Library UniMath.CategoryTheory.RepresentableFunctors.Precategories

Require Export UniMath.CategoryTheory.Core.Categories. Require Export UniMath.CategoryTheory.Core.Isos. Require Export UniMath.CategoryTheory.Core.Functors.
Require Export UniMath.CategoryTheory.Core.NaturalTransformations.
Require Export UniMath.CategoryTheory.Core.Univalence.
Require Export UniMath.CategoryTheory.opp_precat
               UniMath.CategoryTheory.yoneda
               UniMath.CategoryTheory.categories.HSET.Core.
Require Export UniMath.Foundations.Preamble.
Require Export UniMath.Foundations.Sets.
Require Import UniMath.MoreFoundations.Tactics.

Local Open Scope cat.

Notation "a <-- b" := (@precategory_morphisms (opp_precat _) a b) : cat.

Definition src {C:precategory} {a b:C} (f:a -->b) : C := a.
Definition tar {C:precategory} {a b:C} (f:a -->b) : C := b.

Definition hom (C:precategory_data) : ob C → ob C → UU :=
  λ c c', c --> c'.

Definition Hom (C : category) : ob C → ob C → hSet :=
  λ c c', make_hSet (c --> c') (homset_property C _ _ ).

Ltac eqn_logic :=
  abstract (
      repeat (
          try reflexivity;
          try intro; try split; try apply id_right;
          try apply id_left; try apply assoc;
          try apply funextsec;
          try apply homset_property;
          try apply isasetunit;
          try apply isapropunit;
          try refine (two_arg_paths_f _ _);
          try refine (total2_paths_f _ _);
          try refine (nat_trans_ax _ _ _ _);
          try refine (! nat_trans_ax _ _ _ _);
          try apply functor_id;
          try apply functor_comp;
          try apply isaprop_is_nat_trans
        )) using _L_.

Ltac set_logic :=
  abstract (repeat (
                try intro;
                try apply isaset_total2;
                try apply isasetdirprod;
                try apply homset_property;
                try apply impred_isaset;
                try apply isasetaprop)) using _M_.

Notation "[ C , D ]" := (functor_category C D) : cat.

Definition oppositecategory (C : category) : category.
Proof.
  ∃ (opp_precat C).
  unfold category in C.
  exact (λ a b, pr2 C b a).
Defined.

Notation "C '^op'" := (oppositecategory C) (at level 3) : cat.
Definition precategory_obmor (C:precategory) : precategory_ob_mor :=
      precategory_ob_mor_from_precategory_data (
          precategory_data_from_precategory C).

Definition Functor_obmor {C D} (F:functor C D) := pr1 F.
Definition Functor_obj {C D} (F:functor C D) := pr1 (pr1 F).
Definition Functor_mor {C D} (F:functor C D) := pr2 (pr1 F).
Definition Functor_identity {C D} (F:functor C D) := functor_id F.
Definition Functor_compose {C D} (F:functor C D) := @functor_comp _ _ F.

Definition theUnivalenceProperty (C: univalent_category) := pr2 C : is_univalent C.

Lemma category_eq (C D : category) :
  (C:precategory_data ) = (D:precategory_data ) → C =D.
Proof.
  intro e. apply subtypePath. intro. apply isaprop_has_homsets.
  apply subtypePath'.
  { assumption. }
  apply isaprop_is_precategory.
  apply homset_property.
Defined.

embeddings and isomorphism of categories

Definition categoryEmbedding (B C : category) := ∑ F:[B ,C ], fully_faithful F.

Definition embeddingToFunctor (B C : category) :
  categoryEmbedding B C → B ⟶ C
  := pr1.

Coercion embeddingToFunctor : categoryEmbedding >-> functor.

Definition categoryIsomorphism (B C : category) :=
  ∑ F:categoryEmbedding B C , isweq ((pr1 F : B ⟶ C) : ob B → ob C).

