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(** Some facts and definitions concerning choice and description in intuitionistic logic. *)
(** * References: *)
(** [[Bell]] John L. Bell, Choice principles in intuitionistic set theory, unpublished.  [[Bell93]] John L. Bell, Hilbert's Epsilon Operator in Intuitionistic Type Theories, Mathematical Logic Quarterly, volume 39, 1993.  [[Carlström04]] Jesper Carlström, EM + Ext_ + AC_int is equivalent to AC_ext, Mathematical Logic Quaterly, vol 50(3), pp 236-240, 2004.  [[Carlström05]] Jesper Carlström, Interpreting descriptions in intentional type theory, Journal of Symbolic Logic 70(2):488-514, 2005.  [[Werner97]] Benjamin Werner, Sets in Types, Types in Sets, TACS, 1997. *)
Set Implicit Arguments.
Local Unset Intuition Negation Unfolding.
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(** * Definitions *)
(** Choice, reification and description schemes *)
(** We make them all polymorphic. Most of them have existentials as conclusion so they require polymorphism otherwise their first application (e.g. to an existential in [Set]) will fix the level of [A]. *)
(* Set Universe Polymorphism. *)
Section ChoiceSchemes.
Variables A B :Type.
Variable P:A->Prop.
(** ** Constructive choice and description *)
(** AC_rel = relational form of the (non extensional) axiom of choice (a "set-theoretic" axiom of choice) *)
Definition RelationalChoice_on :=
forall R:A->B->Prop,
(forall x : A, exists y : B, R x y) ->
(exists R' : A->B->Prop, subrelation R' R /\ forall x, exists! y, R' x y).
(** AC_fun = functional form of the (non extensional) axiom of choice (a "type-theoretic" axiom of choice) *)
(* Note: This is called Type-Theoretic Description Axiom (TTDA) in [[Werner97]] (using a non-standard meaning of "description"). This is called intensional axiom of choice (AC_int) in [[Carlström04]] *)
Definition FunctionalChoice_on_rel (R:A->B->Prop) :=
(forall x:A, exists y : B, R x y) ->
exists f : A -> B, (forall x:A, R x (f x)).
Definition FunctionalChoice_on :=
forall R:A->B->Prop,
(forall x : A, exists y : B, R x y) ->
(exists f : A->B, forall x : A, R x (f x)).
(** AC_fun_dep = functional form of the (non extensional) axiom of choice, with dependent functions *)
Definition DependentFunctionalChoice_on (A:Type) (B:A -> Type) :=
forall R:forall x:A, B x -> Prop,
(forall x:A, exists y : B x, R x y) ->
(exists f : (forall x:A, B x), forall x:A, R x (f x)).
(** AC_trunc = axiom of choice for propositional truncations (truncation and quantification commute) *)
Definition InhabitedForallCommute_on (A : Type) (B : A -> Type) :=
(forall x, inhabited (B x)) -> inhabited (forall x, B x).
(** DC_fun = functional form of the dependent axiom of choice *)
Definition FunctionalDependentChoice_on :=
forall (R:A->A->Prop),
(forall x, exists y, R x y) -> forall x0,
(exists f : nat -> A, f 0 = x0 /\ forall n, R (f n) (f (S n))).
(** ACw_fun = functional form of the countable axiom of choice *)
Definition FunctionalCountableChoice_on :=
forall (R:nat->A->Prop),
(forall n, exists y, R n y) ->
(exists f : nat -> A, forall n, R n (f n)).
(** AC! = functional relation reification (known as axiom of unique choice in topos theory, sometimes called principle of definite description in the context of constructive type theory, sometimes called axiom of no choice) *)
Definition FunctionalRelReification_on :=
forall R:A->B->Prop,
(forall x : A, exists! y : B, R x y) ->
(exists f : A->B, forall x : A, R x (f x)).
(** AC_dep! = functional relation reification, with dependent functions see AC! *)
Definition DependentFunctionalRelReification_on (A:Type) (B:A -> Type) :=
forall (R:forall x:A, B x -> Prop),
(forall x:A, exists! y : B x, R x y) ->
(exists f : (forall x:A, B x), forall x:A, R x (f x)).
(** AC_fun_repr = functional choice of a representative in an equivalence class *)
(* Note: This is called Type-Theoretic Choice Axiom (TTCA) in [[Werner97]] (by reference to the extensional set-theoretic formulation of choice); Note also a typo in its intended formulation in [[Werner97]]. *)
Definition RepresentativeFunctionalChoice_on :=
forall R:A->A->Prop,
(Equivalence R) ->
(exists f : A->A, forall x : A, (R x (f x)) /\ forall x', R x x' -> f x = f x').
(** AC_fun_setoid = functional form of the (so-called extensional) axiom of choice from setoids *)
Definition SetoidFunctionalChoice_on :=
forall R : A -> A -> Prop,
forall T : A -> B -> Prop,
Equivalence R ->
(forall x x' y, R x x' -> T x y -> T x' y) ->
(forall x, exists y, T x y) ->
exists f : A -> B, forall x : A, T x (f x) /\ (forall x' : A, R x x' -> f x = f x').
