Decidability of dependent function types

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

Created on 2022-02-14.
Last modified on 2024-02-06.

module foundation.decidable-dependent-function-types where
open import foundation.decidable-types
open import foundation.functoriality-dependent-function-types
open import foundation.maybe
open import foundation.universal-property-coproduct-types
open import foundation.universal-property-maybe
open import foundation.universe-levels

open import foundation-core.coproduct-types
open import foundation-core.equivalences


We describe conditions under which dependent products are decidable.

Decidablitilty of dependent products over coproducts

is-decidable-Π-coproduct :
  {l1 l2 l3 : Level} {A : UU l1} {B : UU l2} {C : A + B  UU l3} 
  is-decidable ((a : A)  C (inl a))  is-decidable ((b : B)  C (inr b)) 
  is-decidable ((x : A + B)  C x)
is-decidable-Π-coproduct {C = C} dA dB =
    ( equiv-dependent-universal-property-coproduct C)
    ( is-decidable-product dA dB)

Decidability of dependent products over Maybe

is-decidable-Π-Maybe :
  {l1 l2 : Level} {A : UU l1} {B : Maybe A  UU l2} 
  is-decidable ((x : A)  B (unit-Maybe x))  is-decidable (B exception-Maybe) 
  is-decidable ((x : Maybe A)  B x)
is-decidable-Π-Maybe {B = B} du de =
    ( equiv-dependent-universal-property-Maybe B)
    ( is-decidable-product du de)

Decidability of dependent products over an equivalence

is-decidable-Π-equiv :
  {l1 l2 l3 l4 : Level} {A : UU l1} {B : UU l2} {C : A  UU l3} {D : B  UU l4}
  (e : A  B) (f : (x : A)  C x  D (map-equiv e x)) 
  is-decidable ((x : A)  C x)  is-decidable ((y : B)  D y)
is-decidable-Π-equiv {D = D} e f = is-decidable-equiv' (equiv-Π D e f)

is-decidable-Π-equiv' :
  {l1 l2 l3 l4 : Level} {A : UU l1} {B : UU l2} {C : A  UU l3} {D : B  UU l4}
  (e : A  B) (f : (x : A)  C x  D (map-equiv e x)) 
  is-decidable ((y : B)  D y)  is-decidable ((x : A)  C x)
is-decidable-Π-equiv' {D = D} e f = is-decidable-equiv (equiv-Π D e f)

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