuation'

#Module for adding elements of an array. Leaves error handling to the caller by using exceptions and continuations.
module Adder

#Exception class that offers continuations to the receiver.
    class CcExc < Exception
        def initialize(again, skip, index, sum)
            @again = again
            @skip = skip
            @index = index
            @sum = sum
        end
        def again
            @again.call
        end
        def skip
            @skip.call
        end
        attr_reader :index #where the problem occured
        attr_reader :sum #current sum   
    end

    #Method to get the current continuation
    def Adder.getcc
        cc = nil
        callcc {|c| cc = c}
        cc
    end

    #add all numbers in the array, raise an exception with continuations if an
    #item doesn't have the right type
    def Adder.addAll(array)
        sum = 0;
        array.each_with_index {|dummy,i|
            again = getcc #save continuation before processing the item
            if array[i].is_a? Numeric
                sum += array[i] #process item normally
            else
                #raise exception with previously save continuation (again)
                #and current continuation (skip)
                callcc {|skip| raise CcExc.new again, skip, i, sum}
            end
        }
        sum
    end
end

data = [1,"2",3,"hello",Object,"4",5,"END",6]
begin
    puts "The sum is #{Adder.addAll data}."
rescue Adder::CcExc => e
    puts "Exception raised."
    i = e.index
    case data[i]
        when /^\s*\d/
            data[i] = data[i].to_i
            puts 'Problem fixed. Continue adding.'
            e.again
        when "END"
            puts "'END' found. Stop processing."
            puts "The sum is #{e.sum}"
        else
            puts "'#{data[i]}' of type #{data[i].class} can't be converted " +
                 "to interger. Item skipped."
            e.skip
    end
end

Answer 3

This function should do it (place the code that throws exceptions at foo arg):

def F[T](foo: => T, dealWithError: Exception => T): T =
  try foo
  catch{
    case ex: Exception => dealWithError(ex)}

I use these class + implicit conversion:

  class ORfoo[R](foo: () => R){
    def or(r: R): R =
      try foo() 
      catch{
        case ex: Exception => r
      }
    }

implicit def ORfooWrapper[R](f: => R) = new ORfoo(() => f)

It allows you python-like exception treatment, like "1a".toInt or 5