+- Signs in Generic Declaration in Scala

Question

I was looking at the documentation for PartialFunction in this link:

trait PartialFunction[-A, +B] extends (A) ⇒ B

Maybe someone can help clari

Answer 1

It's covariance and contravariance. https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)

Basically it says for Generic types how inheritance will work. Easy sample from Scala is - trait Seq[+A] Because of the + , the code

val s: Seq[Person] = Seq[Student]()

will compile because Student extends Person. Without the + it won't work

A bit more complex sample -

class C[-A, +B] {
  def foo(param: A): B = ???
}

class Person(val name: String)

class Student(name: String, val university: String) extends Person(name)

val sample: C[Student, Person] = new C[Person, Student]