364 lines
14 KiB
Agda
364 lines
14 KiB
Agda
-- There is no category of categories in our interpretation
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{-# OPTIONS --cubical --allow-unsolved-metas #-}
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module Cat.Categories.Cat where
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open import Agda.Primitive
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open import Cubical
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open import Function
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open import Data.Product renaming (proj₁ to fst ; proj₂ to snd)
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open import Cat.Category
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open import Cat.Category.Functor
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open import Cat.Category.Product
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open import Cat.Category.Exponential
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open import Cat.Category.NaturalTransformation
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open import Cat.Equality
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open Equality.Data.Product
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open Functor using (func→ ; func*)
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open Category using (Object ; 𝟙)
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-- The category of categories
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module _ (ℓ ℓ' : Level) where
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private
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module _ {𝔸 𝔹 ℂ 𝔻 : Category ℓ ℓ'} {F : Functor 𝔸 𝔹} {G : Functor 𝔹 ℂ} {H : Functor ℂ 𝔻} where
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assc : F[ H ∘ F[ G ∘ F ] ] ≡ F[ F[ H ∘ G ] ∘ F ]
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assc = Functor≡ refl refl
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module _ {ℂ 𝔻 : Category ℓ ℓ'} {F : Functor ℂ 𝔻} where
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ident-r : F[ F ∘ identity ] ≡ F
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ident-r = Functor≡ refl refl
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ident-l : F[ identity ∘ F ] ≡ F
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ident-l = Functor≡ refl refl
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RawCat : RawCategory (lsuc (ℓ ⊔ ℓ')) (ℓ ⊔ ℓ')
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RawCat =
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record
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{ Object = Category ℓ ℓ'
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; Arrow = Functor
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; 𝟙 = identity
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; _∘_ = F[_∘_]
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}
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private
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open RawCategory RawCat
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isAssociative : IsAssociative
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isAssociative {f = F} {G} {H} = assc {F = F} {G = G} {H = H}
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-- TODO: Rename `ident'` to `ident` after changing how names are exposed in Functor.
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ident' : IsIdentity identity
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ident' = ident-r , ident-l
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-- NB! `ArrowsAreSets RawCat` is *not* provable. The type of functors,
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-- however, form a groupoid! Therefore there is no (1-)category of
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-- categories. There does, however, exist a 2-category of 1-categories.
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-- Because of the note above there is not category of categories.
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Cat : (unprovable : IsCategory RawCat) → Category (lsuc (ℓ ⊔ ℓ')) (ℓ ⊔ ℓ')
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Category.raw (Cat _) = RawCat
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Category.isCategory (Cat unprovable) = unprovable
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-- Category.raw Cat _ = RawCat
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-- Category.isCategory Cat unprovable = unprovable
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-- The following to some extend depends on the category of categories being a
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-- category. In some places it may not actually be needed, however.
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module CatProduct {ℓ ℓ' : Level} (ℂ 𝔻 : Category ℓ ℓ') where
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private
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:Object: = Object ℂ × Object 𝔻
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:Arrow: : :Object: → :Object: → Set ℓ'
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:Arrow: (c , d) (c' , d') = ℂ [ c , c' ] × 𝔻 [ d , d' ]
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:𝟙: : {o : :Object:} → :Arrow: o o
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:𝟙: = 𝟙 ℂ , 𝟙 𝔻
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_:⊕:_ :
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{a b c : :Object:} →
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:Arrow: b c →
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:Arrow: a b →
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:Arrow: a c
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_:⊕:_ = λ { (bc∈C , bc∈D) (ab∈C , ab∈D) → ℂ [ bc∈C ∘ ab∈C ] , 𝔻 [ bc∈D ∘ ab∈D ]}
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:rawProduct: : RawCategory ℓ ℓ'
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RawCategory.Object :rawProduct: = :Object:
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RawCategory.Arrow :rawProduct: = :Arrow:
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RawCategory.𝟙 :rawProduct: = :𝟙:
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RawCategory._∘_ :rawProduct: = _:⊕:_
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open RawCategory :rawProduct:
