While this axiom makes sense intuitively, are such coinductive extensionality principles known to be sound in general?
Yes. There's an isomorphism between coinductive types and function types. A coinductive type modulo coinductive extensionality is isomorphic to the type of functions from it's index type to its data type modulo function extensionality. So, for example, Stream A modulo EqSt (from Coq.Lists.Streams) is isomorphic to nat -> A modulo function extensionality. Your datatype modulo path_extensionality is roughly isomorphic to { st : nat -> state | forall n, R (st n) (st (S n)) } modulo function extensionality. The trick I'm pulling with your datatype here is to transform it into one that is parameterized instead of indexed; your datatype is morally parameterized over Stream state.
There's unfortunately no way to introduce all the extensionality axioms at once, unless you want to give up coinductives and switch to functions (in which case you can just use the standard library axiom Coq.Logic.FunctionalExtensionality.functional_extensionality_dep). However, you could use a weaker axiom combined with function extensionality: the weaker axiom for a given coinductive type would say that if you go from the coinductive to the corresponding function and then back, you get back what you started with. Another way of phrasing this is that you can axiomatize only one direction of the isomorphism, rather than both directions. This is weaker in the sense that each coinductive extensionality axiom implies function extensionality for the corresponding function type, but this axiom does not.
In case you want some code, here's some code showing the formal isomorphism for standard library streams and for your type (works in Coq 8.13):
Require Import Coq.Lists.Streams.
Require Import Coq.Setoids.Setoid.
Module StreamsExt.
Section __.
Context (A : Type).
Definition t := Stream A.
Definition index := nat.
Definition data := A.
(** inclusion, injection, section, monomorphism *)
(** From a function [nat -> A], we can build a [Stream A] *)
CoFixpoint sect (x : index -> data) : t
:= Streams.Cons (x 0) (sect (fun n => x (S n))).
(* surjection, retraction, epimorphism *)
(** From a [Stream A], we can get the nth element, building a function [nat -> A] *)
Fixpoint retr (x : t) (i : index) : data
:= match i with
| O => Streams.hd x
| S i => retr (Streams.tl x) i
end.
(** Two streams are coextensionally equal, then the corresponding functions are extensionally equal *)
Lemma iff_EqSt_pointwise_eq (s1 s2 : t) : Streams.EqSt s1 s2 <-> forall i, retr s1 i = retr s2 i.
Proof using Type.
split.
{ intros H i; revert s1 s2 H; induction i as [|i IHi]; cbn; intros s1 s2 [H1 H2]; auto. }
{ revert s1 s2; cofix CIH; intros s1 s2 H.
constructor.
{ specialize (H 0); cbn in H. assumption. }
{ specialize (fun i => H (S i)); cbn in H. auto. } }
Qed.
Lemma ext_from_funext
(funext : forall f g : index -> data, (forall x, f x = g x) -> f = g)
(axiom : forall x, sect (retr x) = x)
: forall (s1 s2 : t), EqSt s1 s2 -> s1 = s2.
Proof using Type.
intros s1 s2; rewrite iff_EqSt_pointwise_eq; intro H.
rewrite <- (axiom s1), <- (axiom s2); apply f_equal, funext, H.
Qed.
End __.
End StreamsExt.
Section Paths.
Context {state : Type}.
Variable R : relation state.
CoInductive path (s: state) : Type :=
| step : forall s', R s s' -> path s' -> path s.
CoInductive path_eq : forall {s}, path s -> path s -> Prop :=
| path_eq_intro : forall x y r p p',
path_eq p p' ->
path_eq (step x y r p) (step x y r p').
Definition next_state {s} (p : path s) : state
:= match p with
| @step _ s' _ _ => s'
end.
Definition unstep_rel {s} (p : path s) : R s (next_state p)
:= match p with
| @step _ _ r _ => r
end.
Definition unstep {s} (p : path s) : path (next_state p)
:= match p with
| @step _ _ _ p => p
end.
(* slightly nicer version of path_eq *)
Import EqNotations.
CoInductive path_eq' {s1 s2} (p1 : path s1) (p2 : path s2) (H : s1 = s2) : Prop :=
| path_eq'_intro :
forall (p : next_state p1 = next_state p2),
(rew [fun s => R s _] H in rew p in unstep_rel p1) = unstep_rel p2 ->
path_eq' (unstep p1) (unstep p2) p ->
path_eq' p1 p2 H.
