How to define a function whose output type depends on the input type

Question

Given the following classes:

case class AddRequest(x: Int, y: Int)
case class AddResponse(sum: Int)
case class ToUppercaseRequest(str: String)
case class ToUpper         


        

Answer 1

You can define a common trait for Requests, and a common trait for Responses where the request type is defined for specific response type:

trait Request[R <: Response]
trait Response

case class AddRequest(x: Int, y: Int) extends Request[AddResponse]
case class AddResponse(sum: Int) extends Response
case class ToUppercaseRequest(str: String) extends Request[ToUppercaseResponse]
case class ToUppercaseResponse(upper: String) extends Response Response[ToUppercaseRequest]

Then, process signature would be:

def process[A <: Request[B], B <: Response](req: A): B

When you call process, you'll have to explicitly define the types so that the returned type is what you expect it to be - it can't be inferred specifically enough:

val r1: AddResponse = process[AddRequest, AddResponse](AddRequest(2, 3))
val r2: ToUppercaseResponse = process[ToUppercaseRequest, ToUppercaseResponse](ToUppercaseRequest("aaa"))