Decidable types

Content created by Egbert Rijke, Fredrik Bakke, Jonathan Prieto-Cubides and Eléonore Mangel.

Created on 2022-01-27.
Last modified on 2023-06-10.

module foundation.decidable-types where
open import foundation.coproduct-types
open import foundation.dependent-pair-types
open import foundation.double-negation
open import foundation.empty-types
open import foundation.hilberts-epsilon-operators
open import foundation.negation
open import foundation.propositional-truncations
open import foundation.raising-universe-levels
open import foundation.type-arithmetic-empty-type
open import foundation.unit-type
open import foundation.universe-levels

open import foundation-core.cartesian-product-types
open import foundation-core.equivalences
open import foundation-core.function-types
open import foundation-core.propositions
open import foundation-core.retractions


A type is said to be decidable if we can either construct an element, or we can prove that it is empty. In other words, we interpret decidability via the Curry-Howard interpretation of logic into type theory. A related concept is that a type is either inhabited or empty, where inhabitedness of a type is expressed using the propositional truncation.


The Curry–Howard interpretation of decidability

is-decidable : {l : Level} (A : UU l)  UU l
is-decidable A = A + (¬ A)

is-decidable-fam :
  {l1 l2 : Level} {A : UU l1} (P : A  UU l2)  UU (l1  l2)
is-decidable-fam {A = A} P = (x : A)  is-decidable (P x)

The predicate that a type is inhabited or empty

is-inhabited-or-empty : {l1 : Level}  UU l1  UU l1
is-inhabited-or-empty A = type-trunc-Prop A + is-empty A

Merely decidable types

A type A is said to be merely decidable if it comes equipped with an element of type-trunc-Prop (is-decidable A).

is-merely-Decidable-Prop :
  {l : Level}  UU l  Prop l
is-merely-Decidable-Prop A = trunc-Prop (is-decidable A)

is-merely-decidable : {l : Level}  UU l  UU l
is-merely-decidable A = type-trunc-Prop (is-decidable A)


The unit type and the empty type are decidable

is-decidable-unit : is-decidable unit
is-decidable-unit = inl star

is-decidable-empty : is-decidable empty
is-decidable-empty = inr id


Coproducts of decidable types are decidable

is-decidable-coprod :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable A  is-decidable B  is-decidable (A + B)
is-decidable-coprod (inl a) y = inl (inl a)
is-decidable-coprod (inr na) (inl b) = inl (inr b)
is-decidable-coprod (inr na) (inr nb) = inr (ind-coprod  x  empty) na nb)

Cartesian products of decidable types are decidable

is-decidable-prod :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable A  is-decidable B  is-decidable (A × B)
is-decidable-prod (inl a) (inl b) = inl (pair a b)
is-decidable-prod (inl a) (inr g) = inr (g  pr2)
is-decidable-prod (inr f) (inl b) = inr (f  pr1)
is-decidable-prod (inr f) (inr g) = inr (f  pr1)

is-decidable-prod' :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable A  (A  is-decidable B)  is-decidable (A × B)
is-decidable-prod' (inl a) d with d a
... | inl b = inl (pair a b)
... | inr nb = inr (nb  pr2)
is-decidable-prod' (inr na) d = inr (na  pr1)

is-decidable-left-factor :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable (A × B)  B  is-decidable A
is-decidable-left-factor (inl (pair x y)) b = inl x
is-decidable-left-factor (inr f) b = inr  a  f (pair a b))

is-decidable-right-factor :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable (A × B)  A  is-decidable B
is-decidable-right-factor (inl (pair x y)) a = inl y
is-decidable-right-factor (inr f) a = inr  b  f (pair a b))

Function types of decidable types are decidable

is-decidable-function-type :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable A  is-decidable B  is-decidable (A  B)
is-decidable-function-type (inl a) (inl b) = inl  x  b)
is-decidable-function-type (inl a) (inr g) = inr  h  g (h a))
is-decidable-function-type (inr f) (inl b) = inl (ex-falso  f)
is-decidable-function-type (inr f) (inr g) = inl (ex-falso  f)

is-decidable-function-type' :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} 
  is-decidable A  (A  is-decidable B)  is-decidable (A  B)
is-decidable-function-type' (inl a) d with d a
... | inl b = inl  x  b)
... | inr nb = inr  f  nb (f a))
is-decidable-function-type' (inr na) d = inl (ex-falso  na)

