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bifunctor improvements #1925

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bifunctor improvements #1925

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bifunctor improvements
Alizter Apr 23, 2024
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Alizter Apr 23, 2024
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192 changes: 170 additions & 22 deletions theories/WildCat/Bifunctor.v
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Expand Up @@ -4,21 +4,23 @@ Require Import WildCat.Core WildCat.Prod WildCat.Equiv WildCat.NatTrans WildCat.

(** * Bifunctors between WildCats *)

(** ** Definition *)

Class Is0Bifunctor {A B C : Type} `{IsGraph A, IsGraph B, IsGraph C}
(F : A -> B -> C)
:= {
bifunctor_is0functor01 :: forall a, Is0Functor (F a);
bifunctor_is0functor10 :: forall b, Is0Functor (flip F b);
}.
bifunctor_is0functor01 :: forall a, Is0Functor (F a);
bifunctor_is0functor10 :: forall b, Is0Functor (flip F b);
}.

Class Is1Bifunctor {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F}
:= {
bifunctor_is1functor01 :: forall a : A, Is1Functor (F a);
bifunctor_is1functor10 :: forall b : B, Is1Functor (flip F b);
bifunctor_isbifunctor : forall a0 a1 (f : a0 $-> a1) b0 b1 (g : b0 $-> b1),
fmap (F _) g $o fmap (flip F _) f $== fmap (flip F _) f $o fmap (F _) g
}.
bifunctor_is1functor01 :: forall a : A, Is1Functor (F a);
bifunctor_is1functor10 :: forall b : B, Is1Functor (flip F b);
bifunctor_isbifunctor : forall a0 a1 (f : a0 $-> a1) b0 b1 (g : b0 $-> b1),
fmap (F _) g $o fmap (flip F _) f $== fmap (flip F _) f $o fmap (F _) g
}.

Arguments bifunctor_isbifunctor {A B C} {_ _ _ _ _ _ _ _ _ _ _ _}
F {_ _} {a0 a1} f {b0 b1} g.
Expand Down Expand Up @@ -56,54 +58,180 @@ Definition Build_Is1Bifunctor' {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
: Is1Bifunctor F
:= is1bifunctor_functor_uncurried (uncurry F).

(** [fmap] in the first argument *)
(** ** Bifunctor lemmas *)

(** *** 1-functorial action *)

(** [fmap] in the first argument. *)
Definition fmap10 {A B C : Type} `{IsGraph A, IsGraph B, IsGraph C}
(F : A -> B -> C) `{!Is0Bifunctor F} {a0 a1 : A} (f : a0 $-> a1) (b : B)
: (F a0 b) $-> (F a1 b)
:= fmap (flip F b) f.

(** [fmap] in the second argument *)
(** [fmap] in the second argument. *)
Definition fmap01 {A B C : Type} `{IsGraph A, IsGraph B, IsGraph C}
(F : A -> B -> C) `{!Is0Bifunctor F} (a : A) {b0 b1 : B} (g : b0 $-> b1)
: F a b0 $-> F a b1
:= fmap (F a) g.

(** There are two ways to [fmap] in both arguments of a bifunctor. The bifunctor coherence condition ([bifunctor_isbifunctor]) states precisely that these two routes agree. *)
(** [fmap] in beoth arguments. Note that we made a choice in the order in which to compose. *)
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Definition fmap11 {A B C : Type} `{IsGraph A, IsGraph B, Is01Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F} {a0 a1 : A} (f : a0 $-> a1)
{b0 b1 : B} (g : b0 $-> b1)
: F a0 b0 $-> F a1 b1
:= fmap (F _) g $o fmap (flip F _) f.
:= fmap01 F _ g $o fmap10 F f _.

(** [fmap11] but with the other choice. *)
Definition fmap11' {A B C : Type} `{IsGraph A, IsGraph B, Is01Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F} {a0 a1 : A} (f : a0 $-> a1)
{b0 b1 : B} (g : b0 $-> b1)
: F a0 b0 $-> F a1 b1
:= fmap (flip F _) f $o fmap (F _) g.
:= fmap10 F f _ $o fmap01 F _ g.

