Defining non-unary functions in Cubical mode

Question

I'd like to define a function with two, higher inductive typed, arguments in Cubical mode. I am using the cubical package as my "prelude" library.

I first define a quotient type for integers as a HIT:

{-# OPTIONS --cubical #-}
module _ where

open import Data.Nat renaming (_+_ to _+̂_)
open import Cubical.Core.Prelude

data ℤ : Set where
  _-_ : (x : ℕ) → (y : ℕ) → ℤ
  quot : ∀ {x y x′ y′} → (x ℕ+ y′) ≡ (x′ ℕ+ y) → (x - y) ≡ (x′ - y′)

I can then define a unary function using pattern matching:

_+1 : ℤ → ℤ
(x - y) +1 = suc x - y
quot {x} {y} prf i +1 = quot {suc x} {y} (cong suc prf) i

So far, so good. But what if I want to define a binary function, such as addition?

First, let's get the boring arithmetic proofs out of the way:

import Data.Nat.Properties
open Data.Nat.Properties.SemiringSolver
  using (prove; solve; _:=_; con; var; _:+_; _:*_; :-_; _:-_)

open import Relation.Binary.PropositionalEquality renaming (refl to prefl; _≡_ to _=̂_) using ()
fromPropEq : ∀ {ℓ A} {x y : A} → _=̂_ {ℓ} {A} x y → x ≡ y
fromPropEq prefl = refl

open import Function using (_$_)

reorder :  ∀ x y a b → (x +̂ a) +̂ (y +̂ b) ≡ (x +̂ y) +̂ (a +̂ b)
reorder x y a b = fromPropEq $ solve 4 (λ x y a b → (x :+ a) :+ (y :+ b) := (x :+ y) :+ (a :+ b)) prefl x y a b

inner-lemma : ∀ x y a b a′ b′ → a +̂ b′ ≡ a′ +̂ b → (x +̂ a) +̂ (y +̂ b′) ≡ (x +̂ a′) +̂ (y +̂ b)
inner-lemma x y a b a′ b′ prf = begin
  (x +̂ a) +̂ (y +̂ b′)   ≡⟨ reorder x y a b′ ⟩
  (x +̂ y) +̂ (a +̂ b′)   ≡⟨ cong (x +̂ y +̂_) prf ⟩
  (x +̂ y) +̂ (a′ +̂ b)   ≡⟨ sym (reorder x y a′ b) ⟩
  (x +̂ a′) +̂ (y +̂ b)   ∎

outer-lemma : ∀ x y x′ y′ a b  → x +̂ y′ ≡ x′ +̂ y → (x +̂ a) +̂ (y′ +̂ b) ≡ (x′ +̂ a) +̂ (y +̂ b)
outer-lemma x y x′ y′ a b prf = begin
  (x +̂ a) +̂ (y′ +̂ b)   ≡⟨ reorder x y′ a b ⟩
  (x +̂ y′) +̂ (a +̂ b)   ≡⟨ cong (_+̂ (a +̂ b)) prf ⟩
  (x′ +̂ y) +̂ (a +̂ b)   ≡⟨ sym (reorder x′ y a b) ⟩
  (x′ +̂ a) +̂ (y +̂ b)   ∎

I now try to define _+_ using pattern matching, but I have no idea how to handle the "points in the center of the face", so to speak:

_+_ : ℤ → ℤ → ℤ
(x - y) + (a - b) = (x +̂ a) - (y +̂ b)
(x - y) + quot {a} {b} {a′} {b′} eq₂ j = quot {x +̂ a} {y +̂ b} {x +̂ a′} {y +̂ b′} (inner-lemma x y a b a′ b′ eq₂) j
quot {x} {y} {x′} {y′} eq₁ i + (a - b) = quot {x +̂ a} {y +̂ b} {x′ +̂ a} {y′ +̂ b} (outer-lemma x y x′ y′ a b eq₁) i
quot {x} {y} {x′} {y′} eq₁ i + quot {a} {b} {a′} {b′} eq₂ j = ?

So basically what I have is the following situation:

                 p   Xᵢ
         X  ---------+---> X′

                 p₀  i
   A     X+A --------\---> X′+A
   |     |           |
  q|  q₀ |           | qᵢ
   |     |           |
Aⱼ +    j+          [+]  <--- This is where we want to get to!
   |     |           |
   V     V       p₁  |
   A′    X+A′ -------/---> X′+A′
                     i

with

X = (x - y)
X′ = (x′ - y′)
A = (a - b)
A′ = (a′ - b′)

p : X ≡ X′
p = quot eq₁

q : A ≡ A′
q = quot eq₂

p₀ : X + A ≡ X′ + A
p₀ = quot (outer-lemma x y x′ y′ a b eq₁)

p₁ : X + A′ ≡ X′ + A′
p₁ = quot (outer-lemma x y x′ y′ a′ b′ eq₁)

q₀ : X + A ≡ X + A′
q₀ = quot (inner-lemma x y a b a′ b′ eq₂)

q₁ : X′ + A ≡ X′ + A′
q₁ = quot (inner-lemma x′ y′ a b a′ b′ eq₂)