CoFixpoint path_eq'_of_path_eq {s} {p1 p2 : path s}
: path_eq p1 p2 -> path_eq' p1 p2 eq_refl.
Proof using Type.
intro H; unshelve econstructor; destruct H.
{ cbn. reflexivity. }
{ cbn. reflexivity. }
{ cbn. apply path_eq'_of_path_eq, H. }
Qed.
Lemma next_state_eq_of_path_eq' {s1 s2} {p1 : path s1} {p2 : path s2} {H : s1 = s2}
: path_eq' p1 p2 H -> next_state p1 = next_state p2.
Proof using Type.
intro H'; destruct H'; assumption.
Defined.
Lemma rel_eq_of_path_eq' {s1 s2} {p1 : path s1} {p2 : path s2} {H : s1 = s2}
: forall p : path_eq' p1 p2 H, (rew [fun s => R s _] H in rew next_state_eq_of_path_eq' p in unstep_rel p1) = unstep_rel p2.
Proof using Type.
intro H'; destruct H'; cbn in *; assumption.
Defined.
CoFixpoint path_eq_of_path_eq' {s} {p1 p2 : path s}
: path_eq' p1 p2 eq_refl -> path_eq p1 p2.
Proof using Type.
intro H'; destruct p1, p2, H'; cbn in *; subst; cbn in *.
constructor; auto.
Qed.
Lemma iff_path_eq_path_eq'_eq_refl {s} (p1 p2 : path s)
: path_eq p1 p2 <-> path_eq' p1 p2 eq_refl.
Proof using Type.
split; first [ apply path_eq'_of_path_eq | apply path_eq_of_path_eq' ].
Qed.
Lemma iff_path_eq_path_eq' {s1 s2} (p1 : path s1) (p2 : path s2) (H : s1 = s2)
: path_eq (rew H in p1) p2 <-> path_eq' p1 p2 H.
Proof using Type.
subst; apply iff_path_eq_path_eq'_eq_refl.
Qed.
End Paths.
Module PathsExt.
Section __.
Context {state : Type}.
Variable R : relation state.
Definition t := { s : state & path R s }.
Definition fun_type := { st : nat -> state | forall n, R (st n) (st (S n)) }.
Definition mk_fun_type (st : nat -> state) (H : forall n, R (st n) (st (S n))) : fun_type
:= exist _ st H.
(** inclusion, injection, section, monomorphism *)
CoFixpoint sect' (x : fun_type) : path R (proj1_sig x 0)
:= step
R _ _
(proj2_sig x 0)
(sect' (mk_fun_type (fun i => proj1_sig x (S i)) (fun i => proj2_sig x (S i)))).
Definition sect (x : fun_type) : t := existT _ _ (sect' x).
(* surjection, retraction, epimorphism *)
Fixpoint retr_index (x : t) (i : nat) : state
:= match i with
| O => projT1 x
| S i => retr_index (existT _ _ (unstep _ (projT2 x))) i
end.
Fixpoint retr' (x : t) (i : nat) : R (retr_index x i) (retr_index x (S i))
:= match i with
| O => unstep_rel _ (projT2 x)
| S i => retr' (existT _ _ (unstep _ (projT2 x))) i
end.
Definition retr (x : t) : fun_type := mk_fun_type (retr_index x) (retr' x).
Import EqNotations.
(** Two paths are coextensionally equal, then the corresponding functions are extensionally equal *)
Definition t_eq (s1 s2 : t) : Prop
:= { p : projT1 s1 = projT1 s2 | path_eq' _ (projT2 s1) (projT2 s2) p }.
Definition fun_type_eq (s1 s2 : fun_type) : Prop
:= exists p : (forall i, proj1_sig s1 i = proj1_sig s2 i),
forall i, (rew [fun s => R s _] (p i) in rew (p (S i)) in proj2_sig s1 i) = proj2_sig s2 i.
Lemma iff_path_eq_pointwise_eq (s1 s2 : t) : t_eq s1 s2 <-> fun_type_eq (retr s1) (retr s2).
Proof using Type.
split; cbv [t_eq fun_type_eq].
