为什么scala编译器不能从超类中推断出类型参数？

Question

我想了解为什么scala编译器无法推断传递给超类的类型参数，以便我可以想出解决方法。解决方法建议也非常受欢迎！下面是我卡住的一个人为的例子（代码中解释问题的注释）：

代码也在scala fiddle中。

/** A Svc is a function that responds to requests 
    * @tparam Req[_] a request ADT whose instances specify their response type 
    */ 
trait Svc[Req[_]] { 
    def apply[Resp](req: Req[Resp]): Resp 
} 

/** Service request ADT */ 
sealed trait MyReq[_] 
// two requests have the same response type of String (i.e. MyReq[String]): 
case class GetString(id: String) extends MyReq[String] 
case class GetAltString(id: String) extends MyReq[String] 
// this one is the only MyReq[Int] 
case class GetInt(id: String) extends MyReq[Int] 

/** Type class for marshalling a response for a concrete request type. 
    * This lets us handle marshalling differently for different requests 
    * that have the same response type (such as GetString and GetAltString above). 
    * 
    * @tparam ReqImpl concrete MyReq type. This is required to enforce unique marshaller 
    * per request when there are mutliple request types with the same response type. 
    */ 
trait ReqMarshaller[ReqImpl <: MyReq[Resp], Resp] { 
    def marshal(r: Resp): String 
} 

class MySvc extends Svc[MyReq] { 
    // this apply function compiles and works just fine. 
    override def apply[Resp](req: MyReq[Resp]): Resp = req match { 
    case GetString(id) => id 
    case GetAltString(id) => id + id 
    case GetInt(id) => id.length 
    } 

    // This is the problem. I want to specify the request is a subclass so 
    // we get the specific marshaller for the request type and avoid 
    // ambiguous implicit errors. 
    // However, the Resp type parameter is always inferred as Nothing 
    // instead of the correct response type. 
    def marshal[ReqImpl <: MyReq[Resp], Resp](req: ReqImpl)(
    implicit 
    marshaller: ReqMarshaller[ReqImpl, Resp] 
): String = marshaller.marshal(apply(req)) 

    // this method is just here to show that it won't work as a solution 
    // because it doesn't work when there are multiple request types with 
    // the same response type (causes ambiguous implicits errors) 
    def marshalGeneric[Resp](req: MyReq[Resp])(
    implicit 
    marshaller: ReqMarshaller[_ <: MyReq[Resp], Resp] 
): String = marshaller.marshal(apply(req)) 
} 

implicit val getIntMarshaller: ReqMarshaller[GetInt, Int] = new ReqMarshaller[GetInt, Int] { 
    def marshal(i: Int): String = (i * i).toString 
} 

implicit val getStrMarshaller: ReqMarshaller[GetString, String] = new ReqMarshaller[GetString, String] { 
    def marshal(s: String): String = s 
} 

implicit val getAltStrMarshaller: ReqMarshaller[GetAltString, String] = new ReqMarshaller[GetAltString, String] { 
    def marshal(s: String): String = s + s 
} 

val svc = new MySvc 

val myLength = svc(GetInt("me")) // 2 
println(s"myLength: $myLength") 

svc.marshalGeneric(GetInt("me")) // compiles and works 
//svc.marshal(GetInt("me")) // fails to compile due to infering Resp type as Nothing 
//svc.marshalGeneric(GetAltString("me")) // fails to compile because of ambiguous implicits 

Answer 1

的问题是，Scala是试图以代替一次的推断ReqImpl最先得到来自Resp推断两者ReqImpl和Resp类型参数。由于Resp实际上并未出现在参数列表中，因此推断为Nothing，然后违反了Scala通知类型边界。一种解决方法（我不记得在那里我看到它第一次）是给同等类型req，但其中一个依赖于Resp明确：

def marshal[ReqImpl <: MyReq[Resp], Resp](req: ReqImpl with MyReq[Resp])(
    implicit marshaller: ReqMarshaller[ReqImpl, Resp] 
): String = marshaller.marshal(apply(req)) 

svc.marshal(GetInt("me"))现在编译。

Answer 2

我想你会需要捕获的Req类型参数，并且您apply功能Param类型之间的关系在你的SVC特质。然后你可以相应地修改剩下的东西。

trait Svc[Req[_ <: XX], XX] { 
    def apply[Resp <: XX](req: Req[Resp]): Resp 
} 

Answer 3

这样做的一种方法是明确提及您的ReqImpl是参数化类型（Type infered to Nothing in Scala）。在你的情况下，它看起来就像这样：

def marshal[ReqImpl[Resp] <: MyReq[Resp], Resp](req: ReqImpl[Resp])(
    implicit 
    marshaller: ReqMarshaller[ReqImpl[Resp], Resp] 
): String = marshaller.marshal(apply(req)) 

但有两个问题的方法：

（1）在svc.marshal(GetInt("me"))斯卡拉将推断RepImpl类型为MyReq[Int]，哪一种意义，但ReqMarshaller[GetInt, Int]不匹配。所以，你需要把它定义为：

implicit val getIntMarshaller = new ReqMarshaller[MyReq[Int], Int] { 
    def marshal(i: Int): String = (i * i).toString 
} 

（2）现在你马上有另外一个问题，你不能在同一时间定义两个ReqMarshaller[MyReq[String], String]。也许用相同的类型参数定义两个端点是一个坏主意（只是一个猜测，但某些东西不适合这里，它不适用于Alexey Romanov的解决方案）。

UPDATE

（1）是通过使ReqMarshaller协变解决：

trait ReqMarshaller[+ReqImpl <: MyReq[Resp], Resp] { ... 

（2）仍然失败，暧昧implicits。