```------------------------------------------------------------------------
-- Some decision procedures for equality
------------------------------------------------------------------------

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

open import Equality

module Equality.Decision-procedures
{reflexive} (eq : ∀ {a p} → Equality-with-J a p reflexive) where

open Derived-definitions-and-properties eq
open import Equality.Decidable-UIP eq
open import Prelude
hiding ( module ⊤; module ⊥; module Bool; module ℕ; module Σ
; module List
)

------------------------------------------------------------------------
-- The unit type

module ⊤ where

-- Equality of values of the unit type is decidable.

_≟_ : Decidable-equality ⊤
_ ≟ _ = inj₁ (refl _)

------------------------------------------------------------------------
-- The empty type

module ⊥ {ℓ} where

-- Equality of values of the empty type is decidable.

_≟_ : Decidable-equality (⊥ {ℓ = ℓ})
() ≟ ()

------------------------------------------------------------------------
-- Booleans

module Bool where

-- The values true and false are distinct.

abstract

true≢false : true ≢ false
true≢false true≡false = subst T true≡false _

-- Equality of booleans is decidable.

_≟_ : Decidable-equality Bool
true  ≟ true  = inj₁ (refl _)
false ≟ false = inj₁ (refl _)
true  ≟ false = inj₂ true≢false
false ≟ true  = inj₂ (true≢false ∘ sym)

------------------------------------------------------------------------
-- Natural numbers

module ℕ where

-- Inhabited only for zero.

Zero : ℕ → Set
Zero zero    = ⊤
Zero (suc n) = ⊥

-- Predecessor (except if the argument is zero).

pred : ℕ → ℕ
pred zero    = zero
pred (suc n) = n

abstract

-- Zero is not equal to the successor of any number.

0≢+ : {n : ℕ} → zero ≢ suc n
0≢+ 0≡+ = subst Zero 0≡+ tt

-- The suc constructor is cancellative.

cancel-suc : {m n : ℕ} → suc m ≡ suc n → m ≡ n
cancel-suc = cong pred

-- Equality of natural numbers is decidable.

_≟_ : Decidable-equality ℕ
zero  ≟ zero  = inj₁ (refl _)
suc m ≟ suc n = ⊎-map (cong suc) (λ m≢n → m≢n ∘ cancel-suc) (m ≟ n)
zero  ≟ suc n = inj₂ 0≢+
suc m ≟ zero  = inj₂ (0≢+ ∘ sym)

------------------------------------------------------------------------
-- Σ-types

module Σ {a b} {A : Set a} {B : A → Set b} where

-- Σ preserves decidability of equality.

module Dec (_≟A_ : Decidable-equality A)
(_≟B_ : {x : A} → Decidable-equality (B x)) where

_≟_ : Decidable-equality (Σ A B)
(x₁ , y₁) ≟ (x₂ , y₂) with x₁ ≟A x₂
... | inj₂ x₁≢x₂ = inj₂ (x₁≢x₂ ∘ cong proj₁)
... | inj₁ x₁≡x₂ with subst B x₁≡x₂ y₁ ≟B y₂
...   | inj₁ cast-y₁≡y₂ = inj₁ (Σ-≡,≡→≡ x₁≡x₂ cast-y₁≡y₂)
...   | inj₂ cast-y₁≢y₂ =
inj₂ (cast-y₁≢y₂ ∘
subst (λ p → subst B p y₁ ≡ y₂) (decidable⇒UIP _≟A_ _ _) ∘
proj₂ ∘ Σ-≡,≡←≡)

------------------------------------------------------------------------
-- Binary products

module × {a b} {A : Set a} {B : Set b} where

-- _×_ preserves decidability of equality.

module Dec (_≟A_ : Decidable-equality A)
(_≟B_ : Decidable-equality B) where

_≟_ : Decidable-equality (A × B)
_≟_ = Σ.Dec._≟_ _≟A_ _≟B_

------------------------------------------------------------------------
-- Binary sums

module ⊎ {a b} {A : Set a} {B : Set b} where

abstract

-- The values inj₁ x and inj₂ y are never equal.

inj₁≢inj₂ : {x : A} {y : B} → inj₁ x ≢ inj₂ y
inj₁≢inj₂ = Bool.true≢false ∘
cong {A = A ⊎ B} {B = Bool} [ const true , const false ]

-- The inj₁ and inj₂ constructors are cancellative.

cancel-inj₁ : {x y : A} → _≡_ {A = A ⊎ B} (inj₁ x) (inj₁ y) → x ≡ y
cancel-inj₁ {x = x} = cong {A = A ⊎ B} {B = A} [ id , const x ]

cancel-inj₂ : {x y : B} → _≡_ {A = A ⊎ B} (inj₂ x) (inj₂ y) → x ≡ y
cancel-inj₂ {x = x} = cong {A = A ⊎ B} {B = B} [ const x , id ]

-- _⊎_ preserves decidability of equality.

