Question

如何将隐式val myConnection放入execute（true）函数的范围

def execute[T](active: Boolean)(blockOfCode: => T): Either[Exception, T] = {
  implicit val myConnection = "how to get this implicit val into scope"
  Right(blockOfCode)
}



execute(true){
// myConnection is not in scope
  useMyConnection()   // <- needs implicit value
}

Answer 1

你不能直接这样做。在致电myConnection之前，execute的价值是否真的没有确定？在这种情况下，您可以这样做：

def execute[T](active: Boolean)(blockOfCode: String => T): Either[Exception, T] = {
  val myConnection = "how to get this implicit val into scope"
  Right(blockOfCode(myConnection))
}

execute(true) { implicit connection =>
  useMyConnection() 
}

基本上，您将参数传递给已评估的函数，但是您必须记住在调用站点将其标记为隐式。

如果您有几个这样的含义，您可能希望将它们放在专用的“隐式提供者”类中。 E.g：

class PassedImplicits(implicit val myConnection: String)

def execute[T](active: Boolean)(blockOfCode: PassedImplicits => T): Either[Exception, T] = {
  val myConnection = "how to get this implicit val into scope"
  Right(blockOfCode(new PassedImplicits()(myConnection)))
}

execute(true) { impl =>
  import impl._
  useMyConnection() 
}

如果您想避开import，可以为PassedImplicits中的每个字段提供“隐式getter”并写下这样的内容，然后：

implicit def getMyConnection(implicit impl: PassedImplicits) = impl.myConnection

execute(true) { implicit impl =>
  useMyConnection() 
}

如何将隐式参数导入匿名函数

1 个答案: