Scala如何在元组中指定元组的返回类型

Question

I am trying to specify a return type of tuple within a tuple:

  class First {
    def tupleReturnType(): (Any, Int) = {
      val tup = (1, 2, 3) // This can be variable in length
      val i = 4
      (tup, i)   
    }
  }

and call it for example:

  "First Test" must "understand tuple type" in {
    val first = new First()
    val (tup, i) = first.tupleReturnType()
    assert(tup == (1, 2, 3))
    assert(i == 4)   
  }

I can get this to compile using Any type but I would prefer something specific. Any suggestions? I've researched but not found this specific question elsewhere.

I tried () as a type but got a compile failure.

Answer 1

I believe this is changing in Scala 3 / Dotty, but for now, tuples are instances of TupleN class, with N = 1, 2, 3, ..., 22 . So in order to achieve what you want, you would need some kind of parent type which encapsulates all tuple types.

Here's what Tuple3 looks like:

final case class Tuple3[+T1, +T2, +T3](val _1 : T1, val _2 : T2, val _3 : T3) extends scala.AnyRef with scala.Product3[T1, T2, T3] with scala.Product with scala.Serializable {
  override def toString() : java.lang.String = { /* compiled code */ }
}

Here we can see that the closest common supertype of TupleN instances is scala.Product , so you could say:

def tupleReturnType(): (Product, Int)