爲什麼scala編譯器不能從超類中推斷出類型參數？

Question

我想了解爲什麼scala編譯器無法推斷傳遞給超類的類型參數，以便我可以想出解決方法。解決方法建議也非常受歡迎！下面是我卡住的一個人爲的例子（代碼中解釋問題的註釋）：

代碼也在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

的問題是，Scala是試圖以代替一次的推斷ReqImpl最先得到來自Resp推斷兩者ReqImpl和Resp類型參數。由於Resp實際上並未出現在參數列表中，因此推斷爲Nothing，然後違反了Scala通知類型邊界。一種解決方法（我不記得在那裏我看到它第一次）是給同等類型req，但其中一個依賴於Resp明確：

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"))現在編譯。

Answer 2

我想你會需要捕獲的Req類型參數，並且您apply功能Param類型之間的關係在你的SVC特質。然後你可以相應地修改剩下的東西。

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

Answer 3

這樣做的一種方法是明確提及您的ReqImpl是參數化類型（Type infered to Nothing in Scala）。在你的情況下，它看起來就像這樣：

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

但有兩個問題的方法：

（1）在svc.marshal(GetInt("me"))斯卡拉將推斷RepImpl類型爲MyReq[Int]，哪一種意義，但ReqMarshaller[GetInt, Int]不匹配。所以，你需要把它定義爲：

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

（2）現在你馬上有另外一個問題，你不能在同一時間定義兩個ReqMarshaller[MyReq[String], String]。也許用相同的類型參數定義兩個端點是一個壞主意（只是一個猜測，但某些東西不適合這裏，它不適用於Alexey Romanov的解決方案）。

UPDATE

（1）是通過使ReqMarshaller協變解決：

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

（2）仍然失敗，曖昧implicits。