[WIP] A long way towards proving univalence in the category of hSets

src/Cat/Categories/Sets.agda

@@ -15,7 +15,7 @@ open import Cat.Category
 open import Cat.Category.Functor
 open import Cat.Category.Product
 open import Cat.Wishlist
-open import Cat.Equivalence as Eqv renaming (module NoEta to Eeq) using (AreInverses)
+open import Cat.Equivalence as Eqv renaming (module NoEta to Eeq) using (AreInverses ; module Equiv≃)
 
 module Equivalence = Eeq.Equivalence′
 
@@ -27,12 +27,33 @@ sym≃ = Equivalence.symmetry
 
 infixl 10 _⊙_
 
-inv-coe : {ℓ : Level} {A B : Set ℓ} {a : A}
-  → (p : A ≡ B) → coe (sym p) (coe p a) ≡ a
-inv-coe = {!!}
-inv-coe' : {ℓ : Level} {A B : Set ℓ} {a : A}
-  → (p : B ≡ A) → coe p (coe (sym p) a) ≡ a
-inv-coe' = {!!}
+module _ {ℓ : Level} {A : Set ℓ} {a : A} where
+  id-coe : coe refl a ≡ a
+  id-coe = begin
+    coe refl a ≡⟨ {!!} ⟩
+    a ∎
+
+module _ {ℓ : Level} {A B : Set ℓ} {a : A} where
+  inv-coe : (p : A ≡ B) → coe (sym p) (coe p a) ≡ a
+  inv-coe p =
+    let
+      D : (y : Set ℓ) → _ ≡ y → Set _
+      D _ q = coe (sym q) (coe q a) ≡ a
+      d : D A refl
+      d = begin
+        coe (sym refl) (coe refl a) ≡⟨⟩
+        coe refl (coe refl a)       ≡⟨ id-coe ⟩
+        coe refl a                  ≡⟨ id-coe ⟩
+        a ∎
+    in pathJ D d B p
+  inv-coe' : (p : B ≡ A) → coe p (coe (sym p) a) ≡ a
+  inv-coe' p =
+    let
+      D : (y : Set ℓ) → _ ≡ y → Set _
+      D _ q = coe (sym q) (coe q a) ≡ a
+      k : coe p (coe (sym p) a) ≡ a
+      k = pathJ D (trans id-coe id-coe) B (sym p)
+    in k
 
 module _ (ℓ : Level) where
   private
@@ -125,8 +146,8 @@ module _ (ℓ : Level) where
         iso : (p ≡ q) Eqv.≅ (proj₁ p ≡ proj₁ q)
         iso = f , g , inv
 
-      lem3 : {Q : A → Set ℓb} → ((a : A) → P a ≃ Q a)
-        → Σ A P ≃ Σ A Q
+      lem3 : {Q : A → Set ℓb}
+        → ((a : A) → P a ≃ Q a) → Σ A P ≃ Σ A Q
       lem3 {Q} eA = res
         where
         P→Q : ∀ {a} → P a ≡ Q a
@@ -165,9 +186,25 @@ module _ (ℓ : Level) where
         res = Eeq.fromIsomorphism iso
 
     module _ {ℓa ℓb : Level} {A : Set ℓa} {B : Set ℓb} where
-      postulate
       lem4 : isSet A → isSet B → (f : A → B)
         → isEquiv A B f ≃ isIso f
+      lem4 sA sB f =
+        let
+          obv : isEquiv A B f → isIso f
+          obv = Equiv≃.toIso A B
+          inv : isIso f → isEquiv A B f
+          inv = Equiv≃.fromIso A B
+          re-ve : (x : isEquiv A B f) → (inv ∘ obv) x ≡ x
+          re-ve = Equiv≃.inverse-from-to-iso A B
+          ve-re : (x : isIso f)       → (obv ∘ inv) x ≡ x
+          ve-re = Equiv≃.inverse-to-from-iso A B
+          iso : isEquiv A B f Eqv.≅ isIso f
+          iso = obv , inv ,
+            record
+              { verso-recto = funExt re-ve
+              ; recto-verso = funExt ve-re
+              }
+        in Eeq.fromIsomorphism iso
 
     module _ {hA hB : Object} where
       private
@@ -176,13 +213,24 @@ module _ (ℓ : Level) where
         B = proj₁ hB
         sB = proj₂ hB
 