Definition isomorphismToEmbedding (B C:category) :
  categoryIsomorphism B C → categoryEmbedding B C
  := pr1.

Coercion isomorphismToEmbedding : categoryIsomorphism >-> categoryEmbedding.

Definition isomorphismOnMor {B C:category} (F:categoryIsomorphism B C)
           (b b':B) : Hom B b b' ≃ Hom C (F b) (F b')
  := make_weq _ (pr2 (pr1 F) b b').

make a precategory

Definition makecategory_ob_mor
    (obj : UU)
    (mor : obj → obj → UU) : precategory_ob_mor
  := make_precategory_ob_mor obj (λ i j:obj , mor i j).

Definition makecategory_data
    (obj : UU)
    (mor : obj → obj → UU)
    (identity : ∏ i, mor i i)
    (compose : ∏ i j k (f:mor i j) (g:mor j k ), mor i k)
    : precategory_data
  := make_precategory_data (makecategory_ob_mor obj mor) identity compose.

Local Open Scope cat_deprecated.

Definition makeFunctor {C D:category}
           (obj : C → D)
           (mor : ∏ c c' : C , c --> c' → obj c --> obj c')
           (identity : ∏ c, mor c c (identity c) = identity (obj c))
           (compax : ∏ (a b c : C) (f : a --> b) (g : b --> c ),
                       mor a c (g ∘ f) = mor b c g ∘ mor a b f) :
  C ⟶ D
  := (obj ,, mor ),, identity ,, compax.

notation for dealing with functors, natural transformations, etc.

Definition functor_object_application {B C:category} (F : [B ,C ]) (b:B) : C
  := (F:_⟶_) b.
Notation "F ◾ b" := (functor_object_application F b) : cat.

Definition functor_mor_application {B C:category} {b b':B} (F:[B ,C ]) :
  b --> b' → F ◾ b --> F ◾ b'
  := λ f, # (F:_⟶_) f.
Notation "F ▭ f" := (functor_mor_application F f) : cat.

Definition arrow {C:category} (c : C) (X : [C ^op ,SET ]) : hSet := X ◾ c.
Notation "c ⇒ X" := (arrow c X) : cat.
Definition arrow' {C:category} (c : C) (X : [C ^op ^op ,SET ]) : hSet := X ◾ c.
Notation "X ⇐ c" := (arrow' c X) : cat.
Definition arrow_morphism_composition {C:category} {c' c:C} {X:[C ^op ,SET ]} :
  c'-->c → c ⇒X → c'⇒X
  := λ f x, # (X:_⟶_) f x.
Notation "x ⟲ f" := (arrow_morphism_composition f x) (at level 50, left associativity) : cat.

Definition nattrans_arrow_composition {C:category} {X X':[C ^op ,SET ]} {c:C} :
  c ⇒X → X -->X' → c ⇒X'
  := λ x q, (q:_ ⟹ _) c (x:(X:_⟶_) c:hSet).
Notation "q ⟳ x" := (nattrans_arrow_composition x q) (at level 50, left associativity) : cat.

Definition nattrans_object_application {B C:category} {F F' : [B ,C ]} (b:B) :
  F --> F' → F ◾ b --> F' ◾ b
  := λ p, (p:_ ⟹ _) b.
Notation "p ◽ b" := (nattrans_object_application b p) : cat.

Definition arrow_mor_id {C:category} {c:C} {X:[C ^op ,SET ]} (x:c ⇒X) :
  x ⟲ identity c = x
  := eqtohomot (functor_id X c) x.

Definition arrow_mor_mor_assoc {C:category} {c c' c'':C} {X:[C ^op ,SET ]}
           (g:c''-->c') (f:c'-->c) (x:c ⇒X) :
  x ⟲ (f ∘ g ) = (x ⟲ f ) ⟲ g
  := eqtohomot (functor_comp X f g) x.

Definition nattrans_naturality {B C:category} {F F':[B , C ]} {b b':B}
           (p : F --> F') (f : b --> b') :
  p ◽ b' ∘ F ▭ f = F' ▭ f ∘ p ◽ b
  := nat_trans_ax p _ _ f.

