Scala generics, "simplified", covariance and contravariance.

co.reasoning.scala

// This is an example of how to reason about covariance vs contravariance.

// Between the two co*.reasoning.scala examples, the following compiles but meaning is slightly different due to the class hierarchy.

	type SHORT = A[SMALL]
	type INT = A[MEDIUM]
	type LONG = A[LARGE]
	
	val s:SHORT = new SHORT()
	val i:INT = s
	val l:LONG = i      

contravariance.reasoning.scala

// Compare this to the covariance.reasoning.scala example. Notice the class hierarchy.

	trait SMALL
	trait MEDIUM extends SMALL
	trait LARGE extends MEDIUM

	class A[-T]

// See co.reasoning.scala

covariance.reasoning.scala

// Compare this to the contravariance.reasoning.scala example. Notice the class hierarchy.

	trait LARGE
	trait MEDIUM extends LARGE
	trait SMALL extends MEDIUM
	
	class A[+T]

// See co.reasoning.scala

type.constraints.scala

	class A
	class B extends A
	class C extends B

	// Upper Bounds
	class D[T <: B]  // T must be a B. i.e. for any T, it is a B or a subclass of B.
	
	// new D[A]() // <- type error
	new D[B]()
	new D[C]()

	// Lower Bounds
	class E[T >: B] // B must be a T. i.e. for any T, B must also be that same type.

	new E[A]()
	new E[B]()
	// new E[C]() // <- type error
	

type.contravariance.scala

	trait X
	trait Y extends X
	trait Z extends Y

	// Contravariance
	class A[-T]  // contravariant in T

	// Given:
	val ax : A[X] = new A[X]()
	
	// You can assign any A[T] to an A[Y] as long as T >: Y.
	// i.e. if Y is a T, an A[Y] is an A[T]

	val ayx : A[Y] = ax

	// And you can assign any A[T] to an A[Z] as long as T >: Z.
	// i.e. if T is a Z, an A[Z] is an A[T]

	val azx : A[Z] = ax
	val azy : A[Z] = ayx

type.covariance.scala

	trait X
	trait Y extends X
	trait Z extends Y

	// Covariance
	class A[+T]  // covariant in T

	// Given:
	val az : A[Z] = new A[Z]()

	// You can assign any A[T] to an A[Y] as long as T <: Y.
	// i.e. if T is a Y, any A[T] is an A[Y]

	val ayz : A[Y] = az
	
	// You can assign any A[T] to axy and axz as long as T <: X.
	// i.e. if T is an X, any A[T] is an A[X]

	val axy : A[X] = ayz
	val axz : A[X] = az

nice and concise explanation to generics in Scala, thanks a lot!!