{ intro H; unshelve eexists; intro i; revert s1 s2 H; induction i as [|i IHi]; cbn; intros [s1 s1'] [s2 s2'] [H1 H2];
cbn in *.
{ assumption. }
{ apply IHi; cbn.
unshelve eexists; destruct H2; eassumption. }
{ cbn. destruct H2; assumption. }
{ cbn in *. apply IHi. } }
{ intros [H1 H2].
exists (H1 0).
revert s1 s2 H1 H2; cofix CIH; intros.
unshelve econstructor.
{ exact (H1 1). }
{ exact (H2 O). }
{ apply (CIH _ _ (fun i => H1 (S i)) (fun i => H2 (S i))). } }
Defined.
Lemma ext_from_funext'
(funext_fun_type : forall f g, fun_type_eq f g -> f = g)
(axiom : forall x, sect (retr x) = x)
(funext_fun_type_pr : forall f g p q, f_equal (fun a => proj1_sig a 0) (funext_fun_type f g (ex_intro _ p q)) = p 0)
(axiom_pr1 : forall x, f_equal (@projT1 _ _) (axiom x) = eq_refl)
: forall (p1 p2 : t) (H : t_eq p1 p2), { pf : p1 = p2 | f_equal (@projT1 _ _) pf = proj1_sig H }.
Proof using Type.
intros p1 p2.
intro H; unshelve eexists.
{ apply iff_path_eq_pointwise_eq, funext_fun_type, (f_equal sect) in H.
etransitivity; [ | apply axiom ]; etransitivity; [ symmetry; apply axiom | ].
exact H. }
{ cbv zeta beta.
rewrite !eq_trans_map_distr, <- !eq_sym_map_distr, !axiom_pr1; cbn [eq_sym].
rewrite !eq_trans_refl_l, !eq_trans_refl_r, !f_equal_compose.
cbn [projT1 sect].
cbv [iff_path_eq_pointwise_eq]; cbn.
rewrite funext_fun_type_pr.
cbn.
destruct p1, p2, H; cbn; reflexivity. }
Qed.
Lemma f_equal_projT1_eq_sigT {A P u v p q}
: f_equal (@projT1 _ _) (@eq_sigT A P u v p q) = p.
Proof using Type. destruct u, v; cbn in *; subst; cbn; reflexivity. Qed.
Lemma ext_from_funext''
(funext_nat : forall f g : nat -> state, (forall x, f x = g x) -> f = g)
(funext_rel : forall F (f g : forall n : nat, R (F n) (F (S n))), (forall x, f x = g x) -> f = g)
(axiom : forall x, sect (retr x) = x)
(funext_nat_f_equal : forall f g p x, f_equal (fun f => f x) (funext_nat f g p) = p x)
(axiom_pr1 : forall x, f_equal (@projT1 _ _) (axiom x) = eq_refl)
: forall {s} (p1 p2 : path R s), path_eq R p1 p2 -> p1 = p2.
Proof using Type.
intros s p1 p2 H.
apply iff_path_eq_path_eq'_eq_refl in H.
unshelve epose proof (ext_from_funext' _ _ _ _ (existT _ _ _) (existT _ _ _) (exist _ eq_refl H)) as H'; try eassumption; revgoals.
{ destruct H' as [H0 H']; cbn in *.
induction H0 using (@eq_sigT_ind _ _ _ _); cbn [projT1 projT2] in *.
rewrite f_equal_projT1_eq_sigT in H'; subst; cbn in *.
reflexivity. }
all: revgoals.
{ cbv [fun_type_eq].
intros [f1 f2] [g1 g2] [H1 H2]; cbn in *.
apply eq_sig_uncurried; cbn.
exists (funext_nat _ _ H1).
apply funext_rel; intro n.
etransitivity; [ | apply H2 ].
transitivity (rew [fun s => R s _] f_equal (fun f => f _) (funext_nat _ _ H1) in rew [R _] f_equal (fun f => f _) (funext_nat _ _ H1) in f2 n).