Definition comp_func_on_mor {A B C:category} (F:[A ,B ]) (G:[B ,C ]) {a a':A} (f:a -->a') :
  G □ F ▭ f = G ▭ (F ▭ f ).
Proof.
  reflexivity.
Defined.

Definition nattrans_arrow_mor_assoc {C:category} {c' c:C} {X X':[C ^op ,SET ]}
           (g:c'-->c) (x:c ⇒X) (p:X -->X') :
  p ⟳ (x ⟲ g ) = (p ⟳ x ) ⟲ g
  := eqtohomot (nat_trans_ax p _ _ g) x.

Definition nattrans_arrow_id {C:category} {c:C} {X:[C ^op ,SET ]} (x:c ⇒X) :
  nat_trans_id _ ⟳ x = x
  := idpath _.

Definition nattrans_nattrans_arrow_assoc {C:category} {c:C} {X X' X'':[C ^op ,SET ]}
           (x:c ⇒X) (p:X -->X') (q:X'-->X'') :
  q ⟳ (p ⟳ x ) = (q ∘ p ) ⟳ x
  := idpath _.

Definition nattrans_nattrans_object_assoc {A B C:category}
           (F:[A ,B ]) (G:[B , C ]) {a a' : A} (f : a --> a') :
  G □ F ▭ f = G ▭ (F ▭ f )
  := idpath _.

Lemma functor_on_id {B C:category} (F:[B ,C ]) (b:B) : F ▭ identity b = identity (F ◾ b).
Proof.
  exact (functor_id F b).
Defined.

Lemma functor_on_comp {B C:category} (F:[B ,C ]) {b b' b'':B} (g:b'-->b'') (f:b -->b') :
  F ▭ (g ∘ f ) = F ▭ g ∘ F ▭ f.
Proof.
  exact (functor_comp F f g).
Defined.

natural transformations and isomorphisms

Definition nat_iso {B C:category} (F G:[B ,C ]) := iso F G.

Definition makeNattrans {C D:category} {F G:[C ,D ]}
           (mor : ∏ x : C , F ◾ x --> G ◾ x)
           (eqn : ∏ c c' f, mor c' ∘ F ▭ f = G ▭ f ∘ mor c) :
  F --> G
  := (mor ,,eqn).

Definition makeNattrans_op {C D:category} {F G:[C ^op ,D ]}
           (mor : ∏ x : C , F ◾ x --> G ◾ x)
           (eqn : ∏ c c' f, mor c' ∘ F ▭ f = G ▭ f ∘ mor c) :
  F --> G
  := (mor ,,eqn).

Definition makeNatiso {C D:category} {F G:[C ,D ]}
           (mor : ∏ x : C , iso (F ◾ x) (G ◾ x))
           (eqn : ∏ c c' f, mor c' ∘ F ▭ f = G ▭ f ∘ mor c) :
  nat_iso F G.
Proof.
  refine (makeNattrans mor eqn,,_). apply functor_iso_if_pointwise_iso; intro c. apply pr2.
Defined.

Definition makeNatiso_op {C D:category} {F G:[C ^op ,D ]}
           (mor : ∏ x : C , iso (F ◾ x) (G ◾ x))
           (eqn : ∏ c c' f, mor c' ∘ F ▭ f = G ▭ f ∘ mor c) :
  nat_iso F G.
Proof.
  refine (makeNattrans_op mor eqn,,_). apply functor_iso_if_pointwise_iso; intro c. apply pr2.
Defined.

Lemma move_inv {C:category} {a a' b' b:C} {f : a --> b} {f' : a' --> b'}
      {i : a --> a'} {i' : a' --> a} {j : b --> b'} {j' : b' --> b} :
  is_inverse_in_precat i i' → is_inverse_in_precat j j' →
  j ∘ f = f' ∘ i → j' ∘ f' = f ∘ i'.
Proof.
  intros I J r. rewrite <- id_right. rewrite (! pr1 J). rewrite assoc.
  apply (maponpaths (λ k, j' ∘ k)). rewrite <- assoc. rewrite r.
  rewrite assoc. rewrite (pr2 I). rewrite id_left. reflexivity.
Defined.

