How to return optional information from methods?

Question

The general question is how to return additional information from methods, beside the actual result of the computation. But I want, that this information can silently be ignored.

Take for example the method dropWhile on Iterator. The returned result is the mutated iterator. But maybe sometimes I might be interested in the number of elements dropped.

In the case of dropWhile, this information could be generated externally by adding an index to the iterator and calculating the number of dropped steps afterwards. But in general this is not possible.

I simple solution is to return a tuple with the actual result and optional information. But then I need to handle the tuple whenever I call the method - even if I'm not interested in the optional information.

So the question is, whether there is some clever way of gathering such optional information?

Maybe through Option[X => Unit] parameters with call-back functions that default to None? Is there something more clever?

Answer 1

Just my two cents here…

You could declare this:

case class RichResult[+A, +B](val result: A, val info: B)

with an implicit conversion to A:

implicit def unwrapRichResult[A, B](richResult: RichResult[A, B]): A = richResult.result

Then:

def someMethod: RichResult[Int, String] = /* ... */

val richRes = someMethod
val res: Int = someMethod

Answer 2

It's definitely not more clever, but you could just create a method that drops the additional information.

def removeCharWithCount(str: String, x: Char): (String, Int) =
  (str.replace(x.toString, ""), str.count(x ==))

// alias that drops the additional return information
def removeChar(str: String, x: Char): String =
  removeCharWithCount(str, x)._1