{ generalize (funext_nat _ _ H1); intro; subst; reflexivity. }
{ rewrite <- (funext_nat_f_equal _ _ H1 n), <- (funext_nat_f_equal _ _ H1 (S n)); reflexivity. } }
{ intros; cbn.
destruct f, g; cbn in *.
move funext_nat at bottom.
generalize (funext_nat_f_equal _ _ p).
generalize (funext_nat _ _ p).
clear funext_nat funext_nat_f_equal.
intro; subst; cbn in *.
intro funext_nat_f_equal.
destruct funext_rel; cbn; apply funext_nat_f_equal. }
Qed.
(* quoting myself at https://github.com/HoTT/HoTT/issues/757#issue-76140493 *)
Definition good_funext_of_funext {A B}
(funext : forall f g : A -> B, (forall x, f x = g x) -> f = g)
(funext_dep : forall (f : A -> B) (F G : forall x : A, {x0 : B & f x = x0}),
(forall x, F x = G x) -> F = G)
: { funext : forall f g : A -> B, (forall x, f x = g x) -> f = g
| (forall f g p, funext f g (fun x => f_equal (fun f => f x) p) = p)
/\ (forall f g p x, f_equal (fun f => f x) (funext f g p) = p x) }.
Proof using Type.
exists (fun f g p => eq_trans (eq_sym (funext _ _ (fun _ => eq_refl))) (funext f g p)).
split.
{ intros; subst; cbn; apply eq_trans_sym_inv_l. }
{ intros f g p.
pose (fun x => existT _ (g x) (p x)) as p'.
change p with (fun x => projT2 (p' x)).
change g with (fun x => projT1 (p' x)).
clearbody p'; clear p g.
assert (H'' : forall x, existT _ (f x) eq_refl = p' x).
{ intro x.
destruct (p' x) as [? []]; reflexivity. }
intro x.
apply funext_dep in H''; subst p'; cbn.
rewrite eq_trans_sym_inv_l; reflexivity. }
Qed.
Lemma ext_from_funext_specific
(funext_nat : forall f g : nat -> state, (forall x, f x = g x) -> f = g)
(funext_nat_sig : forall (f : nat -> state) (F G : forall x : nat, {x0 : state & f x = x0}),
(forall x, F x = G x) -> F = G)
(funext_rel : forall F (f g : forall n : nat, R (F n) (F (S n))), (forall x, f x = g x) -> f = g)
(axiom : forall x, projT2 (sect (retr x)) = projT2 x)
: forall {s} (p1 p2 : path R s), path_eq R p1 p2 -> p1 = p2.
Proof using Type.
unshelve eapply ext_from_funext''.
{ unshelve eapply good_funext_of_funext.
{ exact funext_nat. }
{ exact funext_nat_sig. } }
{ intro x.
unshelve eapply eq_sigT.
{ reflexivity. }
{ apply axiom. } }
{ apply funext_rel. }
{ cbn; intros f g p n.
set (fs := good_funext_of_funext _ _).
destruct fs as [fs [H1 H2]].
apply H2. }
{ cbv beta.
intro; apply f_equal_projT1_eq_sigT. }
Qed.
Lemma ext_from_funext_dep
(funext_dep : forall A B (f g : forall a : A, B a), (forall x, f x = g x) -> f = g)
(axiom : forall x, projT2 (sect (retr x)) = projT2 x)
: forall {s} (p1 p2 : path R s), path_eq R p1 p2 -> p1 = p2.
Proof using Type.
apply ext_from_funext_specific; try first [ intros *; apply funext_dep | apply axiom ].
Qed.
Lemma f_equal_id {A x y p} : @f_equal A A id x y p = p.
Proof using Type. destruct p; reflexivity. Qed.
(** A more conceptually separated version of this is at
https://github.com/HoTT/HoTT/blob/master/theories/Metatheory/FunextVarieties.v
*)
Lemma ext_from_funext
(funext : forall A B (f g : A -> B), (forall x, f x = g x) -> f = g)
(axiom : forall x, projT2 (sect (retr x)) = projT2 x)
: forall {s} (p1 p2 : path R s), path_eq R p1 p2 -> p1 = p2.
Proof using Type.
apply ext_from_funext_dep; [ | apply axiom ].
intros A B f g H.
pose (existT (fun F : A -> { x : A & { y : B x & f x = y } } => (fun x => projT1 (F x)) = id)
(fun x : A => existT _ x (existT _ (f x) eq_refl))
eq_refl) as F.
pose (existT (fun G : A -> { x : A & { y : B x & f x = y } } => (fun x => projT1 (G x)) = id)
(fun x : A => existT _ x (existT _ (g x) (H x)))
eq_refl) as G.
cut (F = G).
