理解爲，理解

Question

我有以下特點：

trait Command 
trait Status 
trait Tr{ 
    def getOpt(id: Long): Option[Status] 
} 

及以下地圖：

private val chains: mutable.Map[Long, Tr] = mutable.HashMap() 
private val commandsId: mutable.Map[Command, Long] = mutable.HashMap() 
private val commands: mutable.Map[Long, Command] = mutable.HashMap() 

我想要實現方法：

def getStatus(cmd: Command) = commandsId.get(cmd).flatMap(chains.get).flatMap{tr => 
    tr.status(id) //fail, Id was lost at commandsId.get(cmd) step 
} 

有一個緊湊的形式寫這個？我想換的理解會有所幫助，但我不知道......

Answer 1

commandsId.get(cmd) 
    .flatMap { id => 
     chains.get(id).map(_.status(id)) 
    } 

或者與for理解：

for { 
    id <- commandsId.get(cmd) 
    tr <- chains.get(id) 
    status <- tr.status(id) 
    } yield status