module Cat.Functor.Naturality where
Working with natural transformations🔗
Working with natural transformations can often be more cumbersome than working directly with the underlying families of morphisms; moreover, we often have to convert between a property of natural transformations and a (universally quantified) property of the underlying morphisms. This module collects some notation that will help us with that task.
module _ {o ℓ o' ℓ'} {C : Precategory o ℓ} {D : Precategory o' ℓ'} where private module DD = Cat.Reasoning Cat[ D , D ] module CD = Cat.Reasoning Cat[ C , D ] module D = Cat.Reasoning D module C = Cat.Reasoning C open Functor open _=>_
We’ll refer to the natural-transformation versions of predicates on
morphisms by a superscript ⁿ. A natural
isomorphism is simply an isomorphism in a functor category.
Inversesⁿ : {F G : Functor C D} → F => G → G => F → Type _ Inversesⁿ = CD.Inverses is-invertibleⁿ : {F G : Functor C D} → F => G → Type _ is-invertibleⁿ = CD.is-invertible _≅ⁿ_ : Functor C D → Functor C D → Type _ F ≅ⁿ G = CD.Isomorphism F G
module Inversesⁿ {F G : Functor C D} {α : F => G} {β : G => F} (inv : Inversesⁿ α β) = CD.Inverses inv module is-invertibleⁿ {F G : Functor C D} {α : F => G} (inv : is-invertibleⁿ α) = CD.is-invertible inv module Isoⁿ {F G : Functor C D} (eta : F ≅ⁿ G) = CD._≅_ eta idni : ∀ {F} → F ≅ⁿ F idni = CD.id-iso _∘ni_ : ∀ {F G H} → G ≅ⁿ H → F ≅ⁿ G → F ≅ⁿ H _∘ni_ = CD._∘Iso_ _∙ni_ : ∀ {F G H} → F ≅ⁿ G → G ≅ⁿ H → F ≅ⁿ H f ∙ni g = g ∘ni f _ni⁻¹ : ∀ {F G} → F ≅ⁿ G → G ≅ⁿ F _ni⁻¹ = CD._Iso⁻¹ infixr 30 _∘ni_ _∙ni_ infix 31 _ni⁻¹ ≅ⁿ-pathp : ∀ {a c b d : Functor C D} (p : a ≡ c) (q : b ≡ d) {f : a ≅ⁿ b} {g : c ≅ⁿ d} → (∀ x → PathP (λ i → D.Hom (p i .F₀ x) (q i .F₀ x)) (Isoⁿ.to f .η x) (Isoⁿ.to g .η x)) → PathP (λ i → p i CD.≅ q i) f g ≅ⁿ-pathp p q r = CD.≅-pathp p q (Nat-pathp p q r)
A fundamental lemma that will let us work with natural isomorphisms more conveniently is the following: if is a natural transformation which is componentwise inverted by then is itself a natural transformation This means that, when constructing a natural isomorphism from scratch, we only have to establish naturality in one direction, rather than both.
inverse-is-natural : ∀ {F G : Functor C D} (α : F => G) (β : ∀ x → D.Hom (G .F₀ x) (F .F₀ x) ) → (∀ x → α .η x D.∘ β x ≡ D.id) → (∀ x → β x D.∘ α .η x ≡ D.id) → is-natural-transformation G F β inverse-is-natural {F = F} {G = G} α β invl invr x y f = β y D.∘ G .F₁ f ≡⟨ D.refl⟩∘⟨ D.intror (invl x) ⟩≡ β y D.∘ G .F₁ f D.∘ α .η x D.∘ β x ≡⟨ D.refl⟩∘⟨ D.extendl (sym (α .is-natural x y f)) ⟩≡ β y D.∘ α .η y D.∘ F .F₁ f D.∘ β x ≡⟨ D.cancell (invr y) ⟩≡ F .F₁ f D.∘ β x ∎
We can then create a natural isomorphism from the following data:
record make-natural-iso (F G : Functor C D) : Type (o ⊔ ℓ ⊔ ℓ') where no-eta-equality field eta : ∀ x → D.Hom (F .F₀ x) (G .F₀ x) inv : ∀ x → D.Hom (G .F₀ x) (F .F₀ x) eta∘inv : ∀ x → eta x D.∘ inv x ≡ D.id inv∘eta : ∀ x → inv x D.∘ eta x ≡ D.id natural : ∀ x y f → G .F₁ f D.∘ eta x ≡ eta y D.∘ F .F₁ f open make-natural-iso to-natural-iso : ∀ {F G} → make-natural-iso F G → F ≅ⁿ G to-natural-iso mk .Isoⁿ.to .η = mk .eta to-natural-iso mk .Isoⁿ.to .is-natural x y f = sym (mk .natural x y f) to-natural-iso mk .Isoⁿ.from .η = mk .inv to-natural-iso {F = F} {G} mk .Isoⁿ.from .is-natural = inverse-is-natural {F} {G} (NT _ (λ x y f → sym (mk .natural x y f))) (mk .inv) (mk .eta∘inv) (mk .inv∘eta) to-natural-iso mk .Isoⁿ.inverses .Inversesⁿ.invl = ext (mk .eta∘inv) to-natural-iso mk .Isoⁿ.inverses .Inversesⁿ.invr = ext (mk .inv∘eta)
Moreover, the following family of functions project out the componentwise invertibility, resp. componentwise isomorphism, associated to an invertible natural transformation, resp. natural isomorphism.
