Library UniMath.Induction.M.ComputationalM

Refinement of M-types

M-types can be refined to satisfy the right definitional equalities. This idea is from Felix Rech's Bachelor's thesis, and Felix Rech also developed together with Luis Scoccola a first formalization in UniMath as project work of the UniMath 2017 school. The present formalization was obtained as project work of the UniMath 2019 school and is heavily inspired by that former formalization.
Author: Dominik Kirst (@dominik-kirst) and Ralph Matthes (@rmatthes)

Require Import UniMath.Foundations.All.
Require Import UniMath.MoreFoundations.All.

Require Import UniMath.CategoryTheory.Core.Categories.
Require Import UniMath.CategoryTheory.Core.Functors.
Require Import UniMath.CategoryTheory.FunctorCoalgebras.
Require Import UniMath.CategoryTheory.categories.Type.Core.

Require Import UniMath.Induction.PolynomialFunctors.
Require Import UniMath.Induction.M.Core.
Require Import UniMath.Induction.M.Uniqueness.

The construction is called a refinement: as input we take any final coalgebra for the respective polynomial functor describing an M-type (hence, with the provable coiteration rule), the output is the refined final coalgebra with the equational rule of coiteration holding definitionally: Lemma corec_computation is proved merely by idpath. Of course, both coalgebras are equal - provably (Lemma coalgebras_eq).
Section Refinement.

  Context (A : UU).
  Context (B : A → UU).
  Local Notation F := (polynomial_functor A B).

  Variable M0 : coalgebra F.
  Local Notation carrierM0 := (coalgebra_ob _ M0).
  Local Notation destrM0 := (coalgebra_mor _ M0).

  Variable finalM0 : is_final M0.
  Local Notation corecM0 C := (pr11 (finalM0 C)).

  Local Open Scope cat.
  Local Open Scope functions.


  Definition carrierM := ∑ m0 : carrierM0, ∃ C c, corecM0 C c = m0.


  Definition corecM (C : coalgebra F) (c : coalgebra_ob _ C) : carrierM.
  Proof.
    ∃ (corecM0 C c). apply hinhpr. ∃ C, c. apply idpath.
  Defined.


  Local Definition P (m0 : carrierM0) :=
    ∑ af : F carrierM, destrM0 m0 = # F pr1 af.

in order to show P to be a proposition, a not obviously equivalent formulation is given for which it is easy to show isaprop
  Local Definition P' (m0 : carrierM0) :=
    ∑ ap : ∑ a : A, pr1 (destrM0 m0) = a,
                 ∏ (b : B (pr1 ap)),
                 ∑ mp : ∑ m0' : carrierM0,
                                transportf (λ a, B a → carrierM0)
                                           (pr2 ap)
                                           (pr2 (destrM0 m0)) b =
                                m0',
                                ∃ C c, corecM0 C c = pr1 mp.

the easy auxiliary lemma
  Local Lemma P'_isaprop m0 :
    isaprop (P' m0).
  Proof.
    apply isofhleveltotal2.
    - apply isofhlevelcontr.
      apply iscontrcoconusfromt.
    - intro ap; induction ap as [a p].
      apply impred; intros b.
      apply isofhleveltotal2.
      + apply isofhlevelcontr.
        apply iscontrcoconusfromt.
      + intro mp; induction mp as [m0' q].
        apply isapropishinh.
  Defined.

the crucial lemma
  Local Lemma P_isaprop (m0 : carrierM0) :
    isaprop (P m0).
  Proof.
    use (@isofhlevelweqb _ _ (P' m0) _ (P'_isaprop m0)).
    simple refine (weqcomp (weqtotal2asstor _ _) _).
    simple refine (weqcomp _ (invweq (weqtotal2asstor _ _))).
    apply weqfibtototal; intro a.
    intermediate_weq (
        ∑ f : B a → carrierM,
              ∑ p : pr1 (destrM0 m0) = a,
                    transportf
                      (λ a, B a → carrierM0)
                      p
                      (pr2 (destrM0 m0)) =
                    pr1 ∘ f).
    {
      apply weqfibtototal; intro f.
      apply total2_paths_equiv.
    }
    intermediate_weq (∑ p : pr1 (destrM0 m0) = a,
                            ∑ f : B a → carrierM,
                                  transportf
                                    (λ a, B a → carrierM0)
                                    p
                                    (pr2 (destrM0 m0)) =
                                  pr1 ∘ f).
    { apply weqtotal2comm. }
    apply weqfibtototal; intro p.
    intermediate_weq (∑ fg : ∑ f : B a → carrierM0,
                                   ∏ b, ∃ C c, corecM0 C c = f b,
                        transportf
                          (λ a, B a → carrierM0)
                          p
                          (pr2 (destrM0 m0)) =
                        pr1 fg).
    { use weqbandf.
      - apply weqfuntototaltototal.
      - cbn.
        intro f.
        apply idweq.
    }
    intermediate_weq (∑ f : B a → carrierM0,
                            ∑ _ : ∏ b, ∃ C c, corecM0 C c = f b,
                        transportf
                          (λ a, B a → carrierM0)
                          p
                          (pr2 (destrM0 m0)) =
                        f).
    { apply weqtotal2asstor. }
    intermediate_weq (∑ f : B a → carrierM0,
                            ∑ _ : ∏ b, ∃ C c, corecM0 C c = f b,
                        ∏ b, transportf
                               (λ a, B a → carrierM0)
                               p
                               (pr2 (destrM0 m0)) b =
                             f b).
    { apply weqfibtototal; intro f.
      apply weqfibtototal; intros _.
      apply weqtoforallpaths.
    }
    intermediate_weq (∑ f : B a → carrierM0,
                            ∏ b, ∑ _ : ∃ C c, corecM0 C c = f b,
                        transportf
                          (λ a, B a → carrierM0)
                          p
                          (pr2 (destrM0 m0)) b =
                        f b).
    { apply weqfibtototal; intro f.
      apply invweq.
      apply weqforalltototal.
    }
    intermediate_weq (∏ b, ∑ m0' : carrierM0,
                                   ∑ _ : ∃ C c, corecM0 C c = m0',
                        transportf
                          (λ a, B a → carrierM0)
                          p
                          (pr2 (destrM0 m0)) b =
                        m0').
    { apply invweq.
      apply weqforalltototal.
    }
    apply weqonsecfibers; intro b.
    intermediate_weq (∑ m0' : carrierM0,
                              ∑ _ : transportf
                                      (λ a, B a → carrierM0)
                                      p
                                      (pr2 (destrM0 m0)) b =
                                    m0',
                                    ∃ C c, corecM0 C c = m0').
    {
      apply weqfibtototal; intro m0'.
      apply weqdirprodcomm.
    }
    intermediate_weq (∑ mp : ∑ m0', transportf (λ a, B a → carrierM0) p
                                               (pr2 (destrM0 m0)) b = m0',
                             ∃ C c, corecM0 C c = pr1 mp).
    {
      apply invweq.
      apply weqtotal2asstor.
    }
    use weqbandf.
    - apply weqfibtototal; intro m0'.
      apply idweq.
    - cbn. intro mp.
      apply idweq.
  Defined.


