Convert Option to Either in Scala

Question

Suppose I need to convert Option[Int] to Either[String, Int] in Scala. I'd like to do it like this:

def foo(ox: Option[Int]): Either[String, Int] =
  ox.fold(Left("No number")) {x => Right(x)}

Unfortunately the code above doesn't compile and I need to add type Either[String, Int] explicitly:

ox.fold(Left("No number"): Either[String, Int]) { x => Right(x) }

Is it possible to convert Option to Either this way without adding the type ?
How would you suggest convert Option to Either ?

Answer 1

No, if you do it this way, you can't leave out the type.

The type of Left("No number") is inferred to be Either[String, Nothing]. From just Left("No number") the compiler can't know that you want the second type of the Either to be Int, and type inference doesn't go so far that the compiler will look at the whole method and decide it should be Either[String, Int].

You could do this in a number of different ways. For example with pattern matching:

def foo(ox: Option[Int]): Either[String, Int] = ox match {
  case Some(x) => Right(x)
  case None    => Left("No number")
}

Or with an if expression:

def foo(ox: Option[Int]): Either[String, Int] =
  if (ox.isDefined) Right(ox.get) else Left("No number")

Or with Either.cond:

def foo(ox: Option[Int]): Either[String, Int] =
  Either.cond(ox.isDefined, ox.get, "No number")