如何将隐式参数转化为匿名 function

Question

How do I get the implicit val myConnection into scope of the execute(true) function

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

You can't do this directly. Is the value of myConnection really not determined before you call execute ? In this case, you could do this:

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() 
}

Basically, you pass a parameter to the evaluated function, but then you have to remember to mark it implicit at the call site.

If you have several such implicits, you may want to put them in a dedicated, “implicit provider” class. Eg:

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() 
}

If you want to avoid the import , you can provide “implicit getters” for each of your fields in PassedImplicits and write something like this, then:

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

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