  Local Definition destrM' (m : carrierM) : P (pr1 m).
  Proof.
    induction m as [m0 H]. apply (squash_to_prop H); try apply P_isaprop.
    intros [C [c H1]].
    refine ((# F (corecM C) ∘ (pr2 C)) c,, _). cbn [pr1]. clear H.
    assert (H : is_coalgebra_homo F (corecM0 C)).
    { destruct finalM0 as [[G H] H']. apply H. }
    apply toforallpaths in H.
    apply pathsinv0.
    intermediate_path (destrM0 (corecM0 C c)).
    - apply H.
    - apply maponpaths. assumption.
  Defined.

  Definition destrM (m : carrierM) : F carrierM :=
    pr1 (destrM' m).

  Definition M : coalgebra F :=
    (carrierM,, destrM).


  Lemma corec_computation C c :
    destrM (corecM C c) = # F (corecM C) (pr2 C c).
  Proof.
    apply idpath.
  Defined.


  Lemma eq_corecM0 m0 :
    corecM0 M0 m0 = m0.
  Proof.
    induction finalM0 as [[G H1] H2]. cbn.
    specialize (H2 (coalgebra_homo_id F M0)).
    change (pr1 (G,, H1) m0 = pr1 (coalgebra_homo_id F M0) m0).
    apply (maponpaths (fun X ⇒ pr1 X m0)).
    apply pathsinv0.
    assumption.
  Defined.

  Definition injectM0 m0 :
    ∃ C c, corecM0 C c = m0.
  Proof.
    apply hinhpr. ∃ M0, m0. apply eq_corecM0.
  Defined.

  Lemma carriers_weq :
    carrierM ≃ carrierM0.
  Proof.
    apply (weq_iso pr1 (λ m0, m0,, injectM0 m0)).
    - intros [m H]. cbn. apply maponpaths, ishinh_irrel.
    - intros x. cbn. apply idpath.
  Defined.

  Lemma carriers_eq :
    carrierM = carrierM0.
  Proof.
    apply weqtopaths, carriers_weq.
  Defined.
needs to be transparent


  Local Lemma eq1 (m0 : carrierM0) :
    transportf (λ X, X → F X) carriers_eq destrM m0
    = transportf (λ X, F X) carriers_eq (destrM (transportf (λ X, X) (!carriers_eq) m0)).
  Proof.
    destruct carriers_eq. apply idpath.
  Defined.

  Local Lemma eq2 (m0 : carrierM0) :
    transportf (λ X, X) (!carriers_eq) m0 = m0,, injectM0 m0.
  Proof.
    apply (transportf_pathsinv0' (idfun UU) carriers_eq).
    unfold carriers_eq. rewrite weqpath_transport. apply idpath.
  Defined.

  Local Lemma eq3 m0 :
    destrM (m0,, injectM0 m0) = pr1 (destrM0 m0),, corecM M0 ∘ pr2 (destrM0 m0).
  Proof.
    apply idpath.
  Defined.

  Lemma coalgebras_eq :
    M = M0.
  Proof.
    use total2_paths_f; try apply carriers_eq.
    apply funextfun. intro m0.
    rewrite eq1. rewrite eq2. rewrite eq3.
    cbn. unfold polynomial_functor_obj.
    rewrite transportf_total2_const.
    use total2_paths_f; try apply idpath.
    cbn. apply funextsec. intros b. rewrite <- helper_A.
    unfold carriers_eq. rewrite weqpath_transport.
    cbn. rewrite eq_corecM0. apply idpath.
  Defined.


  Lemma finalM : is_final M.
  Proof.
    rewrite coalgebras_eq. apply finalM0.
  Defined.

End Refinement.