More idiomatic (monadic?) way to express this Scala

Question

I have several blocks of code that follow this pattern:

// Dummy function defs.
def result(i : Int, d : Double, b : Boolean) = {
    if (b) d else i
}

def fA(s : String) = {7}
def fB(s : String, i : Int) = {1.0}
def fC(s : String, i : Int, d : Double) = {true}

// Actual code.
def test(s : String) : Double = {
    try {
        val a = fA(s) 
        try {
            val b = fB(s, a)
            try {
                val c = fC(s, a, b)
                result(a, b, c)
            } catch {
                case _ => result(a, b, false)
            }

        } catch {
            case _ => result(a, 0.0, false)
        }
    } catch {
        case _ => result(0, 0.0, false)
    }
}

Where a, b, & c are calculated in turn by the corresponding functions and then the values are passed to the result function. If at any stage an exception occurs then a default value is used in place of the remaining variables.

Is there a more idiomatic way to express this code. It reminds me of Monads in that it's a series of chained computations which bail out immediately if any computation fails.

Answer 1

These types of problems are just what Try aims to solve a bit more monadically (than nested try/catch blocks).

Try represents a computation that may either result in an exception, or return a successfully computed value. It has two subclasses for these-- Success and Failure.

Very funny that this question popped up when it did-- a few days ago, I finished up some additions and refactoring to scala.util.Try, for the 2.10 release and this SO question helps to illustrate an important use-case for a combinator that we eventually decided to include; transform.

(As of writing this, transform is currently in the nightly and will be in Scala from 2.10-M5 onward, due out today or tomorrow. More info about Try and usage examples can be found in the nightly docs)

With transform (by nesting them), this can be implemented using Trys as follows:

def test(s: String): Double = {
  Try(fA(s)).transform(
    ea => Success(result(0, 0.0, false)), a => Try(fB(s, a)).transform(
      eb => Success(result(a, 0.0, false)), b => Try(fC(s, a, b)).transform(
        ec => Success(result(a, b, false)), c => Try(result(a, b, c))
      )
    )
  ).get
}

Answer 2

I'm not sure you can use monads as at each step you have two alternatives (exception or result) and to be faithful to your original code, on exception you don't want to be calling the fB or fC functions.

I was not able to elegantly remove the duplication of default values so I left it as I think it's clearer. Here is my non-monadic version based on either.fold and control.Exception:

def test(s : String) = {
  import util.control.Exception._
  val args = 
    allCatch.either(fA(s)).fold(err => (0, 0.0, false), a => 
      allCatch.either(fB(s, a)).fold(err => (a, 0.0, false), b =>
        allCatch.either(fC(s, a, b)).fold(err => (a, b, false), c =>
          (a, b, c))))
  (result _).tupled(args)
}