Library UniMath.CategoryTheory.categories.monoids
Category of monoids
Contents
- Precategory of monoids
- Category of monoids
- Forgetful functor to HSET
- Free functor from HSET
- Free/forgetful adjunction
Require Import UniMath.Foundations.PartD.
Require Import UniMath.Foundations.Propositions.
Require Import UniMath.Foundations.Sets.
Require Import UniMath.Foundations.UnivalenceAxiom.
Require Import UniMath.Algebra.BinaryOperations.
Require Import UniMath.Algebra.Monoids.
Require Import UniMath.Algebra.Free_Monoids_and_Groups.
Require Import UniMath.Combinatorics.Lists.
Require Import UniMath.Algebra.IteratedBinaryOperations.
Require Import UniMath.CategoryTheory.Core.Categories.
Require Import UniMath.CategoryTheory.Core.Isos.
Require Import UniMath.CategoryTheory.Core.NaturalTransformations.
Require Import UniMath.CategoryTheory.Core.Univalence.
Require Import UniMath.CategoryTheory.Core.Functors.
Require Import UniMath.CategoryTheory.categories.HSET.Core.
Require Import UniMath.CategoryTheory.Adjunctions.Core.
Local Open Scope cat.
Section def_monoid_precategory.
Definition monoid_fun_space (A B : monoid) : hSet :=
make_hSet (monoidfun A B) (isasetmonoidfun A B).
Definition monoid_precategory_ob_mor : precategory_ob_mor :=
tpair (λ ob : UU, ob → ob → UU) monoid (λ A B : monoid, monoid_fun_space A B).
Definition monoid_precategory_data : precategory_data :=
make_precategory_data
monoid_precategory_ob_mor (λ (X : monoid), ((idmonoidiso X) : monoidfun X X))
(fun (X Y Z : monoid) (f : monoidfun X Y) (g : monoidfun Y Z) ⇒ monoidfuncomp f g).
Local Lemma monoid_id_left {X Y : monoid} (f : monoidfun X Y) :
monoidfuncomp (idmonoidiso X) f = f.
Proof.
use monoidfun_paths. use idpath.
Defined.
Opaque monoid_id_left.
Local Lemma monoid_id_right {X Y : monoid} (f : monoidfun X Y) :
monoidfuncomp f (idmonoidiso Y) = f.
Proof.
use monoidfun_paths. use idpath.
Defined.
Opaque monoid_id_right.
Local Lemma monoid_assoc (X Y Z W : monoid) (f : monoidfun X Y) (g : monoidfun Y Z)
(h : monoidfun Z W) :
monoidfuncomp f (monoidfuncomp g h) = monoidfuncomp (monoidfuncomp f g) h.
Proof.
use monoidfun_paths. use idpath.
Defined.
Opaque monoid_assoc.
Lemma is_precategory_monoid_precategory_data : is_precategory monoid_precategory_data.
Proof.
use make_is_precategory_one_assoc.
- intros a b f. use monoid_id_left.
- intros a b f. use monoid_id_right.
- intros a b c d f g h. use monoid_assoc.
Qed.
Definition monoid_precategory : precategory :=
make_precategory monoid_precategory_data is_precategory_monoid_precategory_data.
Lemma has_homsets_monoid_precategory : has_homsets monoid_precategory.
Proof.
intros X Y. use isasetmonoidfun.
Qed.
End def_monoid_precategory.
Lemma monoid_iso_is_equiv (A B : ob monoid_precategory) (f : iso A B) : isweq (pr1 (pr1 f)).
Proof.
use isweq_iso.
- exact (pr1monoidfun _ _ (inv_from_iso f)).
- intros x.
use (toforallpaths _ _ _ (subtypeInjectivity _ _ _ _ (iso_inv_after_iso f)) x).
intros x0. use isapropismonoidfun.
- intros x.
use (toforallpaths _ _ _ (subtypeInjectivity _ _ _ _ (iso_after_iso_inv f)) x).
intros x0. use isapropismonoidfun.
Defined.
Opaque monoid_iso_is_equiv.
Lemma monoid_iso_equiv (X Y : ob monoid_precategory) : iso X Y → monoidiso X Y.
Proof.
intro f.
use make_monoidiso.
- exact (make_weq (pr1 (pr1 f)) (monoid_iso_is_equiv X Y f)).
- exact (pr2 (pr1 f)).
Defined.
Lemma monoid_equiv_is_iso (X Y : ob monoid_precategory) (f : monoidiso X Y) :
@is_iso monoid_precategory X Y (monoidfunconstr (pr2 f)).
Proof.
use is_iso_qinv.
- exact (monoidfunconstr (pr2 (invmonoidiso f))).
- use make_is_inverse_in_precat.
