Two lists with distinct types, both types having their unique definition of object identity. Find all id:s which exist in both lists or only in the latter.

worksheet.scala

class Ident(identity:Int) {
  def ident = identity
  override def toString = "[id:"+identity+"]"
}

class Id(identity:Int) {
  def id = identity
  override def toString = "[id:"+identity+"]"
}

object diffAndDistinctOfTypes {

  val idents = List(new Ident(1), new Ident(2), new Ident(3)) //> idents  : List[Ident] = List([id:1], [id:2], [id:3])
  val ids = List(new Id(3), new Id(1), new Id(6))             //> ids  : List[Id] = List([id:3], [id:1], [id:6])
 
  def filterLists[IdentityA, IdentityB](
    aList:List[IdentityA],
    bList:List[IdentityB]
  )(distinctEquality:(IdentityA,IdentityB) => Boolean) = {
    aList.filter( ident => {
      bList.filter(id => distinctEquality(ident, id)).length > 0
    })
  }
  
  def inBoth = filterLists(idents, ids) {(a:Ident, b:Id) =>
    a.ident == b.id
  }.collect { case id =>
    id.ident
  }                                               
  
  def onlyInB = filterLists(ids, idents) {(a:Id, b:Ident) =>
    b.ident != a.id
  }.collect { case id =>
    id.id
  }                                               

  val result = inBoth ++ onlyInB distinct                     //> result  : List[Int] = List(1, 3, 6)
}