Scala：返回依赖类型

Question

在我的应用程序中，我有一组闭合的操作，返回相应的设置响应，如下所示。

sealed trait OperationCompletionResponse {
    val state: Int
}
case class ExecutionStartedResponse(state: Int) extends OperationCompletionResponse
case class UpdateRecordedResponse(state: Int) extends OperationCompletionResponse
case class ExecutionTerminatedResponse(state: Int) extends OperationCompletionResponse

sealed trait Operation {
    type R
    def createResponse(state: Int): R
}

case class StartExecutionOperation() extends Operation {
    type R = ExecutionStartedResponse
    override def createResponse(state: Int): ExecutionStartedResponse = ExecutionStartedResponse(state)
}

case class RecordUpdateOperation() extends Operation {
    type R = UpdateRecordedResponse
    override def createResponse(state: Int): UpdateRecordedResponse = UpdateRecordedResponse(state)
}

case class TerminateExecutionOperation() extends Operation {
    type R = ExecutionTerminatedResponse
    override def createResponse(state: Int): ExecutionTerminatedResponse = ExecutionTerminatedResponse(state)
}

---

至于我对类型成员和类型投影的理解，我可以做到以下几点。根据scala编译器，它们是完全有效的语句

val esr:StartExecutionOperation#R = ExecutionStartedResponse(1)
val teo:TerminateExecutionOperation#R = ExecutionTerminatedResponse(-1)
val ruo:RecordUpdateOperation#R = UpdateRecordedResponse(0)

---

但是，我现在想在函数中使用它们;这通常更有用。现在，我如何将输出类型指定为依赖类型？

def updateState[O <: Operation](operation: O) = operation match {
    case StartExecutionOperation() =>  ExecutionStartedResponse(1)
    case TerminateExecutionOperation() => ExecutionTerminatedResponse(-1)
    case RecordUpdateOperation() => UpdateRecordedResponse(0)
}

更具体地说，我不希望在我的情况下函数的返回类型是OperationCompletionResponse，但类似于Operation＃R或operation.R

我怎么能做到这一点？

Answer 1

updateState的路径依赖类型将与operation的类型直接相关。你不想在体内匹配operation，因为这永远不会给你你正在寻找的类型R。

你定义了一个操作，它给你这个R和那个createResponse。因为createResponse期望一个整数参数，你必须以某种方式在updateState中给它。看来你的每个操作都有一些默认状态，所以你可以定义def defaultState: Int Operation然后有

def updateState(op: Operation): op.R = op.createResponse(op.defaultState)` 

如果这不能回答您的问题，请对其进行编辑，以便更具体地了解您要在此状态下尝试实现的目标。