------------------------------------------------------------------------
-- The Agda standard library
--
-- Operations on and properties of decidable relations
------------------------------------------------------------------------

{-# OPTIONS --without-K --safe #-}

module Relation.Nullary.Decidable where

open import Level using (Level)
open import Function.Core
open import Function.Equality    using (_⟨$⟩_; module Π)
open import Function.Equivalence using (_⇔_; equivalence; module Equivalence)
open import Function.Injection   using (Injection; module Injection)
open import Relation.Binary      using (Setoid; module Setoid; Decidable)
open import Relation.Nullary

private
  variable
    p q : Level
    P : Set p
    Q : Set q

------------------------------------------------------------------------
-- Re-exporting the core definitions

open import Relation.Nullary.Decidable.Core public

------------------------------------------------------------------------
-- Maps

map : P ⇔ Q → Dec P → Dec Q
map P⇔Q = map′ (to ⟨$⟩_) (from ⟨$⟩_)
  where open Equivalence P⇔Q

module _ {a₁ a₂ b₁ b₂} {A : Setoid a₁ a₂} {B : Setoid b₁ b₂}
         (inj : Injection A B)
  where

  open Injection inj
  open Setoid A using () renaming (_≈_ to _≈A_)
  open Setoid B using () renaming (_≈_ to _≈B_)

  -- If there is an injection from one setoid to another, and the
  -- latter's equivalence relation is decidable, then the former's
  -- equivalence relation is also decidable.

  via-injection : Decidable _≈B_ → Decidable _≈A_
  via-injection dec x y =
    map′ injective (Π.cong to) (dec (to ⟨$⟩ x) (to ⟨$⟩ y))