```------------------------------------------------------------------------
-- Heterogeneous equality
------------------------------------------------------------------------

{-# OPTIONS --universe-polymorphism #-}

module Relation.Binary.HeterogeneousEquality where

open import Data.Product
open import Function
open import Function.Inverse using (Inverse)
open import Level
open import Relation.Nullary
open import Relation.Binary
open import Relation.Binary.Consequences
open import Relation.Binary.Indexed as I using (_at_)
open import Relation.Binary.PropositionalEquality as PropEq
using (_≡_; refl)

------------------------------------------------------------------------
-- Heterogeneous equality

infix 4 _≅_ _≇_

data _≅_ {a} {A : Set a} (x : A) : ∀ {b} {B : Set b} → B → Set where
refl : x ≅ x

-- Nonequality.

_≇_ : ∀ {a} {A : Set a} → A → ∀ {b} {B : Set b} → B → Set
x ≇ y = ¬ x ≅ y

------------------------------------------------------------------------
-- Conversion

≡-to-≅ : ∀ {a} {A : Set a} {x y : A} → x ≡ y → x ≅ y
≡-to-≅ refl = refl

≅-to-≡ : ∀ {a} {A : Set a} {x y : A} → x ≅ y → x ≡ y
≅-to-≡ refl = refl

------------------------------------------------------------------------
-- Some properties

reflexive : ∀ {a} {A : Set a} → _⇒_ {A = A} _≡_ (λ x y → x ≅ y)
reflexive refl = refl

sym : ∀ {a b} {A : Set a} {B : Set b} {x : A} {y : B} → x ≅ y → y ≅ x
sym refl = refl

trans : ∀ {a b c} {A : Set a} {B : Set b} {C : Set c}
{x : A} {y : B} {z : C} →
x ≅ y → y ≅ z → x ≅ z
trans refl refl = refl

subst : ∀ {a} {A : Set a} {p} → Substitutive {A = A} (λ x y → x ≅ y) p
subst P refl p = p

subst₂ : ∀ {a b p} {A : Set a} {B : Set b} (P : A → B → Set p) →
∀ {x₁ x₂ y₁ y₂} → x₁ ≅ x₂ → y₁ ≅ y₂ → P x₁ y₁ → P x₂ y₂
subst₂ P refl refl p = p

subst-removable : ∀ {a p} {A : Set a}
(P : A → Set p) {x y} (eq : x ≅ y) z →
subst P eq z ≅ z
subst-removable P refl z = refl

≡-subst-removable : ∀ {a p} {A : Set a}
(P : A → Set p) {x y} (eq : x ≡ y) z →
PropEq.subst P eq z ≅ z
≡-subst-removable P refl z = refl

cong : ∀ {a b} {A : Set a} {B : A → Set b} {x y}
(f : (x : A) → B x) → x ≅ y → f x ≅ f y
cong f refl = refl

cong₂ : ∀ {a b c} {A : Set a} {B : A → Set b} {C : ∀ x → B x → Set c}
{x y u v}
(f : (x : A) (y : B x) → C x y) → x ≅ y → u ≅ v → f x u ≅ f y v
cong₂ f refl refl = refl

resp₂ : ∀ {a ℓ} {A : Set a} (∼ : Rel A ℓ) → ∼ Respects₂ (λ x y → x ≅ y)
resp₂ _∼_ = subst⟶resp₂ _∼_ subst

proof-irrelevance : ∀ {a b} {A : Set a} {B : Set b} {x : A} {y : B}
(p q : x ≅ y) → p ≡ q
proof-irrelevance refl refl = refl

isEquivalence : ∀ {a} {A : Set a} →
IsEquivalence {A = A} (λ x y → x ≅ y)
isEquivalence = record
{ refl  = refl
; sym   = sym
; trans = trans
}

setoid : ∀ {a} → Set a → Setoid _ _
