------------------------------------------------------------------------
-- The Agda standard library
--
-- Sums of nullary relations
------------------------------------------------------------------------

module Relation.Nullary.Sum where

open import Data.Sum
open import Data.Empty
open import Level
open import Relation.Nullary

-- Some properties which are preserved by _⊎_.

infixr  _¬-⊎_ _⊎-dec_

_¬-⊎_ : ∀ {p q} {P : Set p} {Q : Set q} →
        ¬ P → ¬ Q → ¬ (P ⊎ Q)
(¬p ¬-⊎ ¬q) (inj₁ p) = ¬p p
(¬p ¬-⊎ ¬q) (inj₂ q) = ¬q q

_⊎-dec_ : ∀ {p q} {P : Set p} {Q : Set q} →
          Dec P → Dec Q → Dec (P ⊎ Q)
yes p ⊎-dec _     = yes (inj₁ p)
_     ⊎-dec yes q = yes (inj₂ q)
no ¬p ⊎-dec no ¬q = no helper
  where
  helper : _ ⊎ _ → ⊥
  helper (inj₁ p) = ¬p p
  helper (inj₂ q) = ¬q q