+ use monoidfun_paths. use funextfun. intros x. use homotinvweqweq.
+ use monoidfun_paths. use funextfun. intros y. use homotweqinvweq.
Defined.
Opaque monoid_equiv_is_iso.
Lemma monoid_equiv_iso (X Y : ob monoid_precategory) : monoidiso X Y → iso X Y.
Proof.
intros f. exact (@make_iso monoid_precategory X Y (monoidfunconstr (pr2 f))
(monoid_equiv_is_iso X Y f)).
Defined.
Lemma monoid_iso_equiv_is_equiv (X Y : monoid_precategory) : isweq (monoid_iso_equiv X Y).
Proof.
use isweq_iso.
- exact (monoid_equiv_iso X Y).
- intros x. use eq_iso. use monoidfun_paths. use idpath.
- intros y. use monoidiso_paths. use subtypePath.
+ intros x0. use isapropisweq.
+ use idpath.
Defined.
Opaque monoid_iso_equiv_is_equiv.
Definition monoid_iso_equiv_weq (X Y : ob monoid_precategory) : (iso X Y) ≃ (monoidiso X Y).
Proof.
use make_weq.
- exact (monoid_iso_equiv X Y).
- exact (monoid_iso_equiv_is_equiv X Y).
Defined.
Lemma monoid_equiv_iso_is_equiv (X Y : ob monoid_precategory) : isweq (monoid_equiv_iso X Y).
Proof.
use isweq_iso.
- exact (monoid_iso_equiv X Y).
- intros y. use monoidiso_paths. use subtypePath.
+ intros x0. use isapropisweq.
+ use idpath.
- intros x. use eq_iso. use monoidfun_paths. use idpath.
Defined.
Opaque monoid_equiv_iso_is_equiv.
Definition monoid_equiv_weq_iso (X Y : ob monoid_precategory) : (monoidiso X Y) ≃ (iso X Y).
Proof.
use make_weq.
- exact (monoid_equiv_iso X Y).
- exact (monoid_equiv_iso_is_equiv X Y).
Defined.
Definition monoid_precategory_isweq (X Y : ob monoid_precategory) :
isweq (λ p : X = Y, idtoiso p).
Proof.
use (@isweqhomot
(X = Y) (iso X Y)
(pr1weq (weqcomp (monoid_univalence X Y) (monoid_equiv_weq_iso X Y)))
_ _ (weqproperty (weqcomp (monoid_univalence X Y) (monoid_equiv_weq_iso X Y)))).
intros e. induction e.
use (pathscomp0 weqcomp_to_funcomp_app).
use total2_paths_f.
- use idpath.
- use proofirrelevance. use isaprop_is_iso.
Defined.
Opaque monoid_precategory_isweq.
Definition monoid_precategory_is_univalent : is_univalent monoid_precategory.
Proof.
use make_is_univalent.
- intros X Y. exact (monoid_precategory_isweq X Y).
- exact has_homsets_monoid_precategory.
Defined.
Definition monoid_category : univalent_category :=
make_univalent_category monoid_precategory monoid_precategory_is_univalent.
End def_monoid_category.
Definition monoid_forgetful_functor : functor monoid_precategory HSET.
Proof.
use make_functor.
- use make_functor_data.
+ intro; exact (pr1setwithbinop (pr1monoid ltac:(assumption))).
+ intros ? ? f; exact (pr1monoidfun _ _ f).
- split.
+
Identity axiom
intro; reflexivity.
+
+
Composition axiom
intros ? ? ? ? ?; reflexivity.
Defined.
Lemma monoid_forgetful_functor_is_faithful : faithful monoid_forgetful_functor.
Proof.
unfold faithful.
intros ? ?.
apply isinclpr1.
apply isapropismonoidfun.
Defined.
Defined.
Lemma monoid_forgetful_functor_is_faithful : faithful monoid_forgetful_functor.
Proof.
unfold faithful.
intros ? ?.
apply isinclpr1.
apply isapropismonoidfun.
Defined.
Definition monoid_free_functor : functor HSET monoid_precategory.
Proof.
use make_functor.
- use make_functor_data.
+ intros s; exact (free_monoid s).
+ intros ? ? f; exact (free_monoidfun f).
- split.
+
Identity axiom
Composition axiom
intros ? ? ? ? ?.
abstract (apply monoidfun_paths, funextfun, (free_monoidfun_comp_homot f g)).
Defined.
abstract (apply monoidfun_paths, funextfun, (free_monoidfun_comp_homot f g)).
Defined.
Definition monoid_free_forgetful_unit :
nat_trans (functor_identity _)
(functor_composite monoid_free_functor monoid_forgetful_functor).
Proof.
use make_nat_trans.
