lean2/hott/algebra/hott.hlean
Leonardo de Moura 9bedbbb739 refactor(library,hott): remove coercions between algebraic structures
They are classes, and mixing coercion with type class resolution is a
recipe for disaster (aka counterintuitive behavior).
2015-11-11 11:57:44 -08:00

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/-
Copyright (c) 2015 Floris van Doorn. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Floris van Doorn
Theorems about algebra specific to HoTT
-/
import .group arity types.pi hprop_trunc
open equiv eq equiv.ops is_trunc
namespace algebra
open Group has_mul has_inv
-- we prove under which conditions two groups are equal
-- group and has_mul are classes. So, lean does not automatically generate
-- coercions between them anymore.
-- So, an application such as (@mul A G g h) in the following definition
-- is type incorrect if the coercion is not added (explicitly or implicitly).
local attribute group.to.has_mul [coercion]
local attribute group.to_has_inv [coercion]
universe variable l
variables {A B : Type.{l}}
definition group_eq {G H : group A} (same_mul' : Π(g h : A), @mul A G g h = @mul A H g h)
: G = H :=
begin
have foo : Π(g : A), @inv A G g = (@inv A G g * g) * @inv A H g,
from λg, !mul_inv_cancel_right⁻¹,
cases G with Gm Gs Gh1 G1 Gh2 Gh3 Gi Gh4,
cases H with Hm Hs Hh1 H1 Hh2 Hh3 Hi Hh4,
rewrite [↑[semigroup.to_has_mul,group.to_has_inv] at (same_mul,foo)] ,
have same_mul : Gm = Hm, from eq_of_homotopy2 same_mul',
cases same_mul,
have same_one : G1 = H1, from calc
G1 = Hm G1 H1 : Hh3
... = H1 : Gh2,
have same_inv : Gi = Hi, from eq_of_homotopy (take g, calc
Gi g = Hm (Hm (Gi g) g) (Hi g) : foo
... = Hm G1 (Hi g) : by rewrite Gh4
... = Hi g : Gh2),
cases same_one, cases same_inv,
have ps : Gs = Hs, from !is_hprop.elim,
have ph1 : Gh1 = Hh1, from !is_hprop.elim,
have ph2 : Gh2 = Hh2, from !is_hprop.elim,
have ph3 : Gh3 = Hh3, from !is_hprop.elim,
have ph4 : Gh4 = Hh4, from !is_hprop.elim,
cases ps, cases ph1, cases ph2, cases ph3, cases ph4, reflexivity
end
definition group_pathover {G : group A} {H : group B} {f : A ≃ B} : (Π(g h : A), f (g * h) = f g * f h) → G =[ua f] H :=
begin
revert H,
eapply (rec_on_ua_idp' f),
intros H resp_mul,
esimp [equiv.refl] at resp_mul, esimp,
apply pathover_idp_of_eq, apply group_eq,
exact resp_mul
end
definition Group_eq {G H : Group} (f : carrier G ≃ carrier H)
(resp_mul : Π(g h : G), f (g * h) = f g * f h) : G = H :=
begin
cases G with Gc G, cases H with Hc H,
apply (apo011 mk (ua f)),
apply group_pathover, exact resp_mul
end
end algebra