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Connect type categories with comprehension categories #170

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merged 16 commits into from
Apr 27, 2020

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fizruk
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@fizruk fizruk commented Apr 16, 2020

Progress:

  • Construct comprehension category from a (non-split) type category:
    • Add displayed category induced by a (non-split) type category
    • Show that it is a fibration;
    • Introduce displayed functor from induced displayed category to codomain displayed category;
    • Prove that the functor is cartesian.
  • Show that constructed comprehension category is full and faithful;
  • Show that constructed comprehension category is univalent;

@fizruk fizruk marked this pull request as ready for review April 26, 2020 21:59
apply identity.
Defined.

Definition typecat_idtoiso_dpr
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This definition does not seem to be needed.

Context {C : precategory}.
Context (TC : typecat_obj_ext_structure C).

Definition typecat_comp_ext_compare
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Maybe a different definition, that returned an isomorphism instead of a morphism, would be more useful.

apply (isofhleveltotal2 2).
+ apply homset_property.
+ intro. apply isasetaprop. apply homset_property.
Defined.
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Maybe use Qed. here?

: typecat_iso_triangle _ A B ≃ @iso_disp C (typecat_disp TC) _ _ (identity_iso Γ) A B
:= (_,, typecat_is_triangle_idtoiso_fiber_disp_isweq A B).

Definition typecat_disp_is_disp_univalent
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This should work the other way round as well: if the displayed category is univalent, then type_cat_idtoiso_triangle should be an equivalence pointwise.

Context (TC : typecat_structure C).

(* NOTE: copied with slight modifications from https://github.com/UniMath/TypeTheory/blob/ad54ca1dad822e9c71acf35c27d0a39983269462/TypeTheory/Displayed_Cats/DisplayedCatFromCwDM.v#L114-L143 *)
Definition pullback_is_cartesian
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Does this lemma need the full typecat_structure? It looks like it only uses the object-extension structure.

@benediktahrens benediktahrens merged commit bf85f4e into UniMath:master Apr 27, 2020
@fizruk fizruk deleted the AKLV/comprehension-cats branch April 27, 2020 08:32
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2 participants