open import Cat.Diagram.Limit.Finite
open import Cat.Instances.Functor
open import Cat.Diagram.Terminal
open import Cat.Diagram.Product
open import Cat.Prelude

open import Data.Bool

open import Order.Base
open import Order.Cat

import Order.Reasoning as Poset

module Cat.Instances.Shape.Interval where

Interval category🔗

The interval category is the category with two points, called (as a form of endearment) 00 and 11, and a single arrow between them. Correspondingly, in shorthand this category is referred to as {01}{\{0 \le 1\}}. Since it has a single (non-trivial) arrow, it is a partial order; In fact, it is the partial order generated by the type of booleans and the natural ordering on them, with \bot \le \top.

open Precategory

Bool-poset : Poset lzero lzero
Bool-poset = to-poset Bool make-bool where
  R : Bool  Bool  Type
  R false false = 
  R false true  = 
  R true  false = 
  R true  true  = 

Note: We define the relation by recursion, rather than by induction, to avoid the issues with computational behaviour with indexed inductive types in Cubical Agda. The interval category is the category underlying the poset of booleans:

0≤1 : Precategory lzero lzero
0≤1 = poset→category Bool-poset

Meets🔗

Note that the category {01}{\{0 \le 1\}} is finitely complete (i.e. it is bounded, and has binary meets for every pair of elements): The top element is \top (go figure), and meets are given by the boolean “and” function”.

0≤1-top : Terminal 0≤1
0≤1-top .top = true

0≤1-top .has⊤ false .centre = tt
0≤1-top .has⊤ false .paths tt = refl

0≤1-top .has⊤ true  .centre = tt
0≤1-top .has⊤ true  .paths tt = refl

0≤1-products :  A B  Product 0≤1 A B
0≤1-products A B .apex = and A B
A ridiculous amount of trivial pattern matching is needed to establish that this cone is universal, but fortunately, we can appeal to thinness to establish commutativity and uniqueness.
0≤1-products false false .π₁ = tt
0≤1-products false true  .π₁ = tt
0≤1-products true  false .π₁ = tt
0≤1-products true  true  .π₁ = tt

0≤1-products false false .π₂ = tt
0≤1-products false true  .π₂ = tt
0≤1-products true  false .π₂ = tt
0≤1-products true  true  .π₂ = tt

0≤1-products A B .has-is-product .⟨_,_⟩ = meet _ _ _ where
  meet :  A B Q (p : Hom 0≤1 Q A) (q : Hom 0≤1 Q B)  Hom 0≤1 Q (and A B)
  meet false false false tt tt = tt
  meet false false true  () ()
  meet false true  false tt tt = tt
  meet false true  true  () tt
  meet true  false false tt tt = tt
  meet true  false true  tt ()
  meet true  true  false tt tt = tt
  meet true  true  true  tt tt = tt
0≤1-products A B .has-is-product .π₁∘factor = Poset.≤-thin Bool-poset _ _
0≤1-products A B .has-is-product .π₂∘factor = Poset.≤-thin Bool-poset _ _
0≤1-products A B .has-is-product .unique _ _ _ = Poset.≤-thin Bool-poset _ _

The space of arrows🔗

The total space of the Hom{\mathbf{Hom}} family of a precategory is referred to as its “space of arrows”. A point in this space is a “free-standing arrow”: it comes equipped with its own domain and codomain. We note that, since a precategory has no upper bound on the h-level of its space of objects, its space of arrows also need not be particularly truncated. However, for a thin category it is a set, and for a univalent category it is a groupoid.

An equivalent description of the space of arrows is as the collection of functors [{01},C][ {\{0 \le 1\}}, \mathcal{C} ]: a functor out of {01}{\{0 \le 1\}} corresponds rather directly to picking out an arrow in C\mathcal{C}. Its domain is the object that \bot maps to, and is codomain is the object that \top maps to.

Arrows : Precategory o   Type (o  )
Arrows C = Σ[ A  C.Ob ] Σ[ B  C.Ob ] (C.Hom A B)
  where module C = Precategory C

We now fix a category and prove the correspondence between the space of arrows Arr(C){ {\mathrm{Arr}}(\mathcal{C})}, as defined above, and the space of functors [{01},C][ {\{0 \le 1\}}, \mathcal{C} ].

module _ {C : Precategory o } where
  import Cat.Reasoning C as C

  arrow→functor : Arrows C  Functor 0≤1 C
  arrow→functor (A , B , f) = fun where
    fun : Functor _ _
    fun .F₀ false = A
    fun .F₀ true = B
    fun .F₁ {false} {false} tt = C.id
    fun .F₁ {false} {true}  tt = f
    fun .F₁ {true}  {false} ()
    fun .F₁ {true}  {true}  tt = C.id

The other direction, turning a functor into an object of Arr, is mostly immediate: we can extract the non-trivial arrow by seeing what the non-trivial arrow 010 \le 1 maps to, and the domain/codomain can be inferred by Agda.

  functor→arrow : Functor 0≤1 C  Arrows C
  functor→arrow F = _ , _ , F₁ F {false} {true} tt

That this function is an equivalence is also straightforward: The only non-trivial step is appealing to functoriality of FF, specifically that it must preserve identity arrows. The converse direction (going functor → arrow → functor) is definitionally the identity.

  arrow≃functor : is-equiv arrow→functor
  arrow≃functor = is-iso→is-equiv (iso functor→arrow rinv linv) where
    rinv : is-right-inverse functor→arrow arrow→functor
    rinv F =
      Functor-path
         { true  refl ; false  refl })
         { {false} {false} tt  sym (F-id F)
           ; {false} {true}  tt  refl
           ; {true}  {false} ()
           ; {true}  {true}  tt  sym (F-id F) })

    linv : is-left-inverse functor→arrow arrow→functor
    linv x = refl

Correspondingly, we define the arrow category Arr(C){ {\mathrm{Arr}}(\mathcal{C})} as the functor category [{01},C][ {\{0 \le 1\}}, \mathcal{C} ].

Arr : Precategory o   Precategory (o  ) 
Arr C = Cat[ 0≤1 , C ]