[英]Scala futures using Future.sequence but having issues with returning a tuple
当我在下面使用Future.sequence
时,我在尝试返回元组时遇到了一些麻烦:
type UserId = Int
type GameId = Int
type Score = Double
case class Game(id: GameId)
def findGame(id: GameId): Future[Game] = ???
def findUserGames(id: UserId): Future[Seq[(GameId, Score)]] = ???
def findGames(id: UserId): Future[Seq[(Game, Score)]] = {
val userGames = findUserGames(id)
// ******* I am stuck here
val games: Future[Seq[Game]] = userGames.flatMap(x => Future.sequence(x.map(y => findGame(y._1))))
}
当 Future.sequence 调用只是返回给我一个Seq[Game]
时,我如何返回一个(Game, Score)
的元组?
另外,如果我们没有Future.sequence
,我们如何能够模仿它的作用? 即将List[Future[Game]]
转换成Future[List[Game]]
?
你可以试试这个:
def findGames(id: UserId): Future[Seq[(Game, Score)]] = {
findUserGames(id).flatMap(gameIdToScoreSeq =>
Future.traverse(gameIdToScoreSeq)(gameIdToScore =>
findGame(gameIdToScore._1).map((_, gameIdToScore._2))
)
)
}
只需跟踪名称是什么,作为oneliner,您就可以做到
def findGames(id: UserId)(implicit ec: ExecutionContext): Future[Seq[(Game, Score)]] =
findUserGames(id).flatMap(games =>
Future.sequence(games.map { case (gameid, score) =>
findGame(gameid).map(game => game -> score)
})
)
你只需要一个 map:
val games: Future[Seq[(Game, Score)]] = userGames.flatMap(x => Future.sequence(x.map(y => findGame(y._1).map(z => (z, y._2)))))
正如你所看到的,它变得有点复杂。 您可以通过使用 for-comprehensions 和模式匹配使您的代码更具可读性(可以说是):
val games2: Future[Seq[(Game, Score)]] = for {
games <- userGames
result <- Future.sequence(games.map {
case (gameId, score) =>
for {
game <- findGame(gameId)
} yield (game, score)
})
} yield result
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