Scala：确定用于泛型的方法结果类型

Question

In a third-party library there is a series of request classes, all of which derive from some common base class, which is generic and takes response class as a parameter:

abstract class AbstractRequest[ResponseType] {
  …
  def execute(): ResponseType
}

class UserList {…}
class UserListRequest extends AbstractRequest[UserList] {…}

class Avatar {…}
class AvatarRequest extends AbstractRequest[Avatar] {…}

…

I want to write some generic method that takes a request instance, executes it several times in some special ways and delegates processing of the responses to a function supplied in arguments:

def specialMultiExecute(request: Req)(responseHandler: Resp => Unit): Unit = …

— to be called like:

val myRequest: UserListRequest = …
specialMultiExecute(myRequest){ userList => … }

The problem is that I need to somehow specify Req and Resp types in the specialMultiExecute declaration. I tried the obvious approach:

def specialMultiExecute[Req <: AbstractRequest[Resp], Resp](request: Req)(responseHandler: Resp => Unit): Unit = …

— but Scala compiler fails to deduct generic argument types (an explicit specification like specialMultiExecute [UserListRequest, UserList] (myRequest){ userList => … } is required).

In C++ in such case I could write a template function with a single template parameter Req , while making Resp to be determined as result type of the method Req::execute :

template<typename Req>
void specialMultiExecute(
    Req request,
    std::function<void (decltype(std::declval<Req>().execute()))> responseHandler
) {…}
//i.e. we use `decltype(std::declval<Req>().execute())` instead of Resp

Is there way to write something similar is Scala?

I mean something like (in Scala-like pseudocode):

def specialMultiExecute[Req <: AbstractRequest](request: Req)(responseHandler:  => Unit): Unit = …

Answer 1

It is a limitation of the type inference mechanism. The simplest way to solve it is to use an implicit evidence that Req is a subtype of AbstractRequest[ResponseType] .

Here is an example.

import scala.language.implicitConversions
import scala.reflect.runtime.universe.TypeTag

abstract class AbstractRequest[ResponseType] {
  def execute(): ResponseType
}

final case class User(id: Int, name: String)
final case class House(id: Int, price: Int)

class UserListRequest extends AbstractRequest[List[User]] {
  override def execute(): List[User] = List(User(id = 3, name = "Sasha"))
  override def toString: String = "UserListRequest"
}

final class RequestWrapper[Req, Resp](val request: Req) extends AnyVal {
  type ResponseType = Resp
}

implicit def request2wrapper[Req, Resp](request: Req)(implicit ev: Req <:< AbstractRequest[Resp]): RequestWrapper[Req, Resp] =
  new RequestWrapper(request)

def specialMultiExecute[Req, Resp](wrapper: RequestWrapper[Req, Resp])
                                  (responseHandler: wrapper.ResponseType => Unit)
                                  (implicit ev: Req <:< AbstractRequest[Resp], TTReq: TypeTag[Req], TTResp: TypeTag[Resp]): Unit = {
  val request: Req = wrapper.request
  val executionResult: Resp = request.execute()
  responseHandler(executionResult)
  println(TTReq)
  println(TTResp)
  println(request)
}

specialMultiExecute(new UserListRequest())(println)
// List(User(3,Sasha))
// TypeTag[UserListRequest]
// TypeTag[List[User]]
// UserListRequest

Reference for <:< . Reference for "Dependent types" .

Edit

Te above code example was modified to allow identification of the concrete Request and Response types being used.