Lemma weq_iff_iso_SET {X Y:SET} (f:X -->Y) : is_iso f ↔ isweq f.
Proof.
  split.
  - intro i. set (F := make_iso f i).
    refine (isweq_iso f (inv_from_iso F)
                   (λ x, eqtohomot (iso_inv_after_iso F) x)
                   (λ y, eqtohomot (iso_after_iso_inv F) y)).
  - exact (λ i Z, weqproperty (weqbfun (Z:hSet) (make_weq f i))).
Defined.

Lemma weq_to_iso_SET {X Y:SET} : iso X Y ≃ ((X:hSet ) ≃ (Y:hSet )).
Proof.
  intros. apply weqfibtototal; intro f. apply weqiff.
  - apply weq_iff_iso_SET.
  - apply isaprop_is_iso.
  - apply isapropisweq.
Defined.

Definition functor_to_opp_opp {C:category} : C ⟶ C ^op ^op
  := makeFunctor (λ c,c) (λ a b f,f) (λ c, idpath _) (λ a b c f g, idpath _).

Definition makeFunctor_op {C D:category}
           (obj : ob C → ob D)
           (mor : ∏ a b : C , b --> a → obj a --> obj b)
           (identity : ∏ c, mor c c (identity c) = identity (obj c))
           (compax : ∏ (a b c : C) (f : b --> a) (g : c --> b ),
                       mor a c (f ∘ g) = mor b c g ∘ mor a b f) :
  C ^op ⟶ D
  := (obj ,, mor ),, identity ,, compax.

Definition opp_ob {C:category} : ob C → ob C ^op
  := λ c, c.

Definition rm_opp_ob {C:category} : ob C ^op → ob C
  := λ c, c.

Definition opp_mor {C:category} {b c:C} : Hom C b c → Hom C ^op c b
  := λ f, f.

Definition rm_opp_mor {C:category} {b c:C} : Hom C ^op b c → Hom C c b
  := λ f, f.

Definition opp_mor_eq {C:category} {a b:C} (f g:a --> b) :
  opp_mor f = opp_mor g → f = g
  := idfun _.

Lemma opp_opp_precat_ob_mor (C : precategory_ob_mor) : C = opp_precat_ob_mor (opp_precat_ob_mor C).
Proof.
  induction C as [ob mor]. reflexivity.
Defined.

Lemma opp_opp_precat_ob_mor_compute (C : precategory_ob_mor) :
  idpath _ = maponpaths precategory_id_comp (opp_opp_precat_ob_mor C).
Proof.
  induction C as [ob mor]. reflexivity.
Defined.

Lemma opp_opp_precat_data (C : precategory_data)
   : C = opp_precat_data (opp_precat_data C).
Proof.
  induction C as [[ob mor] [id co]]. reflexivity.
Defined.

Lemma opp_opp_precat (C:category) : C = C ^op ^op.
Proof.
  apply category_eq.   reflexivity.
Qed.

Definition functorOp {B C : category} : [B , C ] ^op ⟶ [B ^op , C ^op ].
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  { exact functor_opp. }
  { intros H I p. ∃ (λ b, pr1 p b).
    abstract (intros b b' f; simpl; exact (! nat_trans_ax p _ _ f)) using _L_. }
  { abstract (intros H; now apply (nat_trans_eq (homset_property _))). }
  { abstract (intros H J K p q; now apply (nat_trans_eq (homset_property _))). }
Defined.

Definition functorOp' {B C:category} : [B ,C ] ⟶ [B ^op ,C ^op ]^op.
Proof.
  exact (functorOp functorOp).
Defined.