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module ℂ = Category ℂ
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module 𝔻 = Category 𝔻
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open import Cubical.Sigma
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arrowsAreSets : ArrowsAreSets -- {A B : RawCategory.Object :rawProduct:} → isSet (Arrow A B)
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arrowsAreSets = setSig {sA = ℂ.arrowsAreSets} {sB = λ x → 𝔻.arrowsAreSets}
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isIdentity : IsIdentity :𝟙:
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isIdentity
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= Σ≡ (fst ℂ.isIdentity) (fst 𝔻.isIdentity)
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, Σ≡ (snd ℂ.isIdentity) (snd 𝔻.isIdentity)
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postulate univalent : Univalence.Univalent :rawProduct: isIdentity
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instance
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:isCategory: : IsCategory :rawProduct:
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IsCategory.isAssociative :isCategory: = Σ≡ ℂ.isAssociative 𝔻.isAssociative
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IsCategory.isIdentity :isCategory: = isIdentity
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IsCategory.arrowsAreSets :isCategory: = arrowsAreSets
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IsCategory.univalent :isCategory: = univalent
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obj : Category ℓ ℓ'
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Category.raw obj = :rawProduct:
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proj₁ : Functor obj ℂ
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proj₁ = record
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{ raw = record { func* = fst ; func→ = fst }
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; isFunctor = record { isIdentity = refl ; isDistributive = refl }
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}
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proj₂ : Functor obj 𝔻
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proj₂ = record
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{ raw = record { func* = snd ; func→ = snd }
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; isFunctor = record { isIdentity = refl ; isDistributive = refl }
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}
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module _ {X : Category ℓ ℓ'} (x₁ : Functor X ℂ) (x₂ : Functor X 𝔻) where
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private
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x : Functor X obj
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x = record
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{ raw = record
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{ func* = λ x → x₁.func* x , x₂.func* x
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; func→ = λ x → x₁.func→ x , x₂.func→ x
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}
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; isFunctor = record
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{ isIdentity = Σ≡ x₁.isIdentity x₂.isIdentity
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; isDistributive = Σ≡ x₁.isDistributive x₂.isDistributive
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}
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}
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where
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open module x₁ = Functor x₁
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open module x₂ = Functor x₂
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isUniqL : F[ proj₁ ∘ x ] ≡ x₁
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isUniqL = Functor≡ eq* eq→
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where
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eq* : (F[ proj₁ ∘ x ]) .func* ≡ x₁ .func*
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eq* = refl
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eq→ : (λ i → {A : Object X} {B : Object X} → X [ A , B ] → ℂ [ eq* i A , eq* i B ])
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[ (F[ proj₁ ∘ x ]) .func→ ≡ x₁ .func→ ]
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eq→ = refl
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isUniqR : F[ proj₂ ∘ x ] ≡ x₂
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isUniqR = Functor≡ refl refl
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isUniq : F[ proj₁ ∘ x ] ≡ x₁ × F[ proj₂ ∘ x ] ≡ x₂
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isUniq = isUniqL , isUniqR
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isProduct : ∃![ x ] (F[ proj₁ ∘ x ] ≡ x₁ × F[ proj₂ ∘ x ] ≡ x₂)
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isProduct = x , isUniq
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module _ {ℓ ℓ' : Level} (unprovable : IsCategory (RawCat ℓ ℓ')) where
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private
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Catℓ = Cat ℓ ℓ' unprovable
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module _ (ℂ 𝔻 : Category ℓ ℓ') where
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private
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module P = CatProduct ℂ 𝔻
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instance
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isProduct : IsProduct Catℓ P.proj₁ P.proj₂
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isProduct = P.isProduct
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product : Product {ℂ = Catℓ} ℂ 𝔻
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product = record
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{ obj = P.obj
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; proj₁ = P.proj₁
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; proj₂ = P.proj₂
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}
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instance
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hasProducts : HasProducts Catℓ
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hasProducts = record { product = product }
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-- Basically proves that `Cat ℓ ℓ` is cartesian closed.