- intros ?; exact singleton.
- intros ? ? ?.
abstract (apply funextfun; intro; reflexivity).
Defined.
nat_trans (functor_identity _)
(functor_composite monoid_free_functor monoid_forgetful_functor).
Proof.
use make_nat_trans.
- intros ?; exact singleton.
- intros ? ? ?.
abstract (apply funextfun; intro; reflexivity).
Defined.
This amounts to naturality of the counit: mapping commutes with folding
Lemma iterop_list_mon_map {m n : monoid} (f : monoidfun m n) :
∏ l, ((iterop_list_mon ∘ map (pr1monoidfun m n f)) l =
(pr1monoidfun _ _ f ∘ iterop_list_mon) l)%functions.
Proof.
apply list_ind.
- apply pathsinv0, monoidfununel.
- intros x xs H.
unfold funcomp in ×.
refine (maponpaths iterop_list_mon (map_cons _ _ _) @ _).
refine (iterop_list_mon_step _ _ @ _).
refine (_ @ !maponpaths _ (iterop_list_mon_step _ _)).
refine (_ @ !binopfunisbinopfun f _ _).
apply maponpaths.
assumption.
Qed.
∏ l, ((iterop_list_mon ∘ map (pr1monoidfun m n f)) l =
(pr1monoidfun _ _ f ∘ iterop_list_mon) l)%functions.
Proof.
apply list_ind.
- apply pathsinv0, monoidfununel.
- intros x xs H.
unfold funcomp in ×.
refine (maponpaths iterop_list_mon (map_cons _ _ _) @ _).
refine (iterop_list_mon_step _ _ @ _).
refine (_ @ !maponpaths _ (iterop_list_mon_step _ _)).
refine (_ @ !binopfunisbinopfun f _ _).
apply maponpaths.
assumption.
Qed.
The counit of this adjunction is the "folding" function
[a, b, …, z] ↦ a · b · ⋯ · z
(This is known to Haskell programmers as mconcat.)
Definition monoid_free_forgetful_counit :
nat_trans (functor_composite monoid_forgetful_functor monoid_free_functor )
(functor_identity _).
Proof.
use make_nat_trans.
- intros ?.
use tpair.
+ intro; apply iterop_list_mon; assumption.
+ split.
× intros ? ?; apply iterop_list_mon_concatenate.
× reflexivity.
- intros ? ? f; apply monoidfun_paths.
apply funextfun; intro; simpl in ×.
apply (iterop_list_mon_map f).
Defined.
Definition monoid_free_forgetful_adjunction_data :
adjunction_data HSET monoid_precategory .
Proof.
use tpair; [|use tpair]. - exact monoid_free_functor.
- exact monoid_forgetful_functor.
- split.
+ exact monoid_free_forgetful_unit.
+ exact monoid_free_forgetful_counit.
Defined.
Lemma monoid_free_forgetful_adjunction :
form_adjunction' monoid_free_forgetful_adjunction_data.
Proof.
split; intro.
- apply monoidfun_paths.
apply funextfun.
simpl; unfold funcomp.
unfold homot; apply list_ind; [reflexivity|].
intros x xs ?.
unfold funcomp.
rewrite map_cons.
refine (iterop_list_mon_step ((cons _ _) : pr1hSet (free_monoid _))
(map singleton xs) @ _).
apply maponpaths; assumption.
- reflexivity.
Qed.
nat_trans (functor_composite monoid_forgetful_functor monoid_free_functor )
(functor_identity _).
Proof.
use make_nat_trans.
- intros ?.
use tpair.
+ intro; apply iterop_list_mon; assumption.
+ split.
× intros ? ?; apply iterop_list_mon_concatenate.
× reflexivity.
- intros ? ? f; apply monoidfun_paths.
apply funextfun; intro; simpl in ×.
apply (iterop_list_mon_map f).
Defined.
Definition monoid_free_forgetful_adjunction_data :
adjunction_data HSET monoid_precategory .
Proof.
use tpair; [|use tpair]. - exact monoid_free_functor.
- exact monoid_forgetful_functor.
- split.
+ exact monoid_free_forgetful_unit.
+ exact monoid_free_forgetful_counit.
Defined.
Lemma monoid_free_forgetful_adjunction :
form_adjunction' monoid_free_forgetful_adjunction_data.
Proof.
split; intro.
- apply monoidfun_paths.
apply funextfun.
simpl; unfold funcomp.
unfold homot; apply list_ind; [reflexivity|].
intros x xs ?.
unfold funcomp.
rewrite map_cons.
refine (iterop_list_mon_step ((cons _ _) : pr1hSet (free_monoid _))
(map singleton xs) @ _).
apply maponpaths; assumption.
- reflexivity.
Qed.