Scala 2.12，实例化类型参数？

Question

There is already information about what I want to do, but couldn't figure it out yet. I would like to instantiate an element of a type passed as a context bound, something like this:

case class Person(name: String)
case class Dog(name: String)

abstract case class Message[T](result: Map[String, T])

// using Person as a type here
case class PersonMessage(val result: Map[String, Person]) extends Message[Person](result)

// using Dog as a type here
case class DogMessage(val result: Map[String, Dog]) extends Message[Dog](result)

I can certainly instantiate these objects:

val pm: PersonMessage = PersonMessage(Map("joe" -> Person("joe")))
val dm: DogMessage = DogMessage(Map("blacky" -> Dog("blacky")))

but could I do this in a generic function?

// should return a PersonMessage or a DogMessage
def myfunction[T, U <: Message[T]](customName: String): U = {
  U(Map(customName -> T(customName)))
}

val p: PersonMessage = myFunction[Person, PersonMessage]("joe")
val d: DogMessage = myFunction[Dog, DogMessage]("blacky")

This syntax doesn't work, but are there other ways to achieve this? Thanks for any hint.

Answer 1

The issue is that by just defining the type T or U <: Message[T] you have no guarantees about what kind of arguments the constructors take. Why couldn't someone call it with T set to some type that doesn't have a one-string-argument constructor? Instead you can explicitly pass in constructors with appropriate types. Here's how I would handle it:

def myFunction[T, U <: Message[T]](customName: String, mkT: String => T, mkU: Map[String, T] => U): U = {
  mkU(Map(customName -> mkT(customName)))
}

val p: PersonMessage = myFunction("joe", Person, PersonMessage)
val d: DogMessage = myFunction("blacky", Dog, DogMessage)