Definition functorRmOp {B C : category} : [B ^op , C ^op ] ⟶ [B , C ] ^op.
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  { exact functor_opp. }
  { intros H I p. ∃ (λ b, pr1 p b).
    abstract (intros b b' f; simpl; exact (! nat_trans_ax p _ _ f)) using _L_. }
  { abstract (intros H; now apply (nat_trans_eq (homset_property _))) using _L_. }
  { abstract (intros H J K p q; now apply (nat_trans_eq (homset_property _))). }
Defined.

Definition functorMvOp {B C:category} : [B ,C ^op ] ⟶ [B ^op ,C ]^op.
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  { exact functor_opp. }
  { intros H I p. ∃ (λ b, pr1 p b).
    abstract (intros b b' f; simpl; exact (! nat_trans_ax p _ _ f)) using _L_. }
  { abstract (intros H; now apply (nat_trans_eq (homset_property _))). }
  { abstract (intros H J K p q; now apply (nat_trans_eq (homset_property _))). }
Defined.

Lemma functorOpIso {B C:category} : categoryIsomorphism [B , C ]^op [B ^op , C ^op ].
Proof.
  unshelve refine (_,,_).
  { unshelve refine (_,,_).
    { exact functorOp. }
    { intros H H'. unshelve refine (isweq_iso _ _ _ _).
      { simpl. intros p. unshelve refine (makeNattrans _ _).
        { intros b. exact (pr1 p b). }
        { abstract (intros b b' f; simpl; exact (!nat_trans_ax p _ _ f)) using _L_. } }
      { abstract (intro p; apply nat_trans_eq;
                  [ apply homset_property
                  | intro b; reflexivity ]) using _L_. }
      { abstract (intro p; apply nat_trans_eq;
                  [ apply homset_property
                  | intro b; reflexivity ]) using _L_. }}}
  { simpl. unshelve refine (isweq_iso _ _ _ _).
    { exact (functor_opp : B^op ⟶ C^op → B ⟶ C). }
    { abstract (intros H; simpl; apply (functor_eq _ _ (homset_property C));
                unshelve refine (total2_paths_f _ _); reflexivity) using _L_. }
    { abstract (intros H; simpl; apply functor_eq;
                [ exact (homset_property C^op)
                | unshelve refine (total2_paths_f _ _); reflexivity]). } }
Defined.

Definition functorOpEmb {B C:category} : categoryEmbedding [B , C ]^op [B ^op , C ^op ]
  := pr1 functorOpIso.

Lemma functor_op_rm_op_eq {C D:category} (F : C ^op ⟶ D ^op) :
  functorOp (functorRmOp F) = F.
Proof.
  apply functor_eq.
  { apply homset_property. }
  unshelve refine (total2_paths_f _ _); reflexivity.
Qed.

Lemma functor_rm_op_op_eq {C D:category} (F : C ⟶ D) :
  functorRmOp (functorOp F) = F.
Proof.
  apply functor_eq.
  { apply homset_property. }
  unshelve refine (total2_paths_f _ _); reflexivity.
Qed.

Lemma functor_op_op_eq {C D:category} (F : C ⟶ D) :
  functorOp (functorOp F) = F.
Proof.
  apply functor_eq.
  { apply homset_property. }
  unshelve refine (total2_paths_f _ _); reflexivity.
Qed.

Lemma total2_paths1 {A : UU} {B : A → UU} (a:A) {b b':B a} :
  b =b' → tpair B a b = tpair B a b'.
Proof.
  intro e. induction e. reflexivity.
Defined.

Goal ∏ A : UU , ∏ B : A → UU , ∏ p : (∑ a, B a ), p = tpair B (pr1 p) (pr2 p).
  induction p as [a b]. reflexivity.
Defined.

Goal ∏ X Y (f:X →Y ), f = λ x, f x.
  reflexivity.
Defined.