I am using this construction to push out q₀ horizontally by i:

slidingLid : ∀ {ℓ} {A : Set ℓ} {a b c d} (p₀ : a ≡ b) (p₁ : c ≡ d) (q : a ≡ c) → ∀ i → p₀ i ≡ p₁ i
slidingLid p₀ p₁ q i j = comp (λ _ → A)
  (λ{ k (i = i0) → q j
    ; k (j = i0) → p₀ (i ∧ k)
    ; k (j = i1) → p₁ (i ∧ k)
    })
  (inc (q j))

and using this, my attempt at + is as follows:

quot {x} {y} {x′} {y′} eq₁ i + quot {a} {b} {a′} {b′} eq₂ j = Xᵢ+Aⱼ
  where    
    X = (x - y)
    X′ = (x′ - y′)
    A = (a - b)
    A′ = (a′ - b′)

    p : X ≡ X′
    p = quot eq₁

    q : A ≡ A′
    q = quot eq₂

    p₀ : X + A ≡ X′ + A
    p₀ = quot (outer-lemma x y x′ y′ a b eq₁)

    p₁ : X + A′ ≡ X′ + A′
    p₁ = quot (outer-lemma x y x′ y′ a′ b′ eq₁)

    q₀ : X + A ≡ X + A′
    q₀ = quot (inner-lemma x y a b a′ b′ eq₂)

    qᵢ : ∀ i → p₀ i ≡ p₁ i
    qᵢ = slidingLid p₀ p₁ q₀

    q₁ : X′ + A ≡ X′ + A′
    q₁ = quot (inner-lemma x′ y′ a b a′ b′ eq₂)

    Xᵢ+Aⱼ = qᵢ i j

But this fails with the following type error:

quot (inner-lemma x′ y′ a b a′ b′ eq₂) j !=
hcomp
(λ { i ((~ i1 ∨ ~ j ∨ j) = i1)
       → transp (λ j₁ → ℤ) i
         ((λ { i₁ (i1 = i0) → q₀ eq₁ i1 eq₂ j j
             ; i₁ (j = i0) → p₀ eq₁ i1 eq₂ j (i1 ∧ i₁)
             ; i₁ (j = i1) → p₁ eq₁ i1 eq₂ j (i1 ∧ i₁)
             })
          (i ∨ i0) _)
   })
(transp (λ _ → ℤ) i0 (ouc (inc (q₀ eq₁ i1 eq₂ j j))))
of type ℤ

One hint to what might be going wrong is that while these three sides degenerate nicely:

top : ∀ i → qᵢ i i0 ≡ p i + q i0
top i = refl

bottom : ∀ i → qᵢ i i1 ≡ p i + q i1
bottom i = refl

left : qᵢ i0 ≡ q₀
left = refl

the rightmost side doesn't:

right : qᵢ i1 ≡ q₁
right = ? -- refl fails here

I guess because qᵢ is pulled from the left side, so there could still be a hole between the right side and the pushed-all-the-way qᵢ, i.e. this would still be possible, with a hole at O between qᵢ i1 and q₁:

                 p₀
      X+A ------------> X′+A
       |               /|
    q₀ |              / | q₁
       |             |  |
       |             | O|
       |              \ |
       V         p₁    \|
      X+A′ -----------> X′+A′

and intiutively it makes sense, because q₁ is some algebraic statement about natural numbers, and qᵢ i1 is a continuously deformed version of a different algebraic statement about different natural numbers, so there still has to be some kind of connection made between the two; but I don't know where to start on making that connection (i.e. constructing explicitly the 2-path between qᵢ i1 and q₁)

Answer 1

It turns out there really was a possibility of a hole between qᵢ i1 and q₁ with the formalization I've been trying to do. The solution hit me when I went back to the HoTT book to try and solve this more abstractly for all quotient types, not just this particular ℤ type. To quote from section 6.10:

We can also describe this directly, as the higher inductive type A/R generated by

• A function q : A → A/R;

• For each a, b : A such that R(a, b), an equality q(a) = q(b); and

• The 0-truncation constructor: for all x, y : A/R and r,s : x = y, we have r = s.

So what I was missing was that third point: that the lack of higher-typed structure is something that needs to be explicitly modeled.

Using this information, I have added a third constructor to my ℤ:

Same : ℕ → ℕ → ℕ → ℕ → Set
Same x y x′ y′ = x +̂ y′ ≡ x′ +̂ y

data ℤ : Set where
  _-_ : (x : ℕ) → (y : ℕ) → ℤ
  quot : ∀ {x y x′ y′} → Same x y x′ y′ → (x - y) ≡ (x′ - y′)
  trunc : {x y : ℤ} → (p q : x ≡ y) → p ≡ q