(** *** Coherence *)

(** The bifunctor coherence condition becomes a 2-cell between the two choices for [fmap11]. *)
Definition fmap11_coh {A B C : Type}
(F : A -> B -> C) `{Is1Bifunctor A B C F}
{a0 a1 : A} (f : a0 $-> a1) {b0 b1 : B} (g : b0 $-> b1)
: fmap11 F f g $== fmap11' F f g.
Proof.
rapply bifunctor_isbifunctor.
Defined.

(** Sometimes it is convenient to turn an [fmap10] into an [fmap11]. *)
Definition fmap10_is_fmap11 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $-> a1) (b : B)
: fmap10 F f b $== fmap11 F f (Id b)
:= ((fmap_id _ _ $@R _) $@ cat_idl _)^$.
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(** Similarly for an [fmap01]. *)
Definition fmap01_is_fmap11 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
(a : A) {b0 b1 : B} (g : b0 $-> b1)
: fmap01 F a g $== fmap11 F (Id a) g
:= ((_ $@L fmap_id _ _) $@ cat_idr _)^$.

(** 2-functorial action *)

Definition fmap02 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
(a : A) {b0 b1 : B} {g g' : b0 $-> b1} (q : g $== g')
: fmap01 F a g $== fmap01 F a g'
:= fmap2 (F a) q.

Definition fmap12 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $-> a1) {b0 b1 : B} {g g' : b0 $-> b1} (q : g $== g')
: fmap11 F f g $== fmap11 F f g'
:= fmap02 F _ q $@R _.

Definition fmap20 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} {f f' : a0 $-> a1} (p : f $== f') (b : B)
: fmap10 F f b $== fmap10 F f' b
:= fmap2 (flip F b) p.

Definition fmap21 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} {f f' : a0 $-> a1} (p : f $== f') {b0 b1 : B} (g : b0 $-> b1)
: fmap11 F f g $== fmap11 F f' g
:= _ $@L fmap20 F p _.

Definition fmap22 {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} {f : a0 $-> a1} {f' : a0 $-> a1}
{b0 b1 : B} {g : b0 $-> b1} {g' : b0 $-> b1}
(p : f $== f') (q : g $== g')
{a0 a1 : A} {f f' : a0 $-> a1} (p : f $== f')
{b0 b1 : B} {g g' : b0 $-> b1} (q : g $== g')
: fmap11 F f g $== fmap11 F f' g'
:= fmap2 (flip F _) p $@@ fmap2 (F _) q.
:= fmap20 F p b0 $@@ fmap02 F a1 q.

Global Instance iemap11 {A B C : Type} `{HasEquivs A, HasEquivs B, HasEquivs C}
(** *** Identity preservation *)

Definition fmap01_id {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F} (a : A) (b : B)
: fmap01 F a (Id b) $== Id (F a b)
:= fmap_id (F a) b.

Definition fmap10_id {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F} (a : A) (b : B)
: fmap10 F (Id a) b $== Id (F a b)
:= fmap_id (flip F b) a.

Definition fmap11_id {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F} (a : A) (b : B)
: fmap11 F (Id a) (Id b) $== Id (F a b).
Proof.
refine ((_ $@@ _) $@ cat_idr _).
1,2: rapply fmap_id.
Defined.

(** *** Composition preservation *)

Definition fmap01_comp {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $<~> a1) {b0 b1 : B} (g : b0 $<~> b1)
: CatIsEquiv (fmap11 F f g).
(a : A) {b0 b1 b2 : B} (g : b1 $-> b2) (f : b0 $-> b1)
: fmap01 F a (g $o f) $== fmap01 F a g $o fmap01 F a f
:= fmap_comp (F a) f g.

Definition fmap10_comp {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 a2 : A} (g : a1 $-> a2) (f : a0 $-> a1) (b : B)
: fmap10 F (g $o f) b $== fmap10 F g b $o fmap10 F f b
:= fmap_comp (flip F b) f g.

