<@U83GT69BJ> A functor F from category C1 to C2 by...

# functional

[tschuchort]

@tmg A functor F from category C1 to C2 by definition has to map every morphism A -> B for all objects A, B in C1 to a morphism F<A> -> F<B> in C2. Thus, only morphisms from an object to itself are allowed to exist in C1. We need to restrict the category C1 (which is usually the category of Kotlin types and functions between them) to a subcategory of Kotlin types and only functions from a type to itself.