{ intro H'.
change ((rew [fun F => forall x, B (F x)] projT2 F in (fun x => projT1 (projT2 (projT1 F x)))) =
(rew [fun F => forall x, B (F x)] projT2 G in (fun x => projT1 (projT2 (projT1 G x))))).
clearbody F G; subst G; reflexivity. }
{ clearbody F G.
pose (fun F : A -> {x : A & {y : B x & f x = y}} => fun x => projT1 (F x)) as postcomp_projT1.
pose (fun F : A -> A
=> (fun x : A => existT (fun x : A => {y : B x & f x = y}) (F x) (existT _ _ eq_refl))) as inv.
assert (H' : forall F, inv (postcomp_projT1 F) = F).
{ clear -funext.
intro F.
apply funext; intro x.
subst inv postcomp_projT1; cbn.
destruct (F x) as [? [? []]]; cbn; reflexivity. }
simple refine (let H'' : forall F, postcomp_projT1 (inv F) = F := _ in _).
{ reflexivity. }
cut (forall Fid (F : {G : A -> {x : A & {y : B x & f x = y}} & postcomp_projT1 G = Fid}),
F = existT _ (fun x => existT _ _ (existT _ _ eq_refl)) eq_refl).
{ intro contr; etransitivity; [|symmetry]; apply contr. }
{ clear F G.
intros Fid F.
refine (eq_sym (@eq_trans _ _ (existT _ _ (H'' Fid)) _ _ (eq_sym _))).
{ apply eq_sigT_uncurried; subst inv postcomp_projT1; cbn.
exists eq_refl; cbv.
reflexivity. }
{ simple refine (let H''' : forall F, inv (postcomp_projT1 F) = F := fun F => eq_trans (eq_trans (f_equal inv (f_equal postcomp_projT1 (eq_sym (H' F)))) (f_equal inv (H'' _))) (H' F) in
_).
clearbody H''.
apply eq_sigT_uncurried; cbn.
exists (eq_trans (eq_sym (H''' _)) (f_equal inv (projT2 F))).
destruct F as [F ?]; subst.
subst H'''; cbn.
clear -funext H' H''.
clearbody inv postcomp_projT1.
clear -funext H' H''.
match goal with
| [ |- rew ?p in eq_refl = ?H ] => cut (f_equal postcomp_projT1 (eq_sym p) = H); [ generalize p | ]
end.
{ clear.
generalize (H'' (postcomp_projT1 F)).
generalize (inv (postcomp_projT1 F)).
intros; subst; cbn; reflexivity. }
rewrite !eq_sym_involutive, !eq_trans_map_distr.
cut (eq_trans
(f_equal postcomp_projT1 (f_equal inv (H'' (postcomp_projT1 F)))) (f_equal postcomp_projT1 (H' F)) =
eq_trans (f_equal postcomp_projT1 (f_equal inv (f_equal postcomp_projT1 (H' F)))) (H'' (postcomp_projT1 F))).
{ rewrite <- !eq_sym_map_distr.
intro H.
rewrite <- eq_trans_assoc, H, !eq_trans_assoc, eq_trans_sym_inv_l, eq_trans_refl_l.
reflexivity. }
generalize (f_equal postcomp_projT1 (H' F)).
generalize (postcomp_projT1 F).
cut (forall a b (e : postcomp_projT1 (inv a) = b),
eq_trans (f_equal postcomp_projT1 (f_equal inv (H'' a))) e = eq_trans (f_equal postcomp_projT1 (f_equal inv e)) (H'' b)).
{ clear; intros H ??; apply H. }
intros; subst b; cbn.
rewrite eq_trans_refl_l.
rewrite f_equal_compose.
set (F' := fun a => postcomp_projT1 (inv a)).
change (forall F, F' F = F) in H''.
change (f_equal F' (H'' a) = H'' (F' a)).
assert (Hid : F' = id) by (apply funext, H'').
clearbody F'; subst F'.
rewrite f_equal_id; reflexivity. } } }
Qed.
End __.
End PathsExt.
