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(********
Isabelle/HoTT: Univalence
Feb 2019
********)
theory Univalence
imports HoTT_Methods Prod Sum Eq Type_Families
begin
section \<open>Homotopy\<close>
definition homotopic :: "[t, t \<Rightarrow> t, t, t] \<Rightarrow> t" ("(2homotopic[_, _] _ _)" [0, 0, 1000, 1000])
where "homotopic[A, B] f g \<equiv> \<Prod>x: A. f`x =[B x] g`x"
declare homotopic_def [comp]
syntax "_homotopic" :: "[t, idt, t, t, t] \<Rightarrow> t" ("(1_ ~[_: _. _]/ _)" [101, 0, 0, 0, 101] 100)
translations "f ~[x: A. B] g" \<rightleftharpoons> "(CONST homotopic) A (\<lambda>x. B) f g"
(*
syntax "_homotopic'" :: "[t, t] \<Rightarrow> t" ("(2_ ~ _)" [1000, 1000])
ML \<open>val pretty_homotopic = Attrib.setup_config_bool @{binding "pretty_homotopic"} (K true)\<close>
print_translation \<open>
let fun homotopic_tr' ctxt [A, B, f, g] =
if Config.get ctxt pretty_homotopic
then Syntax.const @{syntax_const "_homotopic'"} $ f $ g
else @{const homotopic} $ A $ B $ f $ g
in
[(@{const_syntax homotopic}, homotopic_tr')]
end
\<close>
*)
lemma homotopic_type:
assumes [intro]: "A: U i" "B: A \<leadsto> U i" "f: \<Prod>x: A. B x" "g: \<Prod>x: A. B x"
shows "f ~[x: A. B x] g: U i"
by derive
declare homotopic_type [intro]
schematic_goal fun_eq_imp_homotopic:
assumes [intro]:
"p: f =[\<Prod>x: A. B x] g"
"f: \<Prod>x: A. B x" "g: \<Prod>x: A. B x"
"A: U i" "B: A \<leadsto> U i"
shows "?prf: f ~[x: A. B x] g"
proof (path_ind' f g p)
show "\<And>f. f : \<Prod>(x: A). B x \<Longrightarrow> \<lambda>x: A. refl(f`x): f ~[x: A. B x] f" by derive
qed routine
definition happly :: "[t, t \<Rightarrow> t, t, t, t] \<Rightarrow> t"
where "happly A B f g p \<equiv> indEq (\<lambda>f g. & f ~[x: A. B x] g) (\<lambda>f. \<lambda>(x: A). refl(f`x)) f g p"
syntax "_happly" :: "[idt, t, t, t, t, t] \<Rightarrow> t"
("(2happly[_: _. _] _ _ _)" [0, 0, 0, 1000, 1000, 1000])
translations "happly[x: A. B] f g p" \<rightleftharpoons> "(CONST happly) A (\<lambda>x. B) f g p"
corollary happly_type:
assumes [intro]:
"p: f =[\<Prod>x: A. B x] g"
"f: \<Prod>x: A. B x" "g: \<Prod>x: A. B x"
"A: U i" "B: A \<leadsto> U i"
shows "happly[x: A. B x] f g p: f ~[x: A. B x] g"
unfolding happly_def by (derive lems: fun_eq_imp_homotopic)
section \<open>Equivalence\<close>
text \<open>For now, we define equivalence in terms of bi-invertibility.\<close>
definition biinv :: "[t, t, t] \<Rightarrow> t" ("(2biinv[_, _]/ _)")
where "biinv[A, B] f \<equiv>
(\<Sum>g: B \<rightarrow> A. g o[A] f ~[x:A. A] id A) \<times> (\<Sum>g: B \<rightarrow> A. f o[B] g ~[x: B. B] id B)"
text \<open>
The meanings of the syntax defined above are:
\<^item> @{term "f ~[x: A. B x] g"} expresses that @{term f} and @{term g} are homotopic functions of type @{term "\<Prod>x:A. B x"}.
\<^item> @{term "biinv[A, B] f"} expresses that the function @{term f} of type @{term "A \<rightarrow> B"} is bi-invertible.
