Let's see what is Covariance.
We are going to learn what it is and when to use it.
So, what is variance ? This is a concept related to types and how they are connected to each other. It is about inheritance. We saw before the relationship between a parent class and a child class. In functional programming we also talk about sub-type.
You can see in the start of the exercise what we encountered before related to parent and child class with inheritance.
Then we use a “normal” generic class without the covariance. You see that using a
F[A] as a
F[B] do not with and you are getting
type mismatch with some explanation :
found F[A] , required F[B]. This is expected, the types are different even tho they share a common parent, they are different.
However, why couldn't you use a
F[A] as a
P is the parent of
A ? This time the error is a lot more detailed:
type mismatch; found : F[A] required: F[P] Note: A <: P, but trait F is invariant in type A. You may wish to define A as +A instead. (SLS 4.5)
We recognize one symbol we saw before:
<: which talks about the relationship between the two types.
In the last part of the exercise you make a covariant trait using the syntax:
+A means that the trait is now covariant for
And now, what was failing before is working. We are able to use
Hope that was of help to understand what the
+A means when you encounter it and how to solve some exception you might have seen ! See you next time.