Can a Scala for-yield return None if I pass in an Option to it?

Question

I have the following for-yield loop which takes in a boolean and should either yield Some(string) or None, depending on the boolean:

val theBoolean = false

val x: Option[String] =
for {
  theArg <- theBoolean
} yield {
  if (theArg) {
    "abc"
  } else {
    None
  }
}

This works great if theBoolean is actually a Boolean like false. However if I wanted to pass in an Option[Boolean]:

val theBoolean = Some(false)

it seems like Scala automatically applies a Some() wrapper to the None return - I get a complaint that "Expression of type Option[Serializable] doesn't conform to expected type Option[String]" (with None being the Serializable). The yield is perfectly happy with the same string return though (it doesn't become an Option[Option[String]]

How would I return a None in this case?

Answer 1

A for-comprehension is just syntactic sugar for a series of flatMap, map and filter. Let's desugar your code then:

val theBoolean = Some(false)

val x = theBoolean.map { theArg =>
  if (theArg) {
    "abc"
  } else {
    None
  }
}

As you can see, you're just mapping over the value of the Option, so you'll either return Some(abc), Some(None) or None (in case theBoolean is already None).

The lowest common type of None and "abc" is java.Serializable, so that's why the type of x is inferred as Option[Serializable], which as meaningless as Option[Any].

Possible solutions are:

• using a flatMap

  theBoolean.flatMap(theArg => if (theArg) Some("abc") else None)
  

  or even shorter

  theBoolean.flatMap(if (_) Some("abc") else None)
  

• filtering and mapping

  theBoolean.withFilter(identity).map(_ => "abc")
  

Where I used identity since you're testing the value itself.

Clearly you can always leverage the syntactic sugar provided by a for-comprehension, although it doesn't really make a difference in this case

for {
  theArg <- theBoolean
  if theArg
} yield "abc"