\<close>
lemma biinv_type:
assumes [intro]: "A: U i" "B: U i" "f: A \<rightarrow> B"
shows "biinv[A, B] f: U i"
unfolding biinv_def by derive
declare biinv_type [intro]
schematic_goal id_is_biinv:
assumes [intro]: "A: U i"
shows "?prf: biinv[A, A] (id A)"
unfolding biinv_def proof (rule Sum_routine, compute)
show "<id A, \<lambda>x: A. refl x>: \<Sum>(g: A \<rightarrow> A). (g o[A] id A) ~[x: A. A] (id A)" by derive
show "<id A, \<lambda>x: A. refl x>: \<Sum>(g: A \<rightarrow> A). (id A o[A] g) ~[x: A. A] (id A)" by derive
qed routine
definition equivalence :: "[t, t] \<Rightarrow> t" (infix "\<simeq>" 100)
where "A \<simeq> B \<equiv> \<Sum>f: A \<rightarrow> B. biinv[A, B] f"
schematic_goal equivalence_symmetric:
assumes [intro]: "A: U i"
shows "?prf: A \<simeq> A"
unfolding equivalence_def proof (rule Sum_routine)
show "\<And>f. f : A \<rightarrow> A \<Longrightarrow> biinv[A, A] f : U i" unfolding biinv_def by derive
show "id A: A \<rightarrow> A" by routine
qed (routine add: id_is_biinv)
section \<open>Transport, homotopy, and bi-invertibility\<close>
schematic_goal transport_invl_hom:
assumes [intro]:
"P: A \<leadsto> U j" "A: U i"
"x: A" "y: A" "p: x =[A] y"
shows "?prf:
(transport[A, P, y, x]`(inv[A, x, y]`p)) o[P x] (transport[A, P, x, y]`p) ~[w: P x. P x] id P x"
by (rule happly_type[OF transport_invl], derive)
schematic_goal transport_invr_hom:
assumes [intro]:
"A: U i" "P: A \<leadsto> U j"
"y: A" "x: A" "p: x =[A] y"
shows "?prf:
(transport[A, P, x, y]`p) o[P y] (transport[A, P, y, x]`(inv[A, x, y]`p)) ~[w: P y. P y] id P y"
by (rule happly_type[OF transport_invr], derive)
declare
transport_invl_hom [intro]
transport_invr_hom [intro]
text \<open>
The following result states that the transport of an equality @{term p} is bi-invertible, with inverse given by the transport of the inverse @{text "~p"}.
It is used in the derivation of @{text idtoeqv} in the next section.
\<close>
schematic_goal transport_biinv:
assumes [intro]: "p: A =[U i] B" "A: U i" "B: U i"
shows "?prf: biinv[A, B] (transport[U i, Id, A, B]`p)"
unfolding biinv_def
apply (rule Sum_routine)
prefer 2
apply (rule Sum_routine)
prefer 3 apply (rule transport_invl_hom)
prefer 9
apply (rule Sum_routine)
prefer 3 apply (rule transport_invr_hom)
\<comment> \<open>The remaining subgoals are now handled easily\<close>
by derive
section \<open>Univalence\<close>
schematic_goal type_eq_imp_equiv:
assumes [intro]: "A: U i" "B: U i"
shows "?prf: A =[U i] B \<rightarrow> A \<simeq> B"
unfolding equivalence_def
apply (rule Prod_routine, rule Sum_routine)
prefer 3 apply (derive lems: transport_biinv)
done
(* Copy-paste the derived proof term as the definition of idtoeqv for now;
should really have some automatic export of proof terms, though.
*)
definition idtoeqv :: "[ord, t, t] \<Rightarrow> t" ("(2idtoeqv[_, _, _])") where "
idtoeqv[i, A, B] \<equiv>
\<lambda>p: A =[U i] B.
< transport[U i, Id, A, B]`p, <
< transport[U i, Id, B, A]`(inv[U i, A, B]`p),
happly[x: A. A]
((transport[U i, Id, B, A]`(inv[U i, A, B]`p)) o[A] transport[U i, Id, A, B]`p)
(id A)
(indEq
(\<lambda>A B p.
(transport[U i, Id, B, A]`(inv[U i, A, B]`p)) o[A] transport[U i, Id, A, B]`p
=[A \<rightarrow> A] id A)
(\<lambda>x. refl (id x)) A B p)
>,
< transport[U i, Id, B, A]`(inv[U i, A, B]`p),
happly[x: B. B]
((transport[U i, Id, A, B]`p) o[B] (transport[U i, Id, B, A]`(inv[U i, A, B]`p)))
(id B)
(indEq
(\<lambda>A B p.
transport[U i, Id, A, B]`p o[B] (transport[U i, Id, B, A]`(inv[U i, A, B]`p))
=[B \<rightarrow> B] id B)
(\<lambda>x. refl (id x)) A B p)
>
>
>
"
corollary idtoeqv_type:
assumes [intro]: "A: U i" "B: U i" "p: A =[U i] B"
shows "idtoeqv[i, A, B]: A =[U i] B \<rightarrow> A \<simeq> B"
unfolding idtoeqv_def by (routine add: type_eq_imp_equiv)
declare idtoeqv_type [intro]
(* I'll formalize a more explicit constructor for the inverse equivalence of idtoeqv later;
the following is a placeholder for now.
*)
axiomatization ua :: "[ord, t, t] \<Rightarrow> t" where univalence: "ua i A B: biinv[A, B] idtoeqv[i, A, B]"
end
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