Scala futures and promises API resulted from an attempt to consolidate several different APIs for asynchronous programming, among them, legacy Scala futures, Akka futures, Scalaz futures, and Twitter's Finagle futures. Legacy Scala futures and Akka futures have already converged to the futures and promises API that you've learned about so far in this chapter. Finagle's com.twitter.util.Future type is planned to eventually implement the same interface as scala.concurrent.Future, while the Scalaz scalaz.concurrent.Future type implements a slightly different interface. In this section, we give a brief of Scalaz futures.

To use Scalaz, we add the following dependency to the build.sbt file:

libraryDependencies +=
  "org.scalaz" %% "scalaz-concurrent" % "7.0.6"

We now encode an asynchronous tombola program using Scalaz. The Future type in Scalaz does not have the foreach method. Instead, we use its runAsync method, which asynchronously runs the future computation to...