Scala Option with fold operation

Question

I am trying to use fold or map operation instead of match for Option .

I have a Option val ao: Option[String] = xxxx and a function f: (String => Future[Option[T]])

If I do pattern matching is:

ao match {
  case Some(t) => f(t)
  case None => Future.successful(None)
}

if I do map is:

ao map f getOrElse Future.successful(None)

But when I do fold, I got some following compiler errors:

ao.fold(Future.successful(None))(t => f(t))

about complaining expression Future[Option[T]] doesn't confirm to Future[None.type]

So why map works here but fold not, did I miss something here?

Answer 1

The reason for this is that Scala is trying to derive the return type None.type which is kind of like Nil for lists in the sense that only one object ( None ) exists and is upcast in all situations because it is Option[Nothing] . To get around this, you should explicitly define the types.

Here's the Scala doc for fold :

fold[B](ifEmpty: ⇒ B)(f: (A) ⇒ B): B
Returns the result of applying f to this scala.Option's value if the scala.Option is nonempty. Otherwise, evaluates expression ifEmpty.

The compiler thinks [B] is None.type . so try calling it this way :

ao.fold[Future[Option[T]]](Future.successful(None))(t => f(t))

or calling it with a type ascription :

ao.fold(Future.successful(None: Option[T]))(t => f(t))