(** AC_fun_setoid_gen = functional form of the general form of the (so-called extensional) axiom of choice over setoids *)
(* Note: This is called extensional axiom of choice (AC_ext) in [[Carlström04]]. *)
Definition GeneralizedSetoidFunctionalChoice_on :=
forall R : A -> A -> Prop,
forall S : B -> B -> Prop,
forall T : A -> B -> Prop,
Equivalence R ->
Equivalence S ->
(forall x x' y y', R x x' -> S y y' -> T x y -> T x' y') ->
(forall x, exists y, T x y) ->
exists f : A -> B,
forall x : A, T x (f x) /\ (forall x' : A, R x x' -> S (f x) (f x')).
(** AC_fun_setoid_simple = functional form of the (so-called extensional) axiom of choice from setoids on locally compatible relations *)
Definition SimpleSetoidFunctionalChoice_on A B :=
forall R : A -> A -> Prop,
forall T : A -> B -> Prop,
Equivalence R ->
(forall x, exists y, forall x', R x x' -> T x' y) ->
exists f : A -> B, forall x : A, T x (f x) /\ (forall x' : A, R x x' -> f x = f x').
(** ID_epsilon = constructive version of indefinite description; combined with proof-irrelevance, it may be connected to Carlström's type theory with a constructive indefinite description operator *)
Definition ConstructiveIndefiniteDescription_on :=
forall P:A->Prop,
(exists x, P x) -> { x:A | P x }.
(** ID_iota = constructive version of definite description; combined with proof-irrelevance, it may be connected to Carlström's and Stenlund's type theory with a constructive definite description operator) *)
Definition ConstructiveDefiniteDescription_on :=
forall P:A->Prop,
(exists! x, P x) -> { x:A | P x }.
(** ** Weakly classical choice and description *)
(** GAC_rel = guarded relational form of the (non extensional) axiom of choice *)
Definition GuardedRelationalChoice_on :=
forall P : A->Prop, forall R : A->B->Prop,
(forall x : A, P x -> exists y : B, R x y) ->
(exists R' : A->B->Prop,
subrelation R' R /\ forall x, P x -> exists! y, R' x y).
(** GAC_fun = guarded functional form of the (non extensional) axiom of choice *)
Definition GuardedFunctionalChoice_on :=
forall P : A->Prop, forall R : A->B->Prop,
inhabited B ->
(forall x : A, P x -> exists y : B, R x y) ->
(exists f : A->B, forall x, P x -> R x (f x)).
(** GAC! = guarded functional relation reification *)
Definition GuardedFunctionalRelReification_on :=
forall P : A->Prop, forall R : A->B->Prop,
inhabited B ->
(forall x : A, P x -> exists! y : B, R x y) ->
(exists f : A->B, forall x : A, P x -> R x (f x)).
(** OAC_rel = "omniscient" relational form of the (non extensional) axiom of choice *)
Definition OmniscientRelationalChoice_on :=
forall R : A->B->Prop,
exists R' : A->B->Prop,
subrelation R' R /\ forall x : A, (exists y : B, R x y) -> exists! y, R' x y.
(** OAC_fun = "omniscient" functional form of the (non extensional) axiom of choice (called AC* in Bell [[Bell]]) *)
Definition OmniscientFunctionalChoice_on :=
forall R : A->B->Prop,
inhabited B ->
exists f : A->B, forall x : A, (exists y : B, R x y) -> R x (f x).
(** D_epsilon = (weakly classical) indefinite description principle *)
Definition EpsilonStatement_on :=
forall P:A->Prop,
inhabited A -> { x:A | (exists x, P x) -> P x }.
(** D_iota = (weakly classical) definite description principle *)
Definition IotaStatement_on :=
forall P:A->Prop,
inhabited A -> { x:A | (exists! x, P x) -> P x }.
End ChoiceSchemes.
(** Generalized schemes *)
Notation RelationalChoice :=
(forall A B : Type, RelationalChoice_on A B).
Notation FunctionalChoice :=
(forall A B : Type, FunctionalChoice_on A B).
Notation DependentFunctionalChoice :=
(forall A (B:A->Type), DependentFunctionalChoice_on B).
Notation InhabitedForallCommute :=
(forall A (B : A -> Type), InhabitedForallCommute_on B).
Notation FunctionalDependentChoice :=
(forall A : Type, FunctionalDependentChoice_on A).
Notation FunctionalCountableChoice :=
(forall A : Type, FunctionalCountableChoice_on A).
Notation FunctionalChoiceOnInhabitedSet :=
(forall A B : Type, inhabited B -> FunctionalChoice_on A B).
Notation FunctionalRelReification :=
(forall A B : Type, FunctionalRelReification_on A B).
Notation DependentFunctionalRelReification :=
(forall A (B:A->Type), DependentFunctionalRelReification_on B).
Notation RepresentativeFunctionalChoice :=
(forall A : Type, RepresentativeFunctionalChoice_on A).
Notation SetoidFunctionalChoice :=
(forall A  B: Type, SetoidFunctionalChoice_on A B).
