Scala类型系统

contravariant.scala

class Pen {
  def getInfo = "a pen"
}

class Chalk extends Pen {
  override def getInfo = "a pencil that can be erased by eraser"
}

class WhiteBoardPen extends Pen {
  override def getInfo = "a white board pen that can write on window wall"
}

class Board[-T] {
  def show[U <: T](pen: U) = pen match {
    case p: Pen => println(s"writing with ${p.getInfo}")
    case _ => println("can not write")
  }
}

class Teacher[T] {
  def teachLessons(board: Board[T], pen: T) = board.show(pen)
}

val teacher = new Teacher[Chalk]

teacher.teachLessons(new Board[Pen], new Chalk)

covariation.scala

class ProgrammingLanguage {
  def getInfo = "ProgrammingLanguage"
}

class ObjectOrientedLanguage extends ProgrammingLanguage {
  override def getInfo = "SmallTalk"
}

class Java extends ProgrammingLanguage {
 override def getInfo = "Java"
}

class Book[+T] (val content: T)

def read(a: Book[ProgrammingLanguage]) = {
  println(s"""reading a book of ${a.content.getInfo}""")
}

val pl = new Book[ProgrammingLanguage](new ProgrammingLanguage)
val java = new Book[Java](new Java)

val smallTalk = new Book[ObjectOrientedLanguage](new ObjectOrientedLanguage)

read(pl)


read(java)


read(smallTalk)

non-variation.scala

class ProgrammingLanguage {
  def getInfo = "ProgrammingLanguage"
}

class ObjectOrientedLanguage extends ProgrammingLanguage {
  override def getInfo = "SmallTalk"
}

class Java extends ProgrammingLanguage {
 override def getInfo = "Java"
}

class Book[T] (val content: T)

def read(a: Book[ProgrammingLanguage]) = {
  println(s"""reading a book of ${a.content.getInfo}""")
}

val pl = new Book[ProgrammingLanguage](new ProgrammingLanguage)
val java = new Book[Java](new Java)

val smallTalk = new Book[ObjectOrientedLanguage](new ObjectOrientedLanguage)

read(pl)


read(java)


read(smallTalk)