module Cat.Instances.Shape.Two where

The two-object category🔗

We define the discrete category $2$ on two objects, which is useful for expressing binary products and coproducts as limits and colimits, respectively.

2-object-category : Precategory _ _
2-object-category = Disc' (el! Bool)

A diagram of shape $2$ in $C$ simply consists of two objects of $C .$

module _ {o h} {C : Precategory o h} where
  open Precategory C

  2-object-diagram : Ob → Ob → Functor 2-object-category C
  2-object-diagram a b = Disc-diagram λ where
    true  → a
    false → b

Similarly, a natural transformation between two such diagrams consists of two morphisms in $C .$

  2-object-nat-trans
    : ∀ {F G : Functor 2-object-category C}
    → Hom (F # true) (G # true) → Hom (F # false) (G # false)
    → F => G
  2-object-nat-trans f g = Disc-natural λ where
    true  → f
    false → g

We note that any functor $F : 2 \to C$ is canonically equal, not just naturally isomorphic, to the one we defined above.

  canonical-functors
    : ∀ (F : Functor 2-object-category C)
    → F ≡ 2-object-diagram (F # true) (F # false)
  canonical-functors F = Functor-path p q where
    p : ∀ x → _
    p false = refl
    p true  = refl

    q : ∀ {x y} (f : x ≡ y) → _
    q {false} {false} p =
      F .F₁ p           ≡⟨ ap (F .F₁) (Bool-is-set _ _ _ _) ⟩
      F .F₁ refl        ≡⟨ F .F-id ⟩
      id                ∎
    q {true} {true} p =
      F .F₁ p           ≡⟨ ap (F .F₁) (Bool-is-set _ _ _ _) ⟩
      F .F₁ refl        ≡⟨ F .F-id ⟩
      id                ∎
    q {false} {true} p = absurd (true≠false (sym p))
    q {true} {false} p = absurd (true≠false p)