Origin of The Terms
The origin of these terms is in category theory, where the types in the type system form a category C, in which morphisms represent the subtype relationship.
Defining a function that accepts type p and returns type r creates a new type p → r in the type system; the new function name is associated with that type. This function definition operator is actually a functor F : C × C → C that creates the said type. From the substitution principle above, this functor must be contravariant in the first argument and covariant in the second.
Read more about this topic: Covariance And Contravariance (computer Science)
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