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.
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 }
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.
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