# Library Coq.Logic.Eqdep_dec

We prove that there is only one proof of x=x, i.e eq_refl x. This holds if the equality upon the set of x is decidable. A corollary of this theorem is the equality of the right projections of two equal dependent pairs.
Author: Thomas Kleymann |<tms@dcs.ed.ac.uk>| in Lego adapted to Coq by B. Barras
Credit: Proofs up to K_dec follow an outline by Michael Hedberg
Table of contents:
1. Streicher's K and injectivity of dependent pair hold on decidable types
1.1. Definition of the functor that builds properties of dependent equalities from a proof of decidability of equality for a set in Type
1.2. Definition of the functor that builds properties of dependent equalities from a proof of decidability of equality for a set in Set

# Streicher's K and injectivity of dependent pair hold on decidable types

Set Implicit Arguments.
Section EqdepDec.
Variable A : Type.
Let comp (x y y':A) (eq1:x = y) (eq2:x = y') : y = y' :=
eq_ind _ (fun a => a = y') eq2 _ eq1.
Remark trans_sym_eq : forall (x y:A) (u:x = y), comp u u = eq_refl y.
Variable eq_dec : forall x y:A, x = y \/ x <> y.
Variable x : A.
Let nu (y:A) (u:x = y) : x = y :=
match eq_dec x y with
| or_introl eqxy => eqxy
| or_intror neqxy => False_ind _ (neqxy u)
end.
Let nu_constant : forall (y:A) (u v:x = y), nu u = nu v.
Qed.
Let nu_inv (y:A) (v:x = y) : x = y := comp (nu (eq_refl x)) v.
Remark nu_left_inv : forall (y:A) (u:x = y), nu_inv (nu u) = u.
Theorem eq_proofs_unicity : forall (y:A) (p1 p2:x = y), p1 = p2.
Theorem K_dec :
forall P:x = x -> Prop, P (eq_refl x) -> forall p:x = x, P p.
The corollary
Let proj (P:A -> Prop) (exP:ex P) (def:P x) : P x :=
match exP with
| ex_intro x' prf =>
match eq_dec x' x with
| or_introl eqprf => eq_ind x' P prf x eqprf
| _ => def
end
end.
Theorem inj_right_pair :
forall (P:A -> Prop) (y y':P x),
ex_intro P x y = ex_intro P x y' -> y = y'.
End EqdepDec.
Require Import EqdepFacts.
We deduce axiom K for (decidable) types
Theorem K_dec_type :
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (x:A) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
Theorem K_dec_set :
forall A:Set,
(forall x y:A, {x = y} + {x <> y}) ->
forall (x:A) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
We deduce the eq_rect_eq axiom for (decidable) types
Theorem eq_rect_eq_dec :
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (p:A) (Q:A -> Type) (x:Q p) (h:p = p), x = eq_rect p Q x p h.
We deduce the injectivity of dependent equality for decidable types
Theorem eq_dep_eq_dec :
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (P:A->Type) (p:A) (x y:P p), eq_dep A P p x p y -> x = y.
Theorem UIP_dec :
forall (A:Type),
(forall x y:A, {x = y} + {x <> y}) ->
forall (x y:A) (p1 p2:x = y), p1 = p2.
Unset Implicit Arguments.

## Definition of the functor that builds properties of dependent equalities on decidable sets in Type

The signature of decidable sets in Type
Module Type DecidableType.
Parameter U:Type.
Axiom eq_dec : forall x y:U, {x = y} + {x <> y}.
End DecidableType.
The module DecidableEqDep collects equality properties for decidable set in Type
Module DecidableEqDep (M:DecidableType).
Import M.
Invariance by Substitution of Reflexive Equality Proofs
Lemma eq_rect_eq :
forall (p:U) (Q:U -> Type) (x:Q p) (h:p = p), x = eq_rect p Q x p h.
Injectivity of Dependent Equality
Theorem eq_dep_eq :
forall (P:U->Type) (p:U) (x y:P p), eq_dep U P p x p y -> x = y.
Uniqueness of Identity Proofs (UIP)
Lemma UIP : forall (x y:U) (p1 p2:x = y), p1 = p2.
Uniqueness of Reflexive Identity Proofs
Lemma UIP_refl : forall (x:U) (p:x = x), p = eq_refl x.
Streicher's axiom K
Lemma Streicher_K :
forall (x:U) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
Injectivity of equality on dependent pairs in Type
Lemma inj_pairT2 :
forall (P:U -> Type) (p:U) (x y:P p),
existT P p x = existT P p y -> x = y.
Proof-irrelevance on subsets of decidable sets
Lemma inj_pairP2 :
forall (P:U -> Prop) (x:U) (p q:P x),
ex_intro P x p = ex_intro P x q -> p = q.
End DecidableEqDep.

## Definition of the functor that builds properties of dependent equalities on decidable sets in Set

The signature of decidable sets in Set
Module Type DecidableSet.
Parameter U:Type.
Axiom eq_dec : forall x y:U, {x = y} + {x <> y}.
End DecidableSet.
The module DecidableEqDepSet collects equality properties for decidable set in Set
Module DecidableEqDepSet (M:DecidableSet).
Import M.
Module N:=DecidableEqDep(M).
Invariance by Substitution of Reflexive Equality Proofs
Lemma eq_rect_eq :
forall (p:U) (Q:U -> Type) (x:Q p) (h:p = p), x = eq_rect p Q x p h.
Injectivity of Dependent Equality
Theorem eq_dep_eq :
forall (P:U->Type) (p:U) (x y:P p), eq_dep U P p x p y -> x = y.
Uniqueness of Identity Proofs (UIP)
Lemma UIP : forall (x y:U) (p1 p2:x = y), p1 = p2.
Uniqueness of Reflexive Identity Proofs
Lemma UIP_refl : forall (x:U) (p:x = x), p = eq_refl x.
Streicher's axiom K
Lemma Streicher_K :
forall (x:U) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
Proof-irrelevance on subsets of decidable sets
Lemma inj_pairP2 :
forall (P:U -> Prop) (x:U) (p q:P x),
ex_intro P x p = ex_intro P x q -> p = q.
Injectivity of equality on dependent pairs in Type
Lemma inj_pair2 :
forall (P:U -> Type) (p:U) (x y:P p),
existT P p x = existT P p y -> x = y.
Injectivity of equality on dependent pairs with second component in Type
Notation inj_pairT2 := inj_pair2.
End DecidableEqDepSet.
From decidability to inj_pair2
Lemma inj_pair2_eq_dec : forall A:Type, (forall x y:A, {x=y}+{x<>y}) ->
( forall (P:A -> Type) (p:A) (x y:P p), existT P p x = existT P p y -> x = y ).