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module _ (ℓ : Level) (unprovable : IsCategory (RawCat ℓ ℓ)) where
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private
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open Data.Product
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open import Cat.Categories.Fun
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Catℓ : Category (lsuc (ℓ ⊔ ℓ)) (ℓ ⊔ ℓ)
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Catℓ = Cat ℓ ℓ unprovable
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module _ (ℂ 𝔻 : Category ℓ ℓ) where
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open Fun ℂ 𝔻 renaming (identity to idN)
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private
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:obj: : Object Catℓ
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:obj: = Fun
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:func*: : Functor ℂ 𝔻 × Object ℂ → Object 𝔻
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:func*: (F , A) = func* F A
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module _ {dom cod : Functor ℂ 𝔻 × Object ℂ} where
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private
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F : Functor ℂ 𝔻
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F = proj₁ dom
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A : Object ℂ
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A = proj₂ dom
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G : Functor ℂ 𝔻
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G = proj₁ cod
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B : Object ℂ
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B = proj₂ cod
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:func→: : (pobj : NaturalTransformation F G × ℂ [ A , B ])
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→ 𝔻 [ func* F A , func* G B ]
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:func→: ((θ , θNat) , f) = result
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where
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θA : 𝔻 [ func* F A , func* G A ]
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θA = θ A
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θB : 𝔻 [ func* F B , func* G B ]
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θB = θ B
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F→f : 𝔻 [ func* F A , func* F B ]
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F→f = func→ F f
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G→f : 𝔻 [ func* G A , func* G B ]
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G→f = func→ G f
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l : 𝔻 [ func* F A , func* G B ]
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l = 𝔻 [ θB ∘ F→f ]
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r : 𝔻 [ func* F A , func* G B ]
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r = 𝔻 [ G→f ∘ θA ]
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-- There are two choices at this point,
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-- but I suppose the whole point is that
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-- by `θNat f` we have `l ≡ r`
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-- lem : 𝔻 [ θ B ∘ F .func→ f ] ≡ 𝔻 [ G .func→ f ∘ θ A ]
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-- lem = θNat f
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result : 𝔻 [ func* F A , func* G B ]
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result = l
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_×p_ = product unprovable
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module _ {c : Functor ℂ 𝔻 × Object ℂ} where
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private
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F : Functor ℂ 𝔻
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F = proj₁ c
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C : Object ℂ
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C = proj₂ c
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-- NaturalTransformation F G × ℂ .Arrow A B
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-- :ident: : :func→: {c} {c} (identityNat F , ℂ .𝟙) ≡ 𝔻 .𝟙
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-- :ident: = trans (proj₂ 𝔻.isIdentity) (F .isIdentity)
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-- where
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-- open module 𝔻 = IsCategory (𝔻 .isCategory)
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-- Unfortunately the equational version has some ambigous arguments.
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:ident: : :func→: {c} {c} (NT.identity F , 𝟙 ℂ {A = proj₂ c}) ≡ 𝟙 𝔻
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:ident: = begin
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:func→: {c} {c} (𝟙 (Product.obj (:obj: ×p ℂ)) {c}) ≡⟨⟩
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:func→: {c} {c} (idN F , 𝟙 ℂ) ≡⟨⟩
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𝔻 [ identityTrans F C ∘ func→ F (𝟙 ℂ)] ≡⟨⟩
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𝔻 [ 𝟙 𝔻 ∘ func→ F (𝟙 ℂ)] ≡⟨ proj₂ 𝔻.isIdentity ⟩
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func→ F (𝟙 ℂ) ≡⟨ F.isIdentity ⟩
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𝟙 𝔻 ∎
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where
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open module 𝔻 = Category 𝔻
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open module F = Functor F
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module _ {F×A G×B H×C : Functor ℂ 𝔻 × Object ℂ} where
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F = F×A .proj₁
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A = F×A .proj₂
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G = G×B .proj₁
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B = G×B .proj₂
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H = H×C .proj₁
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C = H×C .proj₂
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-- Not entirely clear what this is at this point:
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_P⊕_ = Category._∘_ (Product.obj (:obj: ×p ℂ)) {F×A} {G×B} {H×C}
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module _
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-- NaturalTransformation F G × ℂ .Arrow A B
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{θ×f : NaturalTransformation F G × ℂ [ A , B ]}
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{η×g : NaturalTransformation G H × ℂ [ B , C ]} where
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private
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θ : Transformation F G
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θ = proj₁ (proj₁ θ×f)
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θNat : Natural F G θ
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θNat = proj₂ (proj₁ θ×f)
