Question

I have the following code:

trait CellT[VAL <: AnyVal] {
  var value: VAL
  type NEXT <: AnyVal
  var next: CellT[NEXT]
}

abstract class Cell[VAL <: AnyVal] extends CellT[VAL] {
  var next = this  // defaults to `this`. ERROR
}

// impl example:
class CellInt extends Cell[Int] {
  var value: Int = 0
}

The error says that

overriding variable next in trait CellT of type
Cell[Cell.this.NEXT]; variable next has
incompatible type

Here it is obvious that this will have the type VAL <: AnyVal which is the same as NEXT <: AnyVal, however, I still get the error. How can I tell Scala that next should be able to return anything of type Cell[A <: AnyVal], but this type should not be necessary the same as the class type parameter [VAL <: AnyVal]??? Otherwise I could just use [VAL] but it will be too restrictive, for example for Cell[Int] it will restrict the next method to return only Cell[Int] type instances. But I want the next to be reassignable to any other Cell[*] types instances.

Answer 1

If you want next to be reassignable to any other Cell[*] types instances, you can use existential types:

trait CellT[VAL <: AnyVal] {
  var value: VAL
  var next: CellT[_ <: AnyVal]
}

abstract class Cell[VAL <: AnyVal] extends CellT[VAL] {
  var next: CellT[_ <: AnyVal] = this // defaults to `this`
  override def toString = "Cell(%s,%s)".format(value, if(next == this) "this" else next.toString)
}

// impl example:
class CellInt extends Cell[Int] {
  var value: Int = 0
}

class CellBoolean extends Cell[Boolean] {
    var value = false
}

var c1 = new CellInt
var c2 = new CellBoolean

println(c1)
c1.value = 1
println(c1)
c1.next = c2
println(c1)

println(c2)
c2.value = true
println(c1)
println(c2)

Output:

Cell(0,this)
Cell(1,this)
Cell(1,Cell(false,this))
Cell(false,this)
Cell(1,Cell(true,this))
Cell(true,this)