This allowed me to prove right (and thus, surface) with no further issues. One slight hiccup is that trying to use pattern matching caused some weird "function is not fibrant" errors, so I ended up going via the following explicit eliminator:

module ℤElim {ℓ} {P : ℤ → Set ℓ}
  (point* : ∀ x y → P (x - y))
  (quot* : ∀ {x y x′ y′} same → PathP (λ i → P (quot {x} {y} {x′} {y′} same i)) (point* x y) (point* x′ y′))
  (trunc* : ∀ {x y} {p q : x ≡ y} → ∀ {fx : P x} {fy : P y} (eq₁ : PathP (λ i → P (p i)) fx fy) (eq₂ : PathP (λ i → P (q i)) fx fy) → PathP (λ i → PathP (λ j → P (trunc p q i j)) fx fy) eq₁ eq₂)
  where

  ℤ-elim : ∀ x → P x
  ℤ-elim (x - y) = point* x y
  ℤ-elim (quot p i) = quot* p i
  ℤ-elim (trunc p q i j) = trunc* (cong ℤ-elim p) (cong ℤ-elim q) i j

and so for reference, the full implementation of _+_ using ℤ-elim:

_+_ : ℤ → ℤ → ℤ
_+_ = ℤ-elim
  (λ x y → ℤ-elim
    (λ a b → (x +̂ a) - (y +̂ b))
    (λ eq₂ → quot (inner-lemma x y eq₂))
    trunc)
  (λ {x} {y} {x′} {y′} eq₁ i → ℤ-elim
    (λ a b → quot (outer-lemma x y eq₁) i)
    (λ {a} {b} {a′} {b′} eq₂ j → lemma {x} {y} {x′} {y′} {a} {b} {a′} {b′} eq₁ eq₂ i j )
    trunc)
  (λ {_} {_} {_} {_} {x+} {y+} eq₁ eq₂ i →
    funExt λ a → λ j → trunc {x+ a} {y+ a} (ap eq₁ a) (ap eq₂ a) i j)
  where
    lemma : ∀ {x y x′ y′ a b a′ b′} → Same x y x′ y′ → Same a b a′ b′ → I → I → ℤ
    lemma {x} {y} {x′} {y′} {a} {b} {a′} {b′} eq₁ eq₂ i j = surface i j
      where
        {-
                         p   Xᵢ
                 X  ---------+---> X′

                         p₀  i
           A     X+A --------\---> X′+A
           |     |           |
          q|  q₀ |           | qᵢ
           |     |           |
        Aⱼ +    j+          [+]  <--- This is where we want to get to!
           |     |           |
           V     V       p₁  |
           A′    X+A′ -------/---> X′+A′
                             i
        -}

        X = x - y
        X′ = x′ - y′
        A = a - b
        A′ = a′ - b′

        X+A   = (x +̂ a) - (y +̂ b)
        X′+A  = (x′ +̂ a) - (y′ +̂ b)
        X+A′  = (x +̂ a′) - (y +̂ b′)
        X′+A′ = (x′ +̂ a′) - (y′ +̂ b′)

        p : X ≡ X′
        p = quot eq₁

        q : A ≡ A′
        q = quot eq₂

        p₀ : X+A ≡ X′+A
        p₀ = quot (outer-lemma x y eq₁)

        p₁ : X+A′ ≡ X′+A′
        p₁ = quot (outer-lemma x y eq₁)

        q₀ : X+A ≡ X+A′
        q₀ = quot (inner-lemma x y eq₂)

        q₁ : X′+A ≡ X′+A′
        q₁ = quot (inner-lemma x′ y′ eq₂)

        qᵢ : ∀ i → p₀ i ≡ p₁ i
        qᵢ = slidingLid p₀ p₁ q₀

        left : qᵢ i0 ≡ q₀
        left = refl

        right : qᵢ i1 ≡ q₁
        right = trunc (qᵢ i1) q₁

        surface : PathP (λ i → p₀ i ≡ p₁ i) q₀ q₁
        surface i = comp (λ j → p₀ i ≡ p₁ i)
          (λ { j (i = i0) → left j
             ; j (i = i1) → right j
             })
          (inc (qᵢ i))

Answer 2

This is a partial answer, in the hope that it will lure someone into solving the next piece of this puzzle.

So, I managed to prove right, and with it, that there is a continuous surface from left to right :

right : qᵢ i1 ≡ q₁
right i = comp
  (λ j → p j + A ≡ p j + A′)
  (λ { j (i = i0) → qᵢ j
     ; j (i = i1) → cong (λ ξ → quot {x} {y} {x′} {y′} eq₁ j + ξ) q
     }
  (inc (left i))

surface : PathP (λ i → p₀ i ≡ p₁ i) q₀ q₁
surface i = comp (λ j → p₀ i ≡ p₁ i)
  (λ { j (i = i0) → q₀
     ; j (i = i1) → right j
     })
  (inc (qᵢ i))

Xᵢ+Aⱼ = surface i j

This definition of Xᵢ+Aⱼ passes the typechecker, but fails during termination checking. Basically, all occurrences of _+_ are marked as problematic; in particular, the ones in the definition of right: both the p j + A and p j + A′ calls, and the congruence function in cong (λ ξ → quot {x} {y} {x′} {y′} eq₁ j + ξ) q.

The first two don't make much sense to me: I have already defined _+- for the midpoint+point and the point+point cases, and the second argument in p j + A and p j + A′ are clearly points.

The third one, I am looking for suggestions on.