scala 我怎样才能扁平化这个 Future[T] 结构？
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How can I flatten this Future[T] structure?
提问by Maurício Linhares
Given the following example:
给出以下示例：
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?
是否有可能将其展平以Future[Option[ResultSet]]在 Scala 的未来结构中的未来内部接收一个，而不是这个未来？
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.
我目前正在使用 Scala 的 2.10 Future 和 Promise，但我找不到解决这个问题的方法。我知道我可以使用嵌套回调，但我宁愿避免这样做，因为代码看起来很糟糕。
The Connectiontrait is defined here.
该Connection特性被定义在这里。
回答by Debilski
Whenever you mapwith an argument of type A => Future[B]you should really be using flatMap.
每当您map使用类型参数时，A => Future[B]您都应该真正使用flatMap.
The code would then be like this:
代码将是这样的：
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 flatMapfor you.
或者，您可以使用 for-comprehension。它flatMap为你使用。
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 }
回答by axel22
You should use a flatMapinstead of a maphere.
您应该flatMap在map这里使用 a而不是 a 。
The flatMapexpects a function funreturning a future gand returns the future hholding the value from the future gthat funreturned.
该flatMap预期函数fun返回一个未来g，并返回将来h拿着从未来的价值g即fun返回。
Also, consider writing this within a for-comprehension, see how here.
另外，请考虑在for-comprehension 中编写此内容，请参阅此处。
