variance

FunctionCoContraVariance.scala

//Big rule → Functions
//“Be liberal in what you accept from others” 
//“Be conservative in what you do” 

//type Function[-A, +B] = Function1[A, B] 
//the inputs of functions are contravariant and output are covariant.

//method arguments are in CONTRAVARIANT position -A (more general input argument)
//return types are in COVARIANT position +B  (more specific return type)

//In most cases, 
//A will be the type parameter of the class
//B will be the type parameter of a method.

//Errors
//Covariant Class
//Covariant type A occurs in contravariant position in type A of value C

//ContraVariant Class
//Contravariant type A occurs in covariant position in type A of value C

//Covariant Class
//Convert input to Contravariant using lower type bounds >:
//Make input the SuperType ( make general)
//Widen the Type

class Sounds3[+A]{
 def sound[C >:A](animal:C) = animal
}

val a3:Sounds3[Cat] = new Sounds3[Tiger]


//ContraVariant Class
//Convert output Covariant using Upper Type Bounds <: 
//Make input SubType ( make specific) 
//Narrowing the Type

class Sounds4[-A]{
 def sound[C <:A](animal:C) = animal
}

val a4:Sounds4[Tiger] = new Sounds4[Cat]