open import Cat.Diagram.Limit.Base
open import Cat.Instances.Discrete
open import Cat.Diagram.Terminal
open import Cat.Prelude

module Cat.Diagram.Product.Indexed {o ℓ} (C : Precategory o ℓ) where

Indexed products🔗

If a category admits a terminal object and binary products, then it admits products of any finite cardinality: iterate the binary product, and use the terminal object when there aren’t any objects. However, these two classes of limit do not let us speak of products of arbitrary cardinality, or, in the univalent context, indexed by types which are not sets.

That’s where $I$-indexed products come in: Rather than having a functor giving the objects to take the limit over, we have an arbitrary map $F$ from $I$ to the space of objects of $\mathcal{C}$. An indexed product for this “diagram” is then an object admitting an universal family of maps $\pi_i : (\prod F) \to F_i$.

record is-indexed-product (F : Idx → C.Ob) (π : ∀ i → C.Hom P (F i))
  : Type (o ⊔ ℓ ⊔ level-of Idx) where
  no-eta-equality
  field
    tuple   : ∀ {Y} → (∀ i → C.Hom Y (F i)) → C.Hom Y P
    commute : ∀ {i} {Y} {f : ∀ i → C.Hom Y (F i)} → π i C.∘ tuple f ≡ f i
    unique  : ∀ {Y} {h : C.Hom Y P} (f : ∀ i → C.Hom Y (F i))
            → (∀ i → π i C.∘ h ≡ f i)
            → h ≡ tuple f

  eta : ∀ {Y} (h : C.Hom Y P) → h ≡ tuple λ i → π i C.∘ h
  eta h = unique _ λ _ → refl

  hom-iso : ∀ {Y} → C.Hom Y P ≃ (∀ i → C.Hom Y (F i))
  hom-iso = (λ f i → π i C.∘ f) , is-iso→is-equiv λ where
    .is-iso.inv → tuple
    .is-iso.rinv x → funext λ i → commute
    .is-iso.linv x → sym (unique _ λ _ → refl)

A category $\mathcal{C}$ admits indexed products (of level $\ell$) if, for any type $I : {{\mathrm{Type}}}\ \ell$ and family $F : I \to \mathcal{C}$, there is an indexed product of $F$.

record Indexed-product (F : Idx → C.Ob) : Type (o ⊔ ℓ ⊔ level-of Idx) where
  no-eta-equality
  field
    {ΠF}      : C.Ob
    π         : ∀ i → C.Hom ΠF (F i)
    has-is-ip : is-indexed-product F π
  open is-indexed-product has-is-ip public

has-indexed-products : ∀ ℓ → Type _
has-indexed-products ℓ = ∀ {I : Type ℓ} (F : I → C.Ob) → Indexed-product F

As limits🔗

In the particular case where $I$ is a groupoid, e.g. because it arises as the space of objects of a univalent category, an indexed product for $F : I \to \mathcal{C}$ is the same thing as a limit over $F$, considered as a functor ${\mathrm{Disc}}{I} \to \mathcal{C}$. We can not lift this restriction: If $I$ is not a groupoid, then its path spaces $x = y$ are not necessarily sets, and so the Disc construction does not apply to it.

module _ {I : Type ℓ'} (i-is-grpd : is-groupoid I) (F : I → C.Ob) where
  open Cone-hom
  open Terminal
  open Cone

  IP→Limit : Indexed-product F → Limit {C = C} (Disc-adjunct {iss = i-is-grpd} F)
  IP→Limit IP = lim where
    module IP = Indexed-product IP

    thelim : Cone _
    thelim .apex = IP.ΠF
    thelim .ψ = IP.π
    thelim .commutes {x} =
      J (λ y p → subst (C.Hom (F x) ⊙ F) p C.id C.∘ (IP.π x) ≡ IP.π y)
        (C.eliml (transport-refl _))

    lim : Limit _
    lim .top = thelim
    lim .has⊤ x .centre .hom = IP.tuple (x .ψ)
    lim .has⊤ x .centre .commutes o = IP.commute
    lim .has⊤ x .paths h = Cone-hom-path _ (sym (IP.unique _ λ i → h .commutes _))

module _ {I : Type ℓ'} (isg : is-groupoid I) (F : Functor (Disc I isg) C) where
  private module F = Functor F
  open is-indexed-product
  open Indexed-product
  open Cone-hom
  open Terminal
  open Cone