setoid A = record
{ Carrier       = A
; _≈_           = λ x y → x ≅ y
; isEquivalence = isEquivalence
}

indexedSetoid : ∀ {a b} {A : Set a} → (A → Set b) → I.Setoid A _ _
indexedSetoid B = record
{ Carrier       = B
; _≈_           = λ x y → x ≅ y
; isEquivalence = record
{ refl  = refl
; sym   = sym
; trans = trans
}
}

≡⇿≅ : ∀ {a b} {A : Set a} (B : A → Set b) {x : A} →
Inverse (PropEq.setoid (B x)) (indexedSetoid B at x)
≡⇿≅ B = record
{ to         = record { _⟨\$⟩_ = id; cong = ≡-to-≅ }
; from       = record { _⟨\$⟩_ = id; cong = ≅-to-≡ }
; inverse-of = record
{ left-inverse-of  = λ _ → refl
; right-inverse-of = λ _ → refl
}
}

decSetoid : ∀ {a} {A : Set a} →
Decidable {A = A} {B = A} (λ x y → x ≅ y) →
DecSetoid _ _
decSetoid dec = record
{ _≈_              = λ x y → x ≅ y
; isDecEquivalence = record
{ isEquivalence = isEquivalence
; _≟_           = dec
}
}

isPreorder : ∀ {a} {A : Set a} →
IsPreorder {A = A} (λ x y → x ≅ y) (λ x y → x ≅ y)
isPreorder = record
{ isEquivalence = isEquivalence
; reflexive     = id
; trans         = trans
}

isPreorder-≡ : ∀ {a} {A : Set a} →
IsPreorder {A = A} _≡_ (λ x y → x ≅ y)
isPreorder-≡ = record
{ isEquivalence = PropEq.isEquivalence
; reflexive     = reflexive
; trans         = trans
}

preorder : ∀ {a} → Set a → Preorder _ _ _
preorder A = record
{ Carrier    = A
; _≈_        = _≡_
; _∼_        = λ x y → x ≅ y
; isPreorder = isPreorder-≡
}

------------------------------------------------------------------------
-- The inspect idiom

-- See Relation.Binary.PropositionalEquality.Inspect.

data Inspect {a} {A : Set a} (x : A) : Set a where
_with-≅_ : (y : A) (eq : y ≅ x) → Inspect x

inspect : ∀ {a} {A : Set a} (x : A) → Inspect x
inspect x = x with-≅ refl

------------------------------------------------------------------------
-- Convenient syntax for equational reasoning

module ≅-Reasoning where

-- The code in Relation.Binary.EqReasoning cannot handle
-- heterogeneous equalities, hence the code duplication here.

infix  4 _IsRelatedTo_
infix  2 _∎
infixr 2 _≅⟨_⟩_ _≡⟨_⟩_
infix  1 begin_

data _IsRelatedTo_ {a} {A : Set a} (x : A) {b} {B : Set b} (y : B) :
Set where
relTo : (x≅y : x ≅ y) → x IsRelatedTo y

begin_ : ∀ {a} {A : Set a} {x : A} {b} {B : Set b} {y : B} →
x IsRelatedTo y → x ≅ y
begin relTo x≅y = x≅y

_≅⟨_⟩_ : ∀ {a} {A : Set a} (x : A) {b} {B : Set b} {y : B}
{c} {C : Set c} {z : C} →
x ≅ y → y IsRelatedTo z → x IsRelatedTo z
_ ≅⟨ x≅y ⟩ relTo y≅z = relTo (trans x≅y y≅z)

_≡⟨_⟩_ : ∀ {a} {A : Set a} (x : A) {y c} {C : Set c} {z : C} →
x ≡ y → y IsRelatedTo z → x IsRelatedTo z
_ ≡⟨ x≡y ⟩ relTo y≅z = relTo (trans (reflexive x≡y) y≅z)

_∎ : ∀ {a} {A : Set a} (x : A) → x IsRelatedTo x
_∎ _ = relTo refl
```