Definition categoryWithStructure (C:category) (P:ob C → UU) : category.
Proof.
  use makecategory.
  - exact (∑ c:C, P c).
  - intros x y. exact (pr1 x --> pr1 y).
  - intros. apply homset_property.
  - intros x. apply identity.
  - intros x y z f g. exact (g ∘ f).
  - intros. apply id_left.
  - intros. apply id_right.
  - intros. apply assoc.
  - intros. apply assoc'.
Defined.

Definition functorWithStructures {C:category} {P Q:ob C → UU}
           (F : ∏ c, P c → Q c) : categoryWithStructure C P ⟶ categoryWithStructure C Q.
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  - exact (λ c, (pr1 c ,, F (pr1 c) (pr2 c))).
  - intros c c' f. exact f.
  - reflexivity.
  - reflexivity.
Defined.

Definition addStructure {B C:category} {P:ob C → UU}
           (F:B ⟶C) (h : ∏ b, P(F b)) : B ⟶ categoryWithStructure C P.
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  - intros b. exact (F b,,h b).
  - intros b b' f. exact (# F f).
  - abstract (intros b; simpl; apply functor_id) using _L_.
  - abstract (intros b b' b'' f g; simpl; apply functor_comp) using _L_.
Defined.

Lemma identityFunction : ∏ (T:SET) (f:T -->T) (t:T:hSet ), f = identity T → f t = t.
Proof.
  intros ? ? ? e. exact (eqtohomot e t).
Defined.

Lemma identityFunction' : ∏ (T:SET) (t:T:hSet ), identity T t = t.
Proof.
  reflexivity.
Defined.

Lemma functor_identity_object {C:category} (c:C) : functor_identity C ◾ c = c.
Proof.
  reflexivity.
Defined.

Lemma functor_identity_arrow {C:category} {c c':C} (f:c -->c') : functor_identity C ▭ f = f.
Proof.
  reflexivity.
Defined.

Definition constantFunctor (C:category) {D:category} (d:D) : [C ,D ].
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  - exact (λ _, d).
  - intros c c' f; simpl. exact (identity d).
  - intros c; simpl. reflexivity.
  - abstract (simpl; intros a b c f g; apply pathsinv0, id_left) using _L_.
Defined.

Definition functor_composite_functor {A B C:category} (F:A ⟶B) :
  [B ,C ] ⟶ [A ,C ].
Proof.
  unshelve refine (makeFunctor _ _ _ _).
  - exact (λ G, G □ F).
  - intros G G' p; simpl.
    unshelve refine (@makeNattrans A C (G □ F) (G' □ F) (λ a, p ◽ (F ◾ a )) _).
    abstract (
        intros a a' f; rewrite 2? nattrans_nattrans_object_assoc;
        exact (nattrans_naturality p (F ▭ f))) using _L_.
  - abstract (
        intros G; now apply (nat_trans_eq (homset_property C))) using _M_.
  - abstract (
        intros G G' G'' p q; now apply (nat_trans_eq (homset_property C))) using _N_.
Defined.

Definition ZeroMaps (C:category) :=
  ∑ (zero : ∏ a b:C , a --> b ),
  (∏ a b c, ∏ f:b --> c , f ∘ zero a b = zero a c )
    ×
    (∏ a b c, ∏ f:c --> b , zero b a ∘ f = zero c a ).

Definition is {C:category} (zero: ZeroMaps C) {a b:C} (f:a -->b)
  := f = pr1 zero _ _.

Definition ZeroMaps_opp (C:category) : ZeroMaps C → ZeroMaps C ^op
  := λ z, (λ a b, pr1 z b a ) ,, pr2 (pr2 z) ,, pr1 (pr2 z).

Definition ZeroMaps_opp_opp (C:category) (zero:ZeroMaps C) :
  ZeroMaps_opp C ^op (ZeroMaps_opp C zero) = zero.
Proof.
  unshelve refine (total2_paths_f _ _).
  - reflexivity.
  - unshelve refine (total2_paths_f _ _); reflexivity.
Defined.