------------------------------------------------------------------------
-- The Agda standard library
--
-- Unary relations
------------------------------------------------------------------------

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

module Relation.Unary where

open import Data.Empty
open import Data.Unit.Base using ()
open import Data.Product
open import Data.Sum.Base using (_⊎_; [_,_])
open import Function.Base
open import Level
open import Relation.Nullary hiding (Irrelevant)
open import Relation.Nullary.Decidable.Core using (True)
open import Relation.Binary.PropositionalEquality.Core using (_≡_)

private
variable
a b c  ℓ₁ ℓ₂ : Level
A : Set a
B : Set b
C : Set c

------------------------------------------------------------------------
-- Definition

-- Unary relations are known as predicates and `Pred A ℓ` can be viewed
-- as some property that elements of type A might satisfy.

-- Consequently `P : Pred A ℓ` can also be seen as a subset of A
-- containing all the elements of A that satisfy property P. This view
-- informs much of the notation used below.

Pred :  {a}  Set a  ( : Level)  Set (a  suc )
Pred A  = A  Set

------------------------------------------------------------------------
-- Special sets

-- The empty set.

: Pred A 0ℓ
= λ _

-- The singleton set.

｛_｝ : A  Pred A _
x  = x ≡_

-- The universal set.

U : Pred A 0ℓ
U = λ _

------------------------------------------------------------------------
-- Membership

infix 4 _∈_ _∉_

_∈_ : A  Pred A   Set _
x  P = P x

_∉_ : A  Pred A   Set _
x  P = ¬ x  P

------------------------------------------------------------------------
-- Subset relations

infix 4 _⊆_ _⊇_ _⊈_ _⊉_ _⊂_ _⊃_ _⊄_ _⊅_ _≐_ _≐′_

_⊆_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q =  {x}  x  P  x  Q

_⊇_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = Q  P

_⊈_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = ¬ (P  Q)

_⊉_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = ¬ (P  Q)

_⊂_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = P  Q × Q  P

_⊃_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = Q  P

_⊄_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = ¬ (P  Q)

_⊅_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = ¬ (P  Q)

_≐_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q = (P  Q) × (Q  P)

-- The following primed variants of _⊆_ can be used when 'x' can't
-- be inferred from 'x ∈ P'.

infix 4 _⊆′_ _⊇′_ _⊈′_ _⊉′_ _⊂′_ _⊃′_ _⊄′_ _⊅′_

_⊆′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊆′ Q =  x  x  P  x  Q

_⊇′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
Q ⊇′ P = P ⊆′ Q

_⊈′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊈′ Q = ¬ (P ⊆′ Q)

_⊉′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊉′ Q = ¬ (P ⊇′ Q)

_⊂′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊂′ Q = P ⊆′ Q × Q ⊈′ P

_⊃′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊃′ Q = Q ⊂′ P

_⊄′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊄′ Q = ¬ (P ⊂′ Q)

_⊅′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ⊅′ Q = ¬ (P ⊃′ Q)

_≐′_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P ≐′ Q = (P ⊆′ Q) × (Q ⊆′ P)

------------------------------------------------------------------------
-- Properties of sets

infix 10 Satisfiable Universal IUniversal

-- Emptiness - no element satisfies P.

Empty : Pred A   Set _
Empty P =  x  x  P

-- Satisfiable - at least one element satisfies P.

Satisfiable : Pred A   Set _
Satisfiable P =  λ x  x  P

syntax Satisfiable P = ∃⟨ P

-- Universality - all elements satisfy P.

Universal : Pred A   Set _
Universal P =  x  x  P

syntax Universal  P = Π[ P ]

-- Implicit universality - all elements satisfy P.

IUniversal : Pred A   Set _
IUniversal P =  {x}  x  P

syntax IUniversal P = ∀[ P ]

-- Decidability - it is possible to determine if an arbitrary element
-- satisfies P.

Decidable : Pred A   Set _
Decidable P =  x  Dec (P x)

-- Erasure: A decidable predicate gives rise to another one, more
-- amenable to η-expansion

⌊_⌋ : {P : Pred A }  Decidable P  Pred A
P?  a = Lift _ (True (P? a))

-- Irrelevance - any two proofs that an element satifies P are
-- indistinguishable.

