How can I flatten this Future[T] structure?

Question

Given the following example:

  val handler : Connection = new DatabaseConnectionHandler()
  val result : Future[Future[Future[Option[ResultSet]]]] = handler.connect
    .map( (parameters) => handler )
    .map( connection => connection.sendQuery("BEGIN TRANSACTION SERIALIZABLE") )
    .map( future => future.map( query => query.rows ) )
    .map( future => handler.sendQuery("COMMIT").map( query => future ) )

Is it possible to flatten it to receive a Future[Option[ResultSet]] at the end instead of this future inside a future inside a future structure in Scala?

I am currently using Scala's 2.10 Future's and Promise's, but I can't find a way to to this. I know I can use nested callbacks but I would rather avoid that since the code is going to look horrible.

The Connection trait is defined here.

Solution 1

Whenever you map with an argument of type A => Future[B] you should really be using flatMap.

The code would then be like this:

  val connection : Connection = new DatabaseConnectionHandler( DefaultConfiguration )
  val result: Future[QueryResult] = connection.connect
    .flatMap( _ => connection.sendQuery("BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ") )
    .flatMap( _ => connection.sendQuery("SELECT 0") )
    .flatMap( _ => connection.sendQuery("COMMIT").map( value => query ) )

Alternatively, you could use for-comprehension. It uses flatMap for you.

val connection : Connection = new DatabaseConnectionHandler( DefaultConfiguration )
val result: Future[QueryResult] = for {
  _ <- connection.connect
  _ <- connection.sendQuery("BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ")
  _ <- connection.sendQuery("SELECT 0")
  queryResult <- connection.sendQuery("COMMIT").map( value => query )
} yield { queryResult }

Solution 2

You should use a flatMap instead of a map here.

The flatMap expects a function fun returning a future g and returns the future h holding the value from the future g that fun returned.

Also, consider writing this within a for-comprehension, see how here.

Comments

[senz]

There is no flatten in scala.concurrent.Future

[HEX]

@senz Yes, scala.concurrent.Future don't have flatten. But you can use f flatMap identity. See here

[Brian Risk]

Are you mixing options and futures in your for-comprehension? Isn't that not allowed?