Contravariance Example

contravariance.scala

/*
Invariant:
String is subclass of Object but in java
List<String> is not a subclass of List<Object>


Covariant
String in scala is a subclass of Object
List[String] is a subclass of List[Object]


Contravariant
Will accept types that are super classes of
the proposed type.
class X[-A]
val x: X[String] = new X[Object]
*/

//cool, but why:

/*** Covariant FAIL ***/

  class Output[+A]{def write(a:A) = /* Do some writing */}
  
  def writeObject(out: Output[Object]) = out.write("hello")
  
  /*
    The code below fails because as defined it only
    accepts types covariant to List[String]. The call to
    "write" attempts to pass a Object[Object] which is
    itself covariant to Output[List[String]] but its the 
    call write that fails because the type passed is defined
    as Output[List[String]] and you are attempting to pass
    it a String.

    Right now we have this resolution:
    writeObject(accepts anything covariant to Object)
    write(accepts anything covariant to List[String])

    
  */
  writeObject(new Output[List[String]])
  
/********************/

/** Contravariant Solution ***/ 
  class Output[-A]{def write(a:A) = /* Do some writing */}
  def writeObject(out: Output[Object]) = out.write("hello")

  //This now causes type mismatch
  //writeObject(new Output[List[String]])
  writeObject(new Output[Any])


  /*
    This now works because write will accept any object that is a 
    super class of Object and write accepts anything of type a
    so you end up with this resolution:
    writeObject(accepts anything that is a supertype to Object)
    write(accepts anything that is a supertype to Object)
  */
   
/******************************/