Irrelevant : Pred A   Set _
Irrelevant P =  {x} (a : P x) (b : P x)  a  b

-- Recomputability - we can rebuild a relevant proof given an
-- irrelevant one.

Recomputable : Pred A   Set _
Recomputable P =  {x}  .(P x)  P x

------------------------------------------------------------------------
-- Operations on sets

infix 10
infixr 9 _⊢_
infixr 8 _⇒_
infixr 7 _∩_
infixr 6 _∪_
infixr 6 _∖_
infix 4 _≬_

-- Complement.

: Pred A   Pred A
P = λ x  x  P

-- Implication.

_⇒_ : Pred A ℓ₁  Pred A ℓ₂  Pred A _
P  Q = λ x  x  P  x  Q

-- Union.

_∪_ : Pred A ℓ₁  Pred A ℓ₂  Pred A _
P  Q = λ x  x  P  x  Q

-- Intersection.

_∩_ : Pred A ℓ₁  Pred A ℓ₂  Pred A _
P  Q = λ x  x  P × x  Q

-- Difference.

_∖_ : Pred A ℓ₁  Pred A ℓ₂  Pred A _
P  Q = λ x  x  P × x  Q

-- Infinitary union.

:  {i} (I : Set i)  (I  Pred A )  Pred A _
I P = λ x  Σ[ i  I ] P i x

syntax  I  i  P) = ⋃[ i  I ] P

-- Infinitary intersection.

:  {i} (I : Set i)  (I  Pred A )  Pred A _
I P = λ x  (i : I)  P i x

syntax  I  i  P) = ⋂[ i  I ] P

-- Positive version of non-disjointness, dual to inclusion.

_≬_ : Pred A ℓ₁  Pred A ℓ₂  Set _
P  Q =  λ x  x  P × x  Q

-- Update.

_⊢_ : (A  B)  Pred B   Pred A
f  P = λ x  P (f x)

------------------------------------------------------------------------
-- Predicate combinators

-- These differ from the set operations above, as the carrier set of the
-- resulting predicates are not the same as the carrier set of the
-- component predicates.

infixr  2 _⟨×⟩_
infixr  2 _⟨⊙⟩_
infixr  1 _⟨⊎⟩_
infixr  0 _⟨→⟩_
infixl  9 _⟨·⟩_
infix  10 _~
infixr  9 _⟨∘⟩_
infixr  2 _//_ _\\_

-- Product.

_⟨×⟩_ : Pred A ℓ₁  Pred B ℓ₂  Pred (A × B) _
(P ⟨×⟩ Q) (x , y) = x  P × y  Q

-- Sum over one element.

_⟨⊎⟩_ : Pred A   Pred B   Pred (A  B) _
P ⟨⊎⟩ Q = [ P , Q ]

-- Sum over two elements.

_⟨⊙⟩_ : Pred A ℓ₁  Pred B ℓ₂  Pred (A × B) _
(P ⟨⊙⟩ Q) (x , y) = x  P  y  Q

-- Implication.

_⟨→⟩_ : Pred A ℓ₁  Pred B ℓ₂  Pred (A  B) _
(P ⟨→⟩ Q) f = P  Q  f

-- Product.

_⟨·⟩_ : (P : Pred A ℓ₁) (Q : Pred B ℓ₂)
(P ⟨×⟩ (P ⟨→⟩ Q))  Q  uncurry (flip _\$_)
(P ⟨·⟩ Q) (p , f) = f p

-- Converse.

_~ : Pred (A × B)   Pred (B × A)
P ~ = P  swap

-- Composition.

_⟨∘⟩_ : Pred (A × B) ℓ₁  Pred (B × C) ℓ₂  Pred (A × C) _
(P ⟨∘⟩ Q) (x , z) =  λ y  (x , y)  P × (y , z)  Q

-- Post-division.

_//_ : Pred (A × C) ℓ₁  Pred (B × C) ℓ₂  Pred (A × B) _
(P // Q) (x , y) = Q  (y ,_)  P  (x ,_)

-- Pre-division.

_\\_ : Pred (A × C) ℓ₁  Pred (A × B) ℓ₂  Pred (B × C) _
P \\ Q = (P ~ // Q ~) ~