Variance, bottom types and type inference in Scala

gistfile1.scala

/*****************************
  * Invariant type with Nothing *
  ****************************/
  
sealed trait Interact[A]
case class Ask(value: String) extends Interact[String]
case class Tell(value: String) extends Interact[Nothing] // Notice Nothing here

val instr = 
  List(Ask("What's your first name?"),
       Ask("What's your last name?"),
       Tell("Hello, ???"))
       
instr: List[Product with Serializable with Interact[_ <: String]] = ???
------------------------------------------------------------------------------------
/*****************************
  * Invariant type with Unit *
  ****************************/
sealed trait Interact[A]
case class Ask(value: String) extends Interact[String]
case class Tell(value: String) extends Interact[Unit] // And Unit here

val instr = 
  List(Ask("What's your first name?"),
       Ask("What's your last name?"),
       Tell("Hello, ???"))
       
instr: List[Product with Serializable with Interact[_ >: Unit with String]] = ???
------------------------------------------------------------------------------------
/********************************
  * Covariant type with Nothing *
  *******************************/
sealed trait Interact[+A]
case class Ask(value: String) extends Interact[String]
case class Tell(value: String) extends Interact[Nothing]

val instr = 
  List(Ask("What's your first name?"),
       Ask("What's your last name?"),
       Tell("Hello, ???"))
       
instr: List[Product with Serializable with Interact[String]] = ???       
------------------------------------------------------------------------------------
/*****************************
  * Covariant type with Unit *
  ****************************/
sealed trait Interact[+A]
case class Ask(value: String) extends Interact[String]
case class Tell(value: String) extends Interact[Unit]  

val instr = 
  List(Ask("What's your first name?"),
       Ask("What's your last name?"),
       Tell("Hello, ???"))
       
instr: List[Product with Serializable with Interact[Any]] = ???
------------------------------------------------------------------------------------
/************************************
  * Contravariant type with Nothing *
  ***********************************/
sealed trait Interact[-A]
case class Ask(value: String) extends Interact[String]
case class Tell(value: String) extends Interact[Nothing]

val instr = 
  List(Ask("What's your first name?"),
       Ask("What's your last name?"),
       Tell("Hello, ???"))
   
instr: List[Product with Serializable with Interact[Nothing]] = ???   
------------------------------------------------------------------------------------
/************************************
  * Contravariant type with Unit *
  ***********************************/
sealed trait Interact[-A]
case class Ask(value: String) extends Interact[String]
case class Tell(value: String) extends Interact[Unit]

val instr = 
  List(Ask("What's your first name?"),
       Ask("What's your last name?"),
       Tell("Hello, ???"))       
       
instr: List[Product with Serializable with Interact[Unit with String]] = ???
  
  

// Example was taken from Runar's talk- http://www.parleys.com/play/53a7d2c3e4b0543940d9e538