Source code on Github
------------------------------------------------------------------------
-- The Agda standard library
--
-- Some properties of reflexive closures
------------------------------------------------------------------------

{-# OPTIONS --safe --cubical-compatible #-}

module Relation.Binary.Construct.Closure.Reflexive.Properties where

open import Data.Product as Prod
open import Data.Sum.Base as Sum
open import Function.Equivalence using (_⇔_; equivalence)
open import Function.Base using (id)
open import Level
open import Relation.Binary hiding (_⇔_)
open import Relation.Binary.Construct.Closure.Reflexive
open import Relation.Binary.PropositionalEquality as PropEq using (_≡_; refl)
open import Relation.Nullary
import Relation.Nullary.Decidable as Dec
open import Relation.Nullary.Negation using (contradiction)
open import Relation.Nullary.Sum using (_⊎-dec_)
open import Relation.Unary using (Pred)

private
  variable
    a b  p q : Level
    A : Set a
    B : Set b

------------------------------------------------------------------------
-- Relational properties

module _ {P : Rel A p} {Q : Rel B q} where

  =[]⇒ :  {f : A  B}  P =[ f ]⇒ Q  ReflClosure P =[ f ]⇒ ReflClosure Q
  =[]⇒ x [ x∼y ] = [ x x∼y ]
  =[]⇒ x refl    = refl

module _ {_~_ : Rel A } where

  private
    _~ᵒ_ = ReflClosure _~_

  fromSum :  {x y}  x  y  x ~ y  x ~ᵒ y
  fromSum (inj₁ refl) = refl
  fromSum (inj₂ y) = [ y ]

  toSum :  {x y}  x ~ᵒ y  x  y  x ~ y
  toSum [ x∼y ] = inj₂ x∼y
  toSum refl = inj₁ refl

  ⊎⇔Refl :  {a b}  (a  b  a ~ b)  a ~ᵒ b
  ⊎⇔Refl = equivalence fromSum toSum

  sym : Symmetric _~_  Symmetric _~ᵒ_
  sym ~-sym [ x∼y ] = [ ~-sym x∼y ]
  sym ~-sym refl    = refl

  trans : Transitive _~_  Transitive _~ᵒ_
  trans ~-trans [ x∼y ] [ x∼y₁ ] = [ ~-trans x∼y x∼y₁ ]
  trans ~-trans [ x∼y ] refl     = [ x∼y ]
  trans ~-trans refl    [ x∼y ]  = [ x∼y ]
  trans ~-trans refl    refl     = refl

  antisym : (_≈_ : Rel A p)  Reflexive _≈_ 
            Asymmetric _~_  Antisymmetric _≈_ _~ᵒ_
  antisym _≈_ ref asym [ x∼y ] [ y∼x ] = contradiction x∼y (asym y∼x)
  antisym _≈_ ref asym [ x∼y ] refl    = ref
  antisym _≈_ ref asym refl    _       = ref

  total : Trichotomous _≡_ _~_  Total _~ᵒ_
  total compare x y with compare x y
  ... | tri< a _    _ = inj₁ [ a ]
  ... | tri≈ _ refl _ = inj₁ refl
  ... | tri> _ _    c = inj₂ [ c ]

  dec : Decidable {A = A} _≡_  Decidable _~_  Decidable _~ᵒ_
  dec ≡-dec ~-dec a b = Dec.map ⊎⇔Refl (≡-dec a b ⊎-dec ~-dec a b)

  decidable : Trichotomous _≡_ _~_  Decidable _~ᵒ_
  decidable ~-tri a b with ~-tri a b
  ... | tri< a~b  _  _ = yes [ a~b ]
  ... | tri≈ _  refl _ = yes refl
  ... | tri> ¬a ¬b   _ = no λ { refl  ¬b refl ; [ p ]  ¬a p }

  respˡ :  {P : REL A B p}  P Respectsˡ _~_  P Respectsˡ _~ᵒ_
  respˡ p-respˡ-~ [ x∼y ] = p-respˡ-~ x∼y
  respˡ _         refl    = id

  respʳ :  {P : REL B A p}  P Respectsʳ _~_  P Respectsʳ _~ᵒ_
  respʳ = respˡ

module _ {_~_ : Rel A } {P : Pred A p} where

  resp : P Respects _~_  P Respects (ReflClosure _~_)
  resp p-resp-~ [ x∼y ] = p-resp-~ x∼y
  resp _        refl    = id

module _ {_~_ : Rel A } {P : Rel A p} where

  resp₂ : P Respects₂ _~_  P Respects₂ (ReflClosure _~_)
  resp₂ = Prod.map respˡ respʳ

------------------------------------------------------------------------
-- Structures

module _ {_~_ : Rel A } where

  private
    _~ᵒ_ = ReflClosure _~_

  isPreorder : Transitive _~_  IsPreorder _≡_ _~ᵒ_
  isPreorder ~-trans = record
    { isEquivalence = PropEq.isEquivalence
    ; reflexive     = λ { refl  refl }
    ; trans         = trans ~-trans
    }

  isPartialOrder : IsStrictPartialOrder _≡_ _~_  IsPartialOrder _≡_ _~ᵒ_
  isPartialOrder O = record
    { isPreorder = isPreorder O.trans
    ; antisym    = antisym _≡_ refl O.asym
    } where module O = IsStrictPartialOrder O

  isDecPartialOrder : IsDecStrictPartialOrder _≡_ _~_  IsDecPartialOrder _≡_ _~ᵒ_
  isDecPartialOrder O = record
    { isPartialOrder = isPartialOrder O.isStrictPartialOrder
    ; _≟_            = O._≟_
    ; _≤?_           = dec O._≟_ O._<?_
    } where module O = IsDecStrictPartialOrder O

  isTotalOrder : IsStrictTotalOrder _≡_ _~_  IsTotalOrder _≡_ _~ᵒ_
  isTotalOrder O = record
    { isPartialOrder = isPartialOrder isStrictPartialOrder
    ; total          = total compare
    } where open IsStrictTotalOrder O

  isDecTotalOrder : IsStrictTotalOrder _≡_ _~_  IsDecTotalOrder _≡_ _~ᵒ_
  isDecTotalOrder O = record
    { isTotalOrder = isTotalOrder O
    ; _≟_          = _≟_
    ; _≤?_         = dec _≟_ _<?_
    } where open IsStrictTotalOrder O