返回值的scala多态类型

Question

How would I make this pattern work? func() fails to compile. I understand the problem with this setup, but what's a pattern that could accomplish basically this?

class A() {
  val a: Int = 123
  val b: String = "xxx"
}

def func[T](key: String, a: A): T = {
  if      (key == "a") a.a     // would make T an Int
  else if (key == "b") a.b     // would make T a String
}

val a = new A()
func[Int]("a", a)
func[String]("b", a)

Answer 1

I'm not quite sure what you are going for, but a few possibilities.

class A() {
  val a: Int = 123
  val b: String = "xxx"
}

def func[T : Manifest](a: A) = implictly[Manifest[T]] match {
  case implicitly[Manifest[Int]]) => a.a
  case implicitly[Manifest[String]) => a.b
}

val a = new A()
func[Int](a)
func[String](a)

or

class A() {
  val a: Int = 123
  val b: String = "xxx"
}

val aKey = (_: A).a
val bKey = (_: A).b

def func[T](key: A => T, a: A) = key(a)

val a = new A()
func(aKey, a)
func(bKey, a)

or even with Shapeless ,

import shapeless._
import syntax.singleton._
import record._

val a = ("a" ->> 123) :: ("b"  ->> "xxx") :: HNil

a("a") // typed as an Int
b("b") // typed as a String

Answer 2

Maybe this is close to what you're after?

class A() {
  val a: Int = 123
  val b: String = "xxx"
}

def func(key: String, a: A): Either[Int,String] = {
  if (key == "a") Left(a.a)
  else            Right(a.b)
}

val a = new A()
func("a", a)  // Left(123)
func("b", a)  // Right("xxx")