Scala:返回从属类型

时间:2019-03-28 08:41:14

标签: scala dependent-type path-dependent-type type-projection

在我的应用程序中,我有一组封闭的操作,它们返回对应的一组响应,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)
}

就我对类型成员和类型投影的理解而言,我可以执行以下操作。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,我现在想在一个函数中使用它们;这通常更有用。现在,如何将输出类型指定为从属类型?

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,我不希望函数的返回类型为 OperationCompletionResponse ,而是类似 Operation#R operation.R < / strong>

我该怎么做?

1 个答案:

答案 0 :(得分:2)

与路径有关的类型updateState将直接链接到operation的类型。您不想在正文中匹配operation,因为它永远不会为您提供您要查找的R类型。

您恰好定义了一个操作R,即createResponse。由于createResponse需要一个整数参数,因此您必须以某种方式在updateState中给它。似乎每个操作都有一些默认状态,因此可以定义def defaultState: Int Operation然后具有

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,以更具体地说明您在此状态下要实现的目标。