Notation GeneralizedSetoidFunctionalChoice :=
(forall A B : Type, GeneralizedSetoidFunctionalChoice_on A B).
Notation SimpleSetoidFunctionalChoice :=
(forall A B : Type, SimpleSetoidFunctionalChoice_on A B).
Notation GuardedRelationalChoice :=
(forall A B : Type, GuardedRelationalChoice_on A B).
Notation GuardedFunctionalChoice :=
(forall A B : Type, GuardedFunctionalChoice_on A B).
Notation GuardedFunctionalRelReification :=
(forall A B : Type, GuardedFunctionalRelReification_on A B).
Notation OmniscientRelationalChoice :=
(forall A B : Type, OmniscientRelationalChoice_on A B).
Notation OmniscientFunctionalChoice :=
(forall A B : Type, OmniscientFunctionalChoice_on A B).
Notation ConstructiveDefiniteDescription :=
(forall A : Type, ConstructiveDefiniteDescription_on A).
Notation ConstructiveIndefiniteDescription :=
(forall A : Type, ConstructiveIndefiniteDescription_on A).
Notation IotaStatement :=
(forall A : Type, IotaStatement_on A).
Notation EpsilonStatement :=
(forall A : Type, EpsilonStatement_on A).
(** Subclassical schemes *)
(** PI = proof irrelevance *)
Definition ProofIrrelevance :=
forall (A:Prop) (a1 a2:A), a1 = a2.
(** IGP = independence of general premises (an unconstrained generalisation of the constructive principle of independence of premises) *)
Definition IndependenceOfGeneralPremises :=
forall (A:Type) (P:A -> Prop) (Q:Prop),
inhabited A ->
(Q -> exists x, P x) -> exists x, Q -> P x.
(** Drinker = drinker's paradox (small form) (called Ex in Bell [[Bell]]) *)
Definition SmallDrinker'sParadox :=
forall (A:Type) (P:A -> Prop), inhabited A ->
exists x, (exists x, P x) -> P x.
(** EM = excluded-middle *)
Definition ExcludedMiddle :=
forall P:Prop, P \/ ~ P.
(** Extensional schemes *)
(** Ext_prop_repr = choice of a representative among extensional propositions *)
Local Notation ExtensionalPropositionRepresentative :=
(forall (A:Type),
exists h : Prop -> Prop,
forall P : Prop, (P <-> h P) /\ forall Q, (P <-> Q) -> h P = h Q).
(** Ext_pred_repr = choice of a representative among extensional predicates *)
Local Notation ExtensionalPredicateRepresentative :=
(forall (A:Type),
exists h : (A->Prop) -> (A->Prop),
forall (P : A -> Prop), (forall x, P x <-> h P x) /\ forall Q, (forall x, P x <-> Q x) -> h P = h Q).
(** Ext_fun_repr = choice of a representative among extensional functions *)
Local Notation ExtensionalFunctionRepresentative :=
(forall (A B:Type),
exists h : (A->B) -> (A->B),
forall (f : A -> B), (forall x, f x = h f x) /\ forall g, (forall x, f x = g x) -> h f = h g).
(** We let also  - IPL_2   = 2nd-order impredicative minimal predicate logic (with ex. quant.) - IPL^2   = 2nd-order functional minimal predicate logic (with ex. quant.) - IPL_2^2 = 2nd-order impredicative, 2nd-order functional minimal pred. logic (with ex. quant.)  with no prerequisite on the non-emptiness of domains *)
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(** * Table of contents *)
(* This is very fragile. *)
(** 1. Definitions  2. IPL_2^2 |- AC_rel + AC! = AC_fun  3.1. typed IPL_2 + Sigma-types + PI |- AC_rel = GAC_rel  and  IPL_2 |- AC_rel + IGP -> GAC_rel and IPL_2 |- GAC_rel = OAC_rel  3.2. IPL^2 |- AC_fun + IGP = GAC_fun = OAC_fun = AC_fun + Drinker  3.3. D_iota -> ID_iota  and  D_epsilon <-> ID_epsilon + Drinker  4. Derivability of choice for decidable relations with well-ordered codomain  5. AC_fun = AC_fun_dep = AC_trunc  6. Non contradiction of constructive descriptions wrt functional choices  7. Definite description transports classical logic to the computational world  8. Choice -> Dependent choice -> Countable choice  9.1. AC_fun_setoid = AC_fun + Ext_fun_repr + EM  9.2. AC_fun_setoid = AC_fun + Ext_pred_repr + PI *)
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(** * AC_rel + AC! = AC_fun  We show that the functional formulation of the axiom of Choice (usual formulation in type theory) is equivalent to its relational formulation (only formulation of set theory) + functional relation reification (aka axiom of unique choice, or, principle of (parametric) definite descriptions) *)
(** This shows that the axiom of choice can be assumed (under its relational formulation) without known inconsistency with classical logic, though functional relation reification conflicts with classical logic *)

Corollary fun_choice_iff_rel_choice_and_functional_rel_reification :
forall A B : Type, FunctionalChoice_on A B <->
RelationalChoice_on A B /\ FunctionalRelReification_on A B.