Why Scala classes cannot have parameter of type object in contructor?

Question

As far as I understood, object is simply singleton instance. I would like to create enumeration type with cell types, and then class Cell with constructor containing coordinates and enumeration type. However, it cannot be done, and I cannot find out why.

object CellType extends Enumeration {
  val Empty, X, O = Value
}

In another file:

class Cell(column: Int, row: Int, cellType : CellType) {
} // CellType is an object, and it doesn't work!

Do you have any ideas how to do it, or at least a reason why Scala forbids objects in contructors?

Error message:

Warning:(3, 37) imported 'CellType' is permanently hidden by definition of object CellType in package model import de.htwg.model.CellType

Answer 1

CellType is not a type, it's an object. It is actually the only object of its type, CellType.type . If you want to have a function argument that can only be this object, you may use an argument of type CellType.type . But then, why even bother putting it as an argument, if it can only be CellType ? Might as well use the object directly, not passing it as an argument.

What you probably want is not the type of CellType , but the type of its enumerated values, which happens to be CellType.Value .

NB: I personally found rather disturbing that the scala language defines on the same level trait s and class es, which are types, and object s, which are instances of a singleton type. You should not be misguided by this apparent analogy, and really consider objects as values, and classes and traits as types.