open import Cat.Prelude

module Cat.Instances.Sets where

The category of Sets🔗

We prove that the category of Sets is univalent. Recall that this means that, fixing a set $A$, the type $\sum_{(B : \mathrm{Set})} (A \cong B)$ is contractible. We first exhibit a contraction directly, using ua, and then provide an alternative proof in terms of univalence for $n$-types.

Direct proof🔗

The direct proof is surprisingly straightforward, in particular because the heavy lifting is done by a plethora of existing lemmas: Iso→Equiv to turn an isomorphism into an equivalence, path→ua-pathp to reduce dependent paths over ua to non-dependent paths, ≅-pathp to characterise dependent paths in _≅_, etc.

module _ {ℓ} where
  import Cat.Reasoning (Sets ℓ) as Sets

We must first rearrange _≅_ to _≃_, for which we can use Iso→Equiv. We must then show that an isomorphism in the category of Sets is the same thing as an isomorphism of types; But the only difference between these types can be patched by happly/funext.

  iso→equiv : {A B : Set ℓ} → A Sets.≅ B → ∣ A ∣ ≃ ∣ B ∣
  iso→equiv x .fst = x .Sets.to
  iso→equiv x .snd = is-iso→is-equiv $ iso x.from (happly x.invl) (happly x.invr)
    where module x = Sets._≅_ x

Using univalence for $n$-types, function extensionality and the computation rule for univalence, it is almost trivial to show that categorical isomorphisms of sets are an identity system.

  Sets-is-category : is-category (Sets ℓ)
  Sets-is-category .to-path i = n-ua (iso→equiv i)
  Sets-is-category .to-path-over p = Sets.≅-pathp refl _ $
    funextP λ a → path→ua-pathp _ refl

Indirect proof🔗

While the proof above is fairly simple, we can give a different formulation, which might be more intuitive. Let’s start by showing that the rearrangement iso→equiv is an equivalence:

  equiv→iso : {A B : Set ℓ} → ∣ A ∣ ≃ ∣ B ∣ → A Sets.≅ B
  equiv→iso (f , f-eqv) =
    Sets.make-iso f (equiv→inverse f-eqv)
      (funext (equiv→counit f-eqv))
      (funext (equiv→unit f-eqv))

  equiv≃iso : {A B : Set ℓ} → (A Sets.≅ B) ≃ (∣ A ∣ ≃ ∣ B ∣)
  equiv≃iso {A} {B} = Iso→Equiv (iso→equiv , iso equiv→iso p q)
    where
      p : is-right-inverse (equiv→iso {A} {B}) iso→equiv
      p x = Σ-prop-path is-equiv-is-prop refl

      q : is-left-inverse (equiv→iso {A} {B}) iso→equiv
      q x = Sets.≅-pathp refl refl refl

We then use univalence for $n$-types to directly establish that $(A \equiv B) \simeq (A \cong B)$:

  is-category'-Sets : ∀ {A B : Set ℓ} → (A ≡ B) ≃ (A Sets.≅ B)
  is-category'-Sets {A} {B} =
    (A ≡ B)         ≃⟨ n-univalence e⁻¹ ⟩≃
    (∣ A ∣ ≃ ∣ B ∣) ≃⟨ equiv≃iso e⁻¹ ⟩≃
    (A Sets.≅ B)    ≃∎