is-invertibleⁿ→is-invertible : ∀ {F G} {α : F => G} → is-invertibleⁿ α → ∀ x → D.is-invertible (α .η x) is-invertibleⁿ→is-invertible inv x = D.make-invertible (CD.is-invertible.inv inv .η x) (CD.is-invertible.invl inv ηₚ x) (CD.is-invertible.invr inv ηₚ x) isoⁿ→iso : ∀ {F G} → F ≅ⁿ G → ∀ x → F .F₀ x D.≅ G .F₀ x isoⁿ→iso α x = D.make-iso (α.to .η x) (α.from .η x) (α.invl ηₚ x) (α.invr ηₚ x) where module α = Isoⁿ α iso→isoⁿ : ∀ {F G} → (is : ∀ x → F .F₀ x D.≅ G .F₀ x) → (∀ {x y} f → G .F₁ f D.∘ is x .D.to ≡ is y .D.to D.∘ F .F₁ f) → F ≅ⁿ G iso→isoⁿ {F} {G} is nat = to-natural-iso mk where mk : make-natural-iso F G mk .eta x = is x .D.to mk .inv x = is x .D.from mk .eta∘inv x = is x .D.invl mk .inv∘eta x = is x .D.invr mk .natural _ _ = nat is-invertibleⁿ→isoⁿ : ∀ {F G} {α : F => G} → is-invertibleⁿ α → F ≅ⁿ G is-invertibleⁿ→isoⁿ nat-inv = CD.invertible→iso _ nat-inv isoⁿ→is-invertible : ∀ {F G} (α : F ≅ⁿ G) → ∀ x → D.is-invertible (α .Isoⁿ.to .η x) isoⁿ→is-invertible α x = D.iso→invertible (isoⁿ→iso α x)
to-inversesⁿ : {F G : Functor C D} {α : F => G} {β : G => F} → (∀ x → α .η x D.∘ β .η x ≡ D.id) → (∀ x → β .η x D.∘ α .η x ≡ D.id) → Inversesⁿ α β to-inversesⁿ p q = CD.make-inverses (ext p) (ext q) to-is-invertibleⁿ : {F G : Functor C D} {α : F => G} → (β : G => F) → (∀ x → α .η x D.∘ β .η x ≡ D.id) → (∀ x → β .η x D.∘ α .η x ≡ D.id) → is-invertibleⁿ α to-is-invertibleⁿ β p q = CD.make-invertible β (ext p) (ext q) inversesⁿ→inverses : ∀ {F G} {α : F => G} {β : G => F} → Inversesⁿ α β → ∀ x → D.Inverses (α .η x) (β .η x) inversesⁿ→inverses inv x = D.make-inverses (CD.Inverses.invl inv ηₚ x) (CD.Inverses.invr inv ηₚ x) isoⁿ→is-invertibleⁿ : ∀ {F G : Functor C D} (i : F ≅ⁿ G) → is-invertibleⁿ (Isoⁿ.to i) isoⁿ→is-invertibleⁿ i = CD.iso→invertible i invertible→invertibleⁿ : ∀ {F G} (eta : F => G) → (∀ x → D.is-invertible (eta .η x)) → is-invertibleⁿ eta invertible→invertibleⁿ eta i = to-is-invertibleⁿ ate (λ x → D.is-invertible.invl (i x)) λ x → D.is-invertible.invr (i x) where ate : _ => _ ate .η x = D.is-invertible.inv (i x) ate .is-natural = inverse-is-natural eta _ (λ x → D.is-invertible.invl (i x)) (λ x → D.is-invertible.invr (i x)) push-eqⁿ : ∀ {F G} (α : F ≅ⁿ G) {a b} {f g : C.Hom a b} → F .F₁ f ≡ F .F₁ g → G .F₁ f ≡ G .F₁ g push-eqⁿ {F = F} {G = G} α {f = f} {g} p = G .F₁ f ≡⟨ D.insertl (α .Isoⁿ.invl ηₚ _) ⟩≡ α .Isoⁿ.to .η _ D.∘ α .Isoⁿ.from .η _ D.∘ G .F₁ f ≡⟨ D.refl⟩∘⟨ α .Isoⁿ.from .is-natural _ _ _ ⟩≡ α .Isoⁿ.to .η _ D.∘ F .F₁ f D.∘ α .Isoⁿ.from .η _ ≡⟨ D.refl⟩∘⟨ p D.⟩∘⟨refl ⟩≡ α .Isoⁿ.to .η _ D.∘ F .F₁ g D.∘ α .Isoⁿ.from .η _ ≡˘⟨ D.refl⟩∘⟨ α .Isoⁿ.from .is-natural _ _ _ ⟩≡˘ α .Isoⁿ.to .η _ D.∘ α .Isoⁿ.from .η _ D.∘ G .F₁ g ≡⟨ D.cancell (α .Isoⁿ.invl ηₚ _) ⟩≡ G .F₁ g ∎