Filling Cubes in a Few Lines of Code (#1053)

* done

* fix Everything

* update

* add an example

* fix

* typos

* alignment

Cubical/Foundations/Everything.agda

@@ -13,6 +13,7 @@ open import Cubical.Foundations.Equiv.BiInvertible public
 open import Cubical.Foundations.Equiv.HalfAdjoint
 open import Cubical.Foundations.Equiv.Dependent
 open import Cubical.Foundations.HLevels public
+open import Cubical.Foundations.HLevels.Extend
 open import Cubical.Foundations.Path public
 open import Cubical.Foundations.Pointed public
 open import Cubical.Foundations.RelationalStructure public

Cubical/Foundations/HLevels/Extend.agda

@@ -0,0 +1,119 @@
+{-
+
+Kan Operations for n-Truncated Types
+
+It provides an efficient way to construct cubes in truncated types.
+
+A draft note on this can be found online at
+https://kangrongji.github.io/files/extend-operations.pdf
+
+-}
+{-# OPTIONS --safe #-}
+module Cubical.Foundations.HLevels.Extend where
+
+open import Cubical.Foundations.Prelude
+open import Cubical.Foundations.HLevels
+
+private
+  variable
+    ℓ : Level
+
+
+-- For conveniently representing the boundary of cubes
+∂ : I → I
+∂ i = i ∨ ~ i
+
+
+-- TODO: Write a macro to generate these stuff.
+
+module _
+  {X : Type ℓ}
+  (h : isContr X)
+  {ϕ : I} where
+
+  extend₀ :
+    (x : Partial _ X)
+    → X [ ϕ ↦ x ]
+  extend₀ = extend h _
+
+
+module _
+  {X : I → Type ℓ}
+  (h : (i : I) → isProp (X i))
+  {ϕ : I} where
+
+  extend₁ :
+    (x : (i : I) → Partial _ (X i))
+    (i : I) → X i [ ϕ ∨ ∂ i ↦ x i ]
+  extend₁ x i = inS (hcomp (λ j → λ
+    { (ϕ = i1) → h i (bottom i) (x i 1=1) j
+    ; (i = i0) → h i (bottom i) (x i 1=1) j
+    ; (i = i1) → h i (bottom i) (x i 1=1) j })
+    (bottom i))
+    where
+    bottom : (i : I) → X i
+    bottom i = isProp→PathP h (x i0 1=1) (x i1 1=1) i
+
+
+module _
+  {X : I → I → Type}
+  (h : (i j : I) → isSet (X i j))
+  {ϕ : I} where
+
+  extend₂ :
+    (x : (i j : I) → Partial _ (X i j))
+    (i j : I) → X i j [ ϕ ∨ ∂ i ∨ ∂ j ↦ x i j ]
+  extend₂ x i j = inS (outS (extend₁PathP p i) j)
+    where
+    isOfHLevel₁PathP : (i : I) (a : X i i0) (b : X i i1)
+      → isProp (PathP (λ j → X i j) a b)
+    isOfHLevel₁PathP i = isOfHLevelPathP' 1 (h i i1)
+
+    extend₁PathP :
+      (p : (i : I) → Partial _ (PathP _ (x i i0 1=1) (x i i1 1=1)))
+      (i : I) → PathP _ (x i i0 1=1) (x i i1 1=1) [ ϕ ∨ ∂ i ↦ p i ]
+    extend₁PathP = extend₁ (λ i → isOfHLevel₁PathP i (x i i0 1=1) (x i i1 1=1)) {ϕ}
+
+    p : (i : I) → Partial _ (PathP _ (x i i0 1=1) (x i i1 1=1))
+    p i (i = i0) = λ j → x i j 1=1
+    p i (i = i1) = λ j → x i j 1=1
+    p i (ϕ = i1) = λ j → x i j 1=1
+
+
+module _
+  (X : I → I → I → Type)
+  (h : (i j k : I) → isGroupoid (X i j k))
+  {ϕ : I} where
+
+  extend₃ :
+    (x : (i j k : I) → Partial _ (X i j k))
+    (i j k : I) → X i j k [ ϕ ∨ ∂ i ∨ ∂ j ∨ ∂ k ↦ x i j k ]
+  extend₃ x i j k = inS (outS (extend₂PathP p i j) k)
+    where
+    isOfHLevel₂PathP : (i j : I) (a : X i j i0) (b : X i j i1)
+      → isSet (PathP (λ k → X i j k) a b)
+    isOfHLevel₂PathP i j = isOfHLevelPathP' 2 (h i j i1)
+
+    extend₂PathP :
+      (p : (i j : I) → Partial _ (PathP _ (x i j i0 1=1) (x i j i1 1=1)))
+      (i j : I) → PathP _ (x i j i0 1=1) (x i j i1 1=1) [ ϕ ∨ ∂ i ∨ ∂ j ↦ p i j ]
+    extend₂PathP = extend₂ (λ i j → isOfHLevel₂PathP i j (x i j i0 1=1) (x i j i1 1=1)) {ϕ}
+
+    p : (i j : I) → Partial _ (PathP (λ k → X i j k) (x i j i0 1=1) (x i j i1 1=1))
+    p i j (i = i0) = λ k → x i j k 1=1
+    p i j (i = i1) = λ k → x i j k 1=1
+    p i j (j = i0) = λ k → x i j k 1=1
+    p i j (j = i1) = λ k → x i j k 1=1
+    p i j (ϕ = i1) = λ k → x i j k 1=1
+
+
+private
+  -- An example showing how to directly fill 3-cubes in an h-proposition.
+  -- It can help when one wants to pattern match certain HITs towards some n-types.
+
+  isProp→Cube :
+    {X : Type ℓ} (h : isProp X)
+    (x : (i j k : I) → Partial _ X)
+    (i j k : I) → X [ ∂ i ∨ ∂ j ∨ ∂ k ↦ x i j k ]
+  isProp→Cube h x i j =
+    extend₁ (λ _ → h) {∂ i ∨ ∂ j} (x i j)