module 1Lab.Resizing where
Propositional resizing🔗
Ordinarily, the collection of all predicates on types lives in the next universe up, This is because predicates are not special in type theory: they are ordinary type families, that just so happen to be valued in propositions. For most purposes we can work with this limitation: this is called predicative mathematics. But, for a few classes of theorems, predicativity is too restrictive: Even if we don’t have a single universe of propositions that can accommodate all predicates, we would still like for universes to be closed under power-sets.
Using some of Agda’s more suspicious features, we can achieve this in
a way which does not break computation too much. Specifically, we’ll use
a combination of NO_UNIVERSE_CHECK, postulates, and rewrite
rules.
We start with the NO_UNIVERSE_CHECK part: We define the
small type of propositions, Ω, to be a
record containing a Type (together with an irrelevant proof
that this type is a proposition), but contrary to the universe checker,
which would like us to put Ω : Type₁, we instead force
Ω : Type.
{-# NO_UNIVERSE_CHECK #-} record Ω : Type where constructor el field ∣_∣ : Type is-tr : is-prop ∣_∣ open Ω public
This type, a priori, only contains the propositions whose underlying
type lives in the first universe. However, we can populate it using a
NO_UNIVERSE_CHECK-powered higher inductive type, the “small
propositional
truncation”:
{-# NO_UNIVERSE_CHECK #-} data □ {ℓ} (A : Type ℓ) : Type where inc : A → □ A squash : (x y : □ A) → x ≡ y
Just like the ordinary propositional truncation, every type can be
squashed, but unlike the ordinary propositional truncation, the □-squashing of a type always
lives in the lowest universe. If
is a proposition in any universe,
is its name in the zeroth universe.
instance H-Level-□ : ∀ {ℓ} {T : Type ℓ} {n} → H-Level (□ T) (suc n) H-Level-□ = prop-instance squash open hlevel-projection Ω-hlevel-proj : hlevel-projection (quote Ω.∣_∣) Ω-hlevel-proj .has-level = quote Ω.is-tr Ω-hlevel-proj .get-level x = pure (quoteTerm (suc zero)) Ω-hlevel-proj .get-argument (arg _ t ∷ _) = pure t Ω-hlevel-proj .get-argument _ = typeError []
We can also prove a univalence principle for Ω:
Ω-ua : {A B : Ω} → (∣ A ∣ → ∣ B ∣) → (∣ B ∣ → ∣ A ∣) → A ≡ B Ω-ua {A} {B} f g i .∣_∣ = ua (prop-ext! f g) i Ω-ua {A} {B} f g i .is-tr = is-prop→pathp (λ i → is-prop-is-prop {A = ua (prop-ext! f g) i}) (A .is-tr) (B .is-tr) i instance abstract H-Level-Ω : ∀ {n} → H-Level Ω (2 + n) H-Level-Ω = basic-instance 2 $ retract→is-hlevel 2 (λ r → el ∣ r ∣ (r .is-tr)) (λ r → el ∣ r ∣ (r .is-tr)) (λ x → Ω-ua (λ x → x) λ x → x) (n-Type-is-hlevel {lzero} 1)
The □ type former is a functor (in
the handwavy sense that it supports a “map” operation), and can be
projected from into propositions of any universe. These functions
compute on incs, as usual.
□-map : ∀ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} → (A → B) → □ A → □ B □-map f (inc x) = inc (f x) □-map f (squash x y i) = squash (□-map f x) (□-map f y) i □-rec : ∀ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} → is-prop B → (A → B) → □ A → B □-rec bp f (inc x) = f x □-rec bp f (squash x y i) = bp (□-rec bp f x) (□-rec bp f y) i elΩ : ∀ {ℓ} (T : Type ℓ) → Ω elΩ T .∣_∣ = □ T elΩ T .is-tr = squash