The negation of a decidable type is decidable

is-decidable-neg :
  {l : Level} {A : UU l}  is-decidable A  is-decidable (¬ A)
is-decidable-neg d = is-decidable-function-type d is-decidable-empty

Decidable types are closed under coinhabited types; retracts, and equivalences

module _
  {l1 l2 : Level} {A : UU l1} {B : UU l2}

  is-decidable-iff :
    (A  B)  (B  A)  is-decidable A  is-decidable B
  is-decidable-iff f g (inl a) = inl (f a)
  is-decidable-iff f g (inr na) = inr  b  na (g b))

module _
  {l1 l2 : Level} {A : UU l1} {B : UU l2}

  is-decidable-retract-of :
    A retract-of B  is-decidable B  is-decidable A
  is-decidable-retract-of (pair i (pair r H)) (inl b) = inl (r b)
  is-decidable-retract-of (pair i (pair r H)) (inr f) = inr (f  i)

  is-decidable-is-equiv :
    {f : A  B}  is-equiv f  is-decidable B  is-decidable A
  is-decidable-is-equiv {f} (pair (pair g G) (pair h H)) =
    is-decidable-retract-of (pair f (pair h H))

  is-decidable-equiv :
    (e : A  B)  is-decidable B  is-decidable A
  is-decidable-equiv e = is-decidable-iff (map-inv-equiv e) (map-equiv e)

is-decidable-equiv' :
  {l1 l2 : Level} {A : UU l1} {B : UU l2} (e : A  B) 
  is-decidable A  is-decidable B
is-decidable-equiv' e = is-decidable-equiv (inv-equiv e)

Decidability implies double negation elimination

double-negation-elim-is-decidable :
  {l : Level} {P : UU l}  is-decidable P  (¬¬ P  P)
double-negation-elim-is-decidable (inl x) p = x
double-negation-elim-is-decidable (inr x) p = ex-falso (p x)

The double negation of is-decidable is always provable

double-negation-is-decidable : {l : Level} {P : UU l}  ¬¬ (is-decidable P)
double-negation-is-decidable {P = P} f =
  map-neg (inr {A = P} {B = ¬ P}) f
    ( map-neg (inl {A = P} {B = ¬ P}) f)

Decidable types have ε-operators

elim-trunc-Prop-is-decidable :
  {l : Level} {A : UU l}  is-decidable A  ε-operator-Hilbert A
elim-trunc-Prop-is-decidable (inl a) x = a
elim-trunc-Prop-is-decidable (inr f) x =
  ex-falso (apply-universal-property-trunc-Prop x empty-Prop f)

is-decidable is an idempotent operation

idempotent-is-decidable :
  {l : Level} (P : UU l)  is-decidable (is-decidable P)  is-decidable P
idempotent-is-decidable P (inl (inl p)) = inl p
idempotent-is-decidable P (inl (inr np)) = inr np
idempotent-is-decidable P (inr np) = inr  p  np (inl p))

Being inhabited or empty is a proposition

  is-prop-is-inhabited-or-empty :
    {l1 : Level} (A : UU l1)  is-prop (is-inhabited-or-empty A)
  is-prop-is-inhabited-or-empty A =
      ( λ t  apply-universal-property-trunc-Prop t empty-Prop)
      ( is-prop-type-trunc-Prop)
      ( is-prop-neg)

is-inhabited-or-empty-Prop : {l1 : Level}  UU l1  Prop l1
pr1 (is-inhabited-or-empty-Prop A) = is-inhabited-or-empty A
pr2 (is-inhabited-or-empty-Prop A) = is-prop-is-inhabited-or-empty A

Any inhabited type is a fixed point for is-decidable

is-fixed-point-is-decidable-is-inhabited :
  {l : Level} {X : UU l}  type-trunc-Prop X  is-decidable X  X
is-fixed-point-is-decidable-is-inhabited {l} {X} t =
  right-unit-law-coprod-is-empty X (¬ X) (is-nonempty-is-inhabited t)

Raising types converves decidability

module _
  (l : Level) {l1 : Level} (A : UU l1)

  is-decidable-raise : is-decidable A  is-decidable (raise l A)
  is-decidable-raise (inl p) = inl (map-raise p)
  is-decidable-raise (inr np) = inr  p'  np (map-inv-raise p'))

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