Notes - Imperative Programming TT23, Type variance
Flashcards
What does the notation
A <: B
mean in Scala?
A <: B
$A$ is a subtype of $B$.
What is the notation for a covariant type $T$ in Scala?
+T
What is the notation for a contravariant type $T$ in Scala?
-T
What does it mean for a type to be invariant?
It is neither covariant or contravariant.
What does it mean for a type $T$ to be covariant?
Whenever A <: B, we have T[A] <: T[B].
What does it mean for a type $T$ to be contravariant?
Whenever A <: B, we have T[B] <: T[A].
Can you state the Liskov substitution principle?
If S <: T then everything you can do with a value of type T, you should also be able to do with a value of type S.