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f : ℂ [ A , B ]
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f = proj₂ θ×f
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η : Transformation G H
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η = proj₁ (proj₁ η×g)
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ηNat : Natural G H η
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ηNat = proj₂ (proj₁ η×g)
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g : ℂ [ B , C ]
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g = proj₂ η×g
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ηθNT : NaturalTransformation F H
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ηθNT = Category._∘_ Fun {F} {G} {H} (η , ηNat) (θ , θNat)
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ηθ = proj₁ ηθNT
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ηθNat = proj₂ ηθNT
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:isDistributive: :
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𝔻 [ 𝔻 [ η C ∘ θ C ] ∘ func→ F ( ℂ [ g ∘ f ] ) ]
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≡ 𝔻 [ 𝔻 [ η C ∘ func→ G g ] ∘ 𝔻 [ θ B ∘ func→ F f ] ]
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:isDistributive: = begin
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𝔻 [ (ηθ C) ∘ func→ F (ℂ [ g ∘ f ]) ]
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≡⟨ ηθNat (ℂ [ g ∘ f ]) ⟩
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𝔻 [ func→ H (ℂ [ g ∘ f ]) ∘ (ηθ A) ]
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≡⟨ cong (λ φ → 𝔻 [ φ ∘ ηθ A ]) (H.isDistributive) ⟩
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𝔻 [ 𝔻 [ func→ H g ∘ func→ H f ] ∘ (ηθ A) ]
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≡⟨ sym isAssociative ⟩
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𝔻 [ func→ H g ∘ 𝔻 [ func→ H f ∘ ηθ A ] ]
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≡⟨ cong (λ φ → 𝔻 [ func→ H g ∘ φ ]) isAssociative ⟩
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𝔻 [ func→ H g ∘ 𝔻 [ 𝔻 [ func→ H f ∘ η A ] ∘ θ A ] ]
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≡⟨ cong (λ φ → 𝔻 [ func→ H g ∘ φ ]) (cong (λ φ → 𝔻 [ φ ∘ θ A ]) (sym (ηNat f))) ⟩
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𝔻 [ func→ H g ∘ 𝔻 [ 𝔻 [ η B ∘ func→ G f ] ∘ θ A ] ]
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≡⟨ cong (λ φ → 𝔻 [ func→ H g ∘ φ ]) (sym isAssociative) ⟩
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𝔻 [ func→ H g ∘ 𝔻 [ η B ∘ 𝔻 [ func→ G f ∘ θ A ] ] ]
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≡⟨ isAssociative ⟩
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𝔻 [ 𝔻 [ func→ H g ∘ η B ] ∘ 𝔻 [ func→ G f ∘ θ A ] ]
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≡⟨ cong (λ φ → 𝔻 [ φ ∘ 𝔻 [ func→ G f ∘ θ A ] ]) (sym (ηNat g)) ⟩
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𝔻 [ 𝔻 [ η C ∘ func→ G g ] ∘ 𝔻 [ func→ G f ∘ θ A ] ]
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≡⟨ cong (λ φ → 𝔻 [ 𝔻 [ η C ∘ func→ G g ] ∘ φ ]) (sym (θNat f)) ⟩
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𝔻 [ 𝔻 [ η C ∘ func→ G g ] ∘ 𝔻 [ θ B ∘ func→ F f ] ] ∎
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where
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open Category 𝔻
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module H = Functor H
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:eval: : Functor ((:obj: ×p ℂ) .Product.obj) 𝔻
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:eval: = record
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{ raw = record
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{ func* = :func*:
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; func→ = λ {dom} {cod} → :func→: {dom} {cod}
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}
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; isFunctor = record
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{ isIdentity = λ {o} → :ident: {o}
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; isDistributive = λ {f u n k y} → :isDistributive: {f} {u} {n} {k} {y}
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}
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}
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module _ (𝔸 : Category ℓ ℓ) (F : Functor ((𝔸 ×p ℂ) .Product.obj) 𝔻) where
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open HasProducts (hasProducts {ℓ} {ℓ} unprovable) renaming (_|×|_ to parallelProduct)
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postulate
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transpose : Functor 𝔸 :obj:
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eq : Catℓ [ :eval: ∘ (parallelProduct transpose (𝟙 Catℓ {A = ℂ})) ] ≡ F
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-- eq : Catℓ [ :eval: ∘ (HasProducts._|×|_ hasProducts transpose (𝟙 Catℓ {o = ℂ})) ] ≡ F
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-- eq' : (Catℓ [ :eval: ∘
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-- (record { product = product } HasProducts.|×| transpose)
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-- (𝟙 Catℓ)
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-- ])
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-- ≡ F
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-- For some reason after `e8215b2c051062c6301abc9b3f6ec67106259758`
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-- `catTranspose` makes Agda hang. catTranspose : ∃![ F~ ] (Catℓ [
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-- :eval: ∘ (parallelProduct F~ (𝟙 Catℓ {o = ℂ}))] ≡ F) catTranspose =
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-- transpose , eq
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postulate :isExponential: : IsExponential Catℓ ℂ 𝔻 :obj: :eval:
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-- :isExponential: : IsExponential Catℓ ℂ 𝔻 :obj: :eval:
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-- :isExponential: = {!catTranspose!}
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-- where
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-- open HasProducts (hasProducts {ℓ} {ℓ} unprovable) using (_|×|_)
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-- :isExponential: = λ 𝔸 F → transpose 𝔸 F , eq' 𝔸 F
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-- :exponent: : Exponential (Cat ℓ ℓ) A B
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:exponent: : Exponential Catℓ ℂ 𝔻
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:exponent: = record
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{ obj = :obj:
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; eval = :eval:
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; isExponential = :isExponential:
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}
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hasExponentials : HasExponentials Catℓ
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hasExponentials = record { exponent = :exponent: }
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