-      postulate
+
       -- lem3 and the equivalence from lem4
       step0 : Σ (A → B) isIso ≃ Σ (A → B) (isEquiv A B)
+      step0 = lem3 (λ f → sym≃ (lem4 sA sB f))
       -- univalence
       step1 : Σ (A → B) (isEquiv A B) ≃ (A ≡ B)
+      step1 =
+        let
+          h : (A ≃ B) ≃ (A ≡ B)
+          h = sym≃ (univalence {A = A} {B})
+          k : Σ _ (isEquiv (A ≃ B) (A ≡ B))
+          k = Eqv.doEta h
+        in {!!}
+
       -- lem2 with propIsSet
       step2 : (A ≡ B) ≃ (hA ≡ hB)
+      step2 = sym≃ (lem2 (λ A → isSetIsProp) hA hB)
+
       -- Go from an isomorphism on sets to an isomorphism on homotopic sets
       trivial? : (hA ≅ hB) ≃ Σ (A → B) isIso
       trivial? = sym≃ (Eeq.fromIsomorphism res)

src/Cat/Equivalence.agda

@@ -58,6 +58,23 @@ module _ {ℓa ℓb ℓ : Level} (A : Set ℓa) (B : Set ℓb) where
     _~_ : Set ℓa → Set ℓb → Set _
     A ~ B = Σ _ iseqv
 
+    inverse-from-to-iso : ∀ {f} (x : _) → (fromIso {f} ∘ toIso {f}) x ≡ x
+    inverse-from-to-iso x = begin
+      (fromIso ∘ toIso) x ≡⟨⟩
+      fromIso (toIso x)   ≡⟨ propIsEquiv _ (fromIso (toIso x)) x ⟩
+      x ∎
+
+    -- `toIso` is abstract - so I probably can't close this proof.
+    -- inverse-to-from-iso : ∀ {f} → toIso {f} ∘ fromIso {f} ≡ idFun _
+    -- inverse-to-from-iso = funExt (λ x → begin
+    --   (toIso ∘ fromIso) x ≡⟨⟩
+    --   toIso (fromIso x)   ≡⟨ cong toIso (propIsEquiv _ (fromIso x) y) ⟩
+    --   toIso y             ≡⟨ {!!} ⟩
+    --   x ∎)
+    --   where
+    --   y : iseqv _
+    --   y = {!!}
+
     fromIsomorphism : A ≅ B → A ~ B
     fromIsomorphism (f , iso) = f , fromIso iso
 
@@ -130,7 +147,26 @@ module _ {ℓa ℓb : Level} (A : Set ℓa) (B : Set ℓb) where
       where
       import Cubical.NType.Properties as P
 
-  module Equiv≃ = Equiv ≃isEquiv
+  module Equiv≃ where
+    open Equiv ≃isEquiv public
+    inverse-to-from-iso : ∀ {f} (x : _) → (toIso {f} ∘ fromIso {f}) x ≡ x
+    inverse-to-from-iso {f} x = begin
+      (toIso ∘ fromIso) x ≡⟨⟩
+      toIso (fromIso x)   ≡⟨ cong toIso (propIsEquiv _ (fromIso x) y) ⟩
+      toIso y             ≡⟨ py ⟩
+      x ∎
+      where
+      helper : (x : Isomorphism _) → Σ _ λ y → toIso y ≡ x
+      helper (f~ , inv) = y , py
+        where
+        module inv = AreInverses inv
+        y  : isEquiv _ _ f
+        y  = {!!}
+        py : toIso y ≡ (f~ , inv)
+        py = {!!}
+      y : isEquiv _ _ _
+      y  = fst (helper x)
+      py = snd (helper x)
 
 module _ {ℓa ℓb : Level} {A : Set ℓa} {B : Set ℓb} where
   open Cubical.PathPrelude using (_≃_)
@@ -210,6 +246,12 @@ module NoEta {ℓa ℓb : Level} {A : Set ℓa} {B : Set ℓb} where
     symmetry : B ≃ A
     symmetry = deEta (Equivalence.symmetry (doEta e))
 
+  -- fromIso      : {f : A → B} → Isomorphism f → isEquiv f
+  -- fromIso = ?
+
+  -- toIso        : {f : A → B} → isEquiv f → Isomorphism f
+  -- toIso = ?
+
   fromIsomorphism : A ≅ B → A ≃ B
   fromIsomorphism (f , iso) = _≃_.con f (Equiv≃.fromIso _ _ iso)