Success as the only option 

Earlier we compared futures to options. The network operation at the center of things may or may not have completed: that’s the temporal uncertainty and it can be thought of an option, and even transformed into one with the completeOption method.

Beyond that we don’t know if a completed future will produce an error or a useful response. We can also think of that uncertainty, and model it in code, as an option. By transferring the uncertainty from the future to a contained option, we make a future that will never fail.

Future#option 

import dispatch._, Defaults._
val str = Http.default(host("example.com") OK as.String).option

The type assigned is str: Future[Option[String]]. When the future completes, its value will be a future of None since the request will fail. The failure exception is captured and discarded by the underlying code.

With this we can write higher level interfaces that encompass the possibility of failure.

Optional weather 

Let’s make the weather interface from the previous section a little more resilient.

case class Location(city: String, state: String)
def weatherSvc(loc: Location) = {
  host("api.wunderground.com") / "api" / "5a7c66db0ba0323a" /
    "conditions" / "q" / loc.state / (loc.city + ".xml")
}
def weatherXml(loc: Location) =
  Http.default(weatherSvc(loc) OK as.xml.Elem).option

Now any connection, status, or parsing error will produce a None.

Optional temperature 

We’ll make a slight change to the extraction method.

def extractTemp(xml: scala.xml.Elem) = {
  val seq = for {
    elem <- xml \\ "temp_c"
  } yield elem.text.toFloat
  seq.headOption
}

Instead of calling head which throws an exception if there are no matching elements, we call headOption. This meshes with a revised temperature method.

def temperature(loc: Location) =
  for (xmlOpt <- weatherXml(loc))
    yield for {
      xml <- xmlOpt
      t <- extractTemp(xml)
    } yield t

This returns the future of some temperature value, or None if an error occurs at any point.

Optional hotness 

And with that, we can rewrite hottest to provide the highest successful result, or None.

def hottest(locs: Location*) = {
  val temps =
    for(loc <- locs)
      yield for (tOpt <- temperature(loc))
        yield for (t <- tOpt)
          yield (t -> loc)
  for (ts <- Future.sequence(temps)) yield {
    val valid = ts.flatten
    for (_ <- valid.headOption)
      yield valid.maxBy { _._1 }
  }
}

If the nested for-expressions throw you for a loop, keep in mind that futures are not themselves Iterable. You’re dealing with unrelated types, even if they share some philosophical opinions. They can’t be haphazardly mixed in the same for-expression.

But as the fors unroll we end up with a future of some city name, or None—exactly what we want. Give it a try with some real and fake city names.

Unknown error 

This version of temperature ranking is much more resilient than the last, but it still leaves something to be desired. We don’t know from the result value which cities, if any, were excluded from consideration, and we don’t know why.

In the next section we’ll explore Either, a favorite type of those who plan for both failure and success.