Definition fmap11_comp {A B C : Type} `{Is1Cat A, Is1Cat B, Is1Cat C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 a2 : A} (g : a1 $-> a2) (f : a0 $-> a1)
{b0 b1 b2 : B} (k : b1 $-> b2) (h : b0 $-> b1)
: fmap11 F (g $o f) (k $o h) $== fmap11 F g k $o fmap11 F f h.
Proof.
rapply compose_catie'.
exact (iemap (flip F _) f).
refine ((fmap10_comp F _ _ _ $@@ fmap01_comp F _ _ _) $@ _).
refine (cat_assoc _ _ _ $@ (_ $@L _) $@ (cat_assoc _ _ _)^$).
refine ((cat_assoc _ _ _)^$ $@ (_ $@R _) $@ cat_assoc _ _ _).
rapply fmap11_coh.
Defined.

(** *** Equivalence preservation *)

Global Instance iemap10 {A B C : Type} `{HasEquivs A, Is1Cat B, HasEquivs C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $<~> a1) (b : B)
: CatIsEquiv (fmap10 F f b)
:= iemap (flip F b) f.

Global Instance iemap01 {A B C : Type} `{Is1Cat A, HasEquivs B, HasEquivs C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
(a : A) {b0 b1 : B} (g : b0 $<~> b1)
: CatIsEquiv (fmap01 F a g)
:= iemap (F a) g.

Global Instance iemap11 {A B C : Type} `{HasEquivs A, HasEquivs B, HasEquivs C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $<~> a1) {b0 b1 : B} (g : b0 $<~> b1)
: CatIsEquiv (fmap11 F f g)
:= compose_catie' _ _.

Definition emap10 {A B C : Type} `{HasEquivs A, Is1Cat B, HasEquivs C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $<~> a1) (b : B)
: F a0 b $<~> F a1 b
:= Build_CatEquiv (fmap10 F f b).

Definition emap01 {A B C : Type} `{Is1Cat A, HasEquivs B, HasEquivs C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
(a : A) {b0 b1 : B} (g : b0 $<~> b1)
: F a b0 $<~> F a b1
:= Build_CatEquiv (fmap01 F a g).

Definition emap11 {A B C : Type} `{HasEquivs A, HasEquivs B, HasEquivs C}
(F : A -> B -> C) `{!Is0Bifunctor F, !Is1Bifunctor F}
{a0 a1 : A} (f : a0 $<~> a1) {b0 b1 : B} (g : b0 $<~> b1)
: F a0 b0 $<~> F a1 b1
:= Build_CatEquiv (fmap11 F f g).

(** *** Uncurrying *)

(** Any 0-bifunctor [A -> B -> C] can be made into a functor from the product category [A * B -> C] in two ways. *)
Global Instance is0functor_uncurry_bifunctor {A B C : Type}
`{IsGraph A, IsGraph B, Is01Cat C} (F : A -> B -> C) `{!Is0Bifunctor F}
Expand Down Expand Up @@ -133,6 +261,26 @@ Proof.
exact (_ $@L bifunctor_isbifunctor F _ _ $@R _).
Defined.

(** ** Flipping bifunctors *)

Definition is0bifunctor_flip {A B C : Type}
(F : A -> B -> C) `{Is0Bifunctor A B C F} : Is0Bifunctor (flip F).
Proof.
snrapply Build_Is0Bifunctor; exact _.
Defined.
Hint Immediate is0bifunctor_flip : typeclass_instances.

Definition is1bifunctor_flip {A B C : Type}
(F : A -> B -> C) `{Is1Bifunctor A B C F} : Is1Bifunctor (flip F).
Proof.
snrapply Build_Is1Bifunctor.
1,2: exact _.
intros a0 a1 f b0 b1 g.
symmetry.
rapply bifunctor_isbifunctor.
Defined.
Hint Immediate is1bifunctor_flip : typeclass_instances.

(** ** Composition of bifunctors *)

(** There are 4 different ways to compose a functor with a bifunctor. *)
Expand Down
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