funktoriell

Funktoriell is a term used in Swedish, Norwegian and related Scandinavian languages to describe properties or constructions that are functorial. In category theory, a functor provides a structured way to translate objects and arrows from one category to another, preserving the essential relationships between them.

Formally, a functor F: C → D assigns to every object X in C an object F(X) in D and to every morphism f: X → Y a morphism F(f): F(X) → F(Y). It preserves identities and composition: F(id_X) = id_{F(X)} and F(g ∘ f) = F(g) ∘ F(f) for all composable morphisms f, g. When these assignments respect the direction of morphisms, the functor is covariant; if they reverse direction (using the opposite category), the functor is contravariant.

Common examples illustrate funktoriell behavior. The power-set functor P: Set → Set sends a function f: X → Y to P(f): P(X) → P(Y) by mapping a subset S ⊆ X to its image f[S]. The Hom-functor Hom(A, -): Set → Set maps X to Hom(A, X) and f: X → Y to composition with f, providing a covariant example in the second argument. Other functorial constructions include forgetful functors from algebraic structures to sets and product or tensor constructions that assemble new objects from existing ones.

In programming languages influenced by category theory, such as Haskell, a Functor is a type constructor equipped with a map function that preserves identity and composition, enabling generic programming over data structures. Funktoriell concepts form the basis for advanced notions such as natural transformations and adjunctions, which study how different functors relate while preserving structure.