module Data.Sum.Properties where
As warmup, we have that both constructors are embeddings:
inl-inj : {x y : A} → inl {B = B} x ≡ inl y → x ≡ y inl-inj {A = A} {x = x} path = ap f path where f : A ⊎ B → A f (inl x) = x f (inr _) = x inr-inj : {A : Type b} {x y : B} → inr {A = A} x ≡ inr y → x ≡ y inr-inj {B = B} {x = x} path = ap f path where f : A ⊎ B → B f (inl _) = x f (inr x) = x inl≠inr : {A : Type a} {B : Type b} {x : A} {y : B} → ¬ inl x ≡ inr y inl≠inr path = subst (λ { (inl x) → ⊤ ; (inr x) → ⊥ }) path tt inr≠inl : {A : Type a} {B : Type b} {x : A} {y : B} → ¬ inr x ≡ inl y inr≠inl path = inl≠inr (sym path)
Closure under h-levels🔗
If
and
are
for
then so is their coproduct. The way we prove this is by characterising
the entire path space of A ⊎ B in terms of the path spaces
for A and B, using a recursive definition:
module ⊎Path where Code : A ⊎ B → A ⊎ B → Type (level-of A ⊔ level-of B) Code {B = B} (inl x) (inl y) = Lift (level-of B) (x ≡ y) Code (inl x) (inr y) = Lift _ ⊥ Code (inr x) (inl y) = Lift _ ⊥ Code {A = A} (inr x) (inr y) = Lift (level-of A) (x ≡ y)
Given a Code for a
path in A ⊎ B, we can turn it into a legitimate path. Agda
automatically lets us ignore the cases where the Code computes to the empty type.
decode : {x y : A ⊎ B} → Code x y → x ≡ y decode {x = inl x} {y = inl x₁} code = ap inl (lower code) decode {x = inr x} {y = inr x₁} code = ap inr (lower code)
In the inverse direction, we have a procedure for turning paths into codes:
encode : {x y : A ⊎ B} → x ≡ y → Code x y encode {x = inl x} {y = inl y} path = lift (inl-inj path) encode {x = inl x} {y = inr y} path = absurd (inl≠inr path) encode {x = inr x} {y = inl y} path = absurd (inr≠inl path) encode {x = inr x} {y = inr y} path = lift (inr-inj path)
Now we must establish that encode and decode are inverses. In the
one direction, we can use path induction:
decode-encode : {x y : A ⊎ B} (p : x ≡ y) → decode (encode p) ≡ p decode-encode = J (λ _ p → decode (encode p) ≡ p) d-e-refl where d-e-refl : {x : A ⊎ B} → decode (encode (λ i → x)) ≡ (λ i → x) d-e-refl {x = inl x} = refl d-e-refl {x = inr x} = refl
In the other direction, the proof is by case analysis, and everything
computes wonderfully to make the right-hand sides fillable by refl:
encode-decode : {x y : A ⊎ B} (p : Code x y) → encode (decode p) ≡ p encode-decode {x = inl x} {y = inl y} p = refl encode-decode {x = inr x} {y = inr y} p = refl
Thus, we have an equivalence between codes for paths in
A ⊎ B and actual paths A ⊎ B. Since
Code has a
nice computational structure, we can establish its h-level by
induction:
Code≃Path : {x y : A ⊎ B} → (x ≡ y) ≃ Code x y Code≃Path = Iso→Equiv (encode , iso decode encode-decode decode-encode)
open ⊎Path Code-is-hlevel : {x y : A ⊎ B} {n : Nat} → is-hlevel A (2 + n) → is-hlevel B (2 + n) → is-hlevel (Code x y) (suc n) Code-is-hlevel {x = inl x} {inl y} {n} ahl bhl = Lift-is-hlevel (suc n) (ahl x y) Code-is-hlevel {x = inr x} {inr y} {n} ahl bhl = Lift-is-hlevel (suc n) (bhl x y)
In the two cases where x and y match, we
can use the fact that Lift preserves h-levels
and the assumption that A (or B) have the
given h-level.
Code-is-hlevel {x = inl x} {inr y} {n} ahl bhl = Lift-is-hlevel (suc n) (is-prop→is-hlevel-suc λ x → absurd x) Code-is-hlevel {x = inr x} {inl y} {n} ahl bhl = Lift-is-hlevel (suc n) (is-prop→is-hlevel-suc λ x → absurd x)
In the mismatched cases, we use the fact that propositions have any successor h-level
to prove that ⊥ is also at the same h-level as
A and B. Thus, we have:
⊎-is-hlevel : (n : Nat) → is-hlevel A (2 + n) → is-hlevel B (2 + n) → is-hlevel (A ⊎ B) (2 + n) ⊎-is-hlevel n ahl bhl x y = Equiv→is-hlevel (1 + n) Code≃Path (Code-is-hlevel ahl bhl) instance H-Level-⊎ : ∀ {n} ⦃ _ : 2 ≤ n ⦄ ⦃ _ : H-Level A n ⦄ ⦃ _ : H-Level B n ⦄ → H-Level (A ⊎ B) n H-Level-⊎ {n = suc (suc n)} ⦃ s≤s (s≤s p) ⦄ = hlevel-instance $ ⊎-is-hlevel _ (hlevel (2 + n)) (hlevel (2 + n))
Note that, in general, being a proposition and contractible are not preserved under
coproducts. Consider the case where (A, a) and
(B, b) are both contractible (this generalises to
propositions): Then their coproduct has two distinct points,
inl a and inr b. However, the coproduct of
disjoint propositions is a proposition:
disjoint-⊎-is-prop : is-prop A → is-prop B → ¬ (A × B) → is-prop (A ⊎ B) disjoint-⊎-is-prop Ap Bp notab (inl x) (inl y) = ap inl (Ap x y) disjoint-⊎-is-prop Ap Bp notab (inl x) (inr y) = absurd (notab (x , y)) disjoint-⊎-is-prop Ap Bp notab (inr x) (inl y) = absurd (notab (y , x)) disjoint-⊎-is-prop Ap Bp notab (inr x) (inr y) = ap inr (Bp x y)
Discreteness🔗
If A and B are discrete, then so is
A ⊎ B.
instance Discrete-⊎ : ⦃ _ : Discrete A ⦄ ⦃ _ : Discrete B ⦄ → Discrete (A ⊎ B) Discrete-⊎ {x = inl x} {inl y} = invmap (ap inl) inl-inj (x ≡? y) Discrete-⊎ {x = inl x} {inr y} = no inl≠inr Discrete-⊎ {x = inr x} {inl y} = no inr≠inl Discrete-⊎ {x = inr x} {inr y} = invmap (ap inr) inr-inj (x ≡? y)