Bargaining with futures
Applying a future is like taking a hostage. Your demands might be met in time, but until they are you’re sitting around doing nothing other than guarding a prisoner.
So we don’t like to take hostages or apply futures, but what good is a future if you can’t do anything with its value? Luckily, you can do plenty. You just have to be flexible about when things happen.
A future is like an option that doesn’t know what it is yet; that doesn’t stop it from transforming into something else. We could transform an option of a string into an option of its length. Same goes for futures.
import dispatch._, Defaults._val svc = url("http://api.hostip.info/country.php")val country = Http(svc OK as.String)val length = for (c <- country) yield c.length
length value is a future of integer.
If you pasted the above into a console, you probably saw something like this in the output:
country: scala.concurrent.Future[String] = scala.concurrent.impl.Promise$DefaultPromise@4929b5a5 length: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@581fa0fe
Not too helpful right? The
scala> country.print res0: String = Future(US)
If the future value isn’t available,
scala> Http(svc OK as.String).print res1: String = Future(-incomplete-)
Futuremethods in this documentation are provided implicitly by
val lengthNow = length.completeOption.getOrElse(-1)
But most of the time, you want to operate on values that are known to be available. In the next pages we’ll see how far we can go in this direction by transforming futures.