Question

scala> sealed trait Gender
defined trait Gender

scala> case object Male extends Gender
defined module Male

scala> case object Female extends Gender
defined module Female

scala> Map(Male -> Male, Female -> Female, Male -> Female, Female -> Male)
res2: scala.collection.immutable.Map[Product with Gender,Product with Gender] =
        Map((Male,Female), (Female,Male))

Why in the above code, the type of res2 is Map[Product with Gender, Product with Gender] instead of Map[Gender, Gender]? And why of the four entries I supplied to map, only two got added?

Answer 1

1) Product is the lowest-level superclass of both Male and Female. This is because all case classes extend Product. Both have trait Gender, which Scala recognizes, so it includes that too. This is Scala’s best guess for type inference because it is the most specific type inferable (Map[Gender,Gender] is more general). If you want the type to be Map[Gender,Gender], you can tell it that explicitly:

scala> val x: Map[Gender,Gender] = Map(Male -> Male, Female -> Female, Male -> Female, Female -> Male)
x: Map[Gender,Gender] = Map(Male -> Female, Female -> Male)

2) The nature of a Map is that it has only one value for each key. When you add Male -> Male you are mapping Male to Male. When you add Male -> Female you are overwriting the original mapping so that now Male maps to Female. If you want to have all of these mappings, then it’s probably easier to just to a list of pairs since that doesn’t enforce any uniqueness constraints.