□-elim : ∀ {ℓ ℓ'} {A : Type ℓ} {P : □ A → Type ℓ'} → (∀ x → is-prop (P x)) → (∀ x → P (inc x)) → ∀ x → P x □-elim pprop go (inc x) = go x □-elim pprop go (squash x y i) = is-prop→pathp (λ i → pprop (squash x y i)) (□-elim pprop go x) (□-elim pprop go y) i instance Inductive-□ : ∀ {ℓ ℓ' ℓm} {A : Type ℓ} {P : □ A → Type ℓ'} ⦃ i : Inductive (∀ x → P (inc x)) ℓm ⦄ → ⦃ _ : ∀ {x} → H-Level (P x) 1 ⦄ → Inductive (∀ x → P x) ℓm Inductive-□ ⦃ i ⦄ = record { methods = i .Inductive.methods ; from = λ f → □-elim (λ x → hlevel 1) (i .Inductive.from f) } □-out : ∀ {ℓ} {A : Type ℓ} → is-prop A → □ A → A □-out ap = □-rec ap (λ x → x) □-out! : ∀ {ℓ} {A : Type ℓ} → ⦃ _ : H-Level A 1 ⦄ → □ A → A □-out! = rec! λ x → x □-rec-set : ∀ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} → is-set B → (f : A → B) → (∀ x y → f x ≡ f y) → □ A → B □-rec-set B-set f f-const a = fst $ □-elim (λ _ → is-constant→image-is-prop B-set f f-const) (λ a → f a , inc (a , refl)) a □-idempotent : ∀ {ℓ} {A : Type ℓ} → is-prop A → □ A ≃ A □-idempotent aprop = prop-ext squash aprop (□-out aprop) inc □-ap : ∀ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} → □ (A → B) → □ A → □ B □-ap (inc f) (inc g) = inc (f g) □-ap (inc f) (squash g g' i) = squash (□-ap (inc f) g) (□-ap (inc f) g') i □-ap (squash f f' i) g = squash (□-ap f g) (□-ap f' g) i □-bind : ∀ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} → □ A → (A → □ B) → □ B □-bind (inc x) f = f x □-bind (squash x x' i) f = squash (□-bind x f) (□-bind x' f) i instance Map-□ : Map (eff □) Map-□ .Map.map = □-map Idiom-□ : Idiom (eff □) Idiom-□ .Idiom.pure = inc Idiom-□ .Idiom._<*>_ = □-ap Bind-□ : Bind (eff □) Bind-□ .Bind._>>=_ = □-bind is-set→locally-small : ∀ {ℓ} {A : Type ℓ} → is-set A → is-identity-system {A = A} (λ x y → □ (x ≡ y)) (λ x → inc refl) is-set→locally-small a-set .to-path = □-rec (a-set _ _) id is-set→locally-small a-set .to-path-over p = is-prop→pathp (λ _ → squash) _ _ to-is-true : ∀ {P Q : Ω} ⦃ _ : H-Level ∣ Q ∣ 0 ⦄ → ∣ P ∣ → P ≡ Q to-is-true prf = Ω-ua (λ _ → hlevel 0 .centre) (λ _ → prf) tr-□ : ∀ {ℓ} {A : Type ℓ} → ∥ A ∥ → □ A tr-□ (inc x) = inc x tr-□ (squash x y i) = squash (tr-□ x) (tr-□ y) i □-tr : ∀ {ℓ} {A : Type ℓ} → □ A → ∥ A ∥ □-tr (inc x) = inc x □-tr (squash x y i) = squash (□-tr x) (□-tr y) i
Connectives🔗
The universe of small propositions contains true, false, conjunctions, disjunctions, and implications.
⊤Ω : Ω ∣ ⊤Ω ∣ = ⊤ ⊤Ω .is-tr = hlevel 1 ⊥Ω : Ω ∣ ⊥Ω ∣ = ⊥ ⊥Ω .is-tr = hlevel 1 _∧Ω_ : Ω → Ω → Ω ∣ P ∧Ω Q ∣ = ∣ P ∣ × ∣ Q ∣ (P ∧Ω Q) .is-tr = hlevel 1 _∨Ω_ : Ω → Ω → Ω ∣ P ∨Ω Q ∣ = ∥ ∣ P ∣ ⊎ ∣ Q ∣ ∥ (P ∨Ω Q) .is-tr = hlevel 1 _→Ω_ : Ω → Ω → Ω ∣ P →Ω Q ∣ = ∣ P ∣ → ∣ Q ∣ (P →Ω Q) .is-tr = hlevel 1 ¬Ω_ : Ω → Ω ¬Ω P = P →Ω ⊥Ω
Furthermore, we can quantify over types of arbitrary size and still
land in Ω.
∃Ω : ∀ {ℓ} (A : Type ℓ) → (A → Ω) → Ω ∣ ∃Ω A P ∣ = □ (Σ[ x ∈ A ] ∣ P x ∣) ∃Ω A P .is-tr = squash ∀Ω : ∀ {ℓ} (A : Type ℓ) → (A → Ω) → Ω ∣ ∀Ω A P ∣ = □ (∀ (x : A) → ∣ P x ∣) ∀Ω A P .is-tr = squash syntax ∃Ω A (λ x → B) = ∃Ω[ x ∈ A ] B syntax ∀Ω A (λ x → B) = ∀Ω[ x ∈ A ] B
These connectives and quantifiers are only provided for completeness; if you find yourself building nested propositions, it is generally a good idea to construct the large proposition by hand, and then use truncation to turn it into a small proposition.
module _ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} (f : A → B) where Ω-image : Type ℓ' Ω-image = Σ[ b ∈ B ] □ (fibre f b) Ω-corestriction : A → Ω-image Ω-corestriction a = f a , inc (a , refl) opaque Ω-corestriction-is-surjective : is-surjective Ω-corestriction Ω-corestriction-is-surjective = elim! λ y x fx=y → pure (x , Σ-prop-path! fx=y)