  Proj→Cone : ∀ {Y} → (∀ i → C.Hom Y (F.₀ i)) → Cone F
  Proj→Cone f .apex = _
  Proj→Cone f .ψ = f
  Proj→Cone f .commutes {x} =
    J (λ y p → F.₁ p C.∘ f x ≡ f y) (C.eliml F.F-id)

  Limit→IP : Limit {C = C} F → Indexed-product F.₀
  Limit→IP lim = the-ip where
    module lim = Cone (lim .top)

    the-ip : Indexed-product _
    the-ip .ΠF = lim.apex
    the-ip .π = lim.ψ
    the-ip .has-is-ip .tuple f = lim .has⊤ (Proj→Cone f) .centre .hom
    the-ip .has-is-ip .commute = lim .has⊤ (Proj→Cone _) .centre .commutes _
    the-ip .has-is-ip .unique {h = h} f p i =
      lim .has⊤ (Proj→Cone f) .paths other (~ i) .hom
      where
        other : Cone-hom _ _ _
        other .hom = h
        other .commutes i = p i

Uniqueness🔗

As is traditional with universal constructions, “having an indexed product for a diagram” is property of a category, not structure: Put another way, for any particular diagram, in a univalent category, there is (at most) a contractible space of indexed products of that diagram. And again as is traditional with universal constructions, the proof is surprisingly straightforward!

Indexed-product-unique
  : ∀ {ℓ′} {I : Type ℓ′} (F : I → C.Ob)
  → is-category C → is-prop (Indexed-product F)
Indexed-product-unique {I = I} F c-cat x y = p where
  module x = Indexed-product x
  module y = Indexed-product y

All we have to do — “all” — is exhibit an isomorphism between the apices which commutes with the projection function in one direction, and with the product with morphisms in the other. That’s it! The isomorphism is induced by the universal properties, and is readily seen to commute with both projections:

  apices : x.ΠF C.≅ y.ΠF
  apices = C.make-iso (y.tuple x.π) (x.tuple y.π)
    ( y.unique y.π (λ i → C.pulll y.commute ∙ x.commute)
    ∙ sym (y.unique y.π λ i → C.idr _) )
    ( x.unique x.π (λ i → C.pulll x.commute ∙ y.commute)
    ∙ sym (x.unique x.π λ i → C.idr _))

By the characterisation of paths-over in Hom-sets, we get paths-over between the projection maps and the product maps:

  module apices = C._≅_ apices
  abstract
    pres : ∀ j → PathP (λ i → C.Hom (c-cat .to-path apices i) (F j)) (x.π j) (y.π j)
    pres j = Univalent.Hom-pathp-refll-iso c-cat x.commute

    pres′ : ∀ {Y} (f : ∀ j → C.Hom Y (F j))
      → PathP (λ i → C.Hom Y (c-cat .to-path apices i)) (x.tuple f) (y.tuple f)
    pres′ f =
      Univalent.Hom-pathp-reflr-iso c-cat (y.unique f λ j → C.pulll y.commute ∙ x.commute)

And after some munging (dealing with the axioms), that’s exactly what we need to prove that indexed products are unique.

  open Indexed-product
  open is-indexed-product
  p : x ≡ y
  p i .ΠF = c-cat .to-path apices i
  p i .π j = pres j i
  p i .has-is-ip .tuple f = pres′ f i
  p i .has-is-ip .commute {i = j} {f = f} =
    is-prop→pathp (λ i → C.Hom-set _ (F j) (pres j i C.∘ pres′ f i) _)
     (x .has-is-ip .commute) (y .has-is-ip .commute) i
  p i .has-is-ip .unique {h = h} f =
    is-prop→pathp
      (λ i → Π-is-hlevel {A = C.Hom _ (c-cat .to-path apices i)} 
       λ h → Π-is-hlevel {A = ∀ j → pres j i C.∘ h ≡ f j} 
       λ p → C.Hom-set _ (c-cat .to-path apices i) h (pres′ f i))
      (λ h → x.unique {h = h} f) (λ h → y.unique {h = h} f) i h