Variance

A good example, the book "Programming Scala" by Dean Wampler & Alex Payne. 

Page 283

trait Function2[-T1, -T2, +R] extends AnyRef

The last type parameter, +R, the is the return type. It is covariant. The leading two type

parameters are for the first and second function arguments, respectively. They are

contravariant.

 

class CSuper { def msuper() = println("CSuper") }
class C extends CSuper { def m() = println("C") }
class CSub extends C { def msub() = println("CSub") }

var f: C => C = (c: C) => new C //
f = (c: CSuper) => new CSub // c: CSuper is valid, because the argument
                            // C is contravariant, so CSuper is a valid substitution
                            // while the return value is covariant, so CSub is a valid replacement for C.
f = (c: CSuper) => new C    // Similar to the previous case, but we simply return a C.
f = (c: C) => new CSub      // Similar to the previous cases, but we simply pass a C.
f = (c: CSub) => new CSuper // An error! A function that takes a CSub argument is invalid, because of
                            // contravariance, and the return type CSuper is invalid due to covariance.