[英]Scala futures using Future.sequence but having issues with returning a tuple
當我在下面使用Future.sequence
時,我在嘗試返回元組時遇到了一些麻煩:
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))))
}
當 Future.sequence 調用只是返回給我一個Seq[Game]
時,我如何返回一個(Game, Score)
的元組?
另外,如果我們沒有Future.sequence
,我們如何能夠模仿它的作用? 即將List[Future[Game]]
轉換成Future[List[Game]]
?
你可以試試這個:
def findGames(id: UserId): Future[Seq[(Game, Score)]] = {
findUserGames(id).flatMap(gameIdToScoreSeq =>
Future.traverse(gameIdToScoreSeq)(gameIdToScore =>
findGame(gameIdToScore._1).map((_, gameIdToScore._2))
)
)
}
只需跟蹤名稱是什么,作為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)
})
)
你只需要一個 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,它變得有點復雜。 您可以通過使用 for-comprehensions 和模式匹配使您的代碼更具可讀性(可以說是):
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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