module Dec (_≟A_ : Decidable-equality A)
(_≟B_ : Decidable-equality B) where

_≟_ : Decidable-equality (A ⊎ B)
inj₁ x ≟ inj₁ y = ⊎-map (cong (inj₁ {B = B})) (λ x≢y → x≢y ∘ cancel-inj₁) (x ≟A y)
inj₂ x ≟ inj₂ y = ⊎-map (cong (inj₂ {A = A})) (λ x≢y → x≢y ∘ cancel-inj₂) (x ≟B y)
inj₁ x ≟ inj₂ y = inj₂ inj₁≢inj₂
inj₂ x ≟ inj₁ y = inj₂ (inj₁≢inj₂ ∘ sym)

------------------------------------------------------------------------
-- Lists

module List {a} {A : Set a} where

abstract

-- The values [] and x ∷ xs are never equal.

[]≢∷ : ∀ {x : A} {xs} → [] ≢ x ∷ xs
[]≢∷ = Bool.true≢false ∘
cong (λ { [] → true; (_ ∷ _) → false })

-- The _∷_ constructor is cancellative in both arguments.

private

head? : A → List A → A
head? _ (x ∷ _) = x

tail? : List A → List A
tail? []       = []
tail? (_ ∷ xs) = xs

cancel-∷-head : ∀ {x y : A} {xs ys} → x ∷ xs ≡ y ∷ ys → x ≡ y

cancel-∷-tail : ∀ {x y : A} {xs ys} → x ∷ xs ≡ y ∷ ys → xs ≡ ys
cancel-∷-tail = cong tail?

abstract

-- An η-like result for the cancellation lemmas.

unfold-∷ : ∀ {x y : A} {xs ys} (eq : x ∷ xs ≡ y ∷ ys) →
eq ≡ cong₂ _∷_ (cancel-∷-head eq) (cancel-∷-tail eq)
unfold-∷ {x} eq =
eq                                             ≡⟨ sym \$ trans-reflʳ eq ⟩
trans eq (refl _)                              ≡⟨ sym \$ cong (trans eq) sym-refl ⟩
trans eq (sym (refl _))                        ≡⟨ sym \$ trans-reflˡ _ ⟩
trans (refl _) (trans eq (sym (refl _)))       ≡⟨ sym \$ cong-roughly-id (λ xs → head? x xs ∷ tail? xs)
non-empty eq tt tt lemma₁ ⟩
cong (λ xs → head? x xs ∷ tail? xs) eq         ≡⟨ lemma₂ _∷_ (head? x) tail? eq ⟩∎
cong₂ _∷_ (cong (head? x) eq) (cong tail? eq)  ∎
where
non-empty : List A → Bool
non-empty []      = false
non-empty (_ ∷ _) = true

lemma₁ : (xs : List A) →
if non-empty xs then ⊤ else ⊥ →
head? x xs ∷ tail? xs ≡ xs
lemma₁ []      ()
lemma₁ (_ ∷ _) _ = refl _

lemma₂ : {A B C D : Set a} {x y : A}
(f : B → C → D) (g : A → B) (h : A → C) →
(eq : x ≡ y) →
cong (λ x → f (g x) (h x)) eq ≡
cong₂ f (cong g eq) (cong h eq)
lemma₂ f g h =
elim (λ eq → cong (λ x → f (g x) (h x)) eq ≡
cong₂ f (cong g eq) (cong h eq))
(λ x → cong (λ x → f (g x) (h x)) (refl x)          ≡⟨ cong-refl (λ x → f (g x) (h x)) ⟩
refl (f (g x) (h x))                         ≡⟨ sym \$ cong₂-refl f ⟩
cong₂ f (refl (g x)) (refl (h x))            ≡⟨ sym \$ cong₂ (cong₂ f) (cong-refl g) (cong-refl h) ⟩∎
cong₂ f (cong g (refl x)) (cong h (refl x))  ∎)

-- List preserves decidability of equality.

module Dec (_≟A_ : Decidable-equality A) where

_≟_ : Decidable-equality (List A)
[]       ≟ []       = inj₁ (refl [])
[]       ≟ (_ ∷ _)  = inj₂ []≢∷
(_ ∷ _)  ≟ []       = inj₂ ([]≢∷ ∘ sym)
(x ∷ xs) ≟ (y ∷ ys) with x ≟A y
... | inj₂ x≢y = inj₂ (x≢y ∘ cancel-∷-head)
... | inj₁ x≡y with xs ≟ ys
...   | inj₁ xs≡ys = inj₁ (cong₂ _∷_ x≡y xs≡ys)
...   | inj₂ xs≢ys = inj₂ (xs≢ys ∘ cancel-∷-tail)

------------------------------------------------------------------------
-- Finite sets

module Fin where

-- Equality of finite numbers is decidable.

_≟_ : ∀ {n} → Decidable-equality (Fin n)
_≟_ {zero}  = λ ()
_≟_ {suc n} = ⊎.Dec._≟_ (λ _ _ → inj₁ (refl tt)) (_≟_ {n})
```