lean2/tests/lean/run/group5.lean
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) 2014 Jeremy Avigad. All rights reserved.
-- Released under Apache 2.0 license as described in the file LICENSE.
-- Authors: Jeremy Avigad, Leonardo de Moura
-- algebra.group
-- =============
-- Various structures with 1, *, inv, including groups.
import logic.eq
import data.unit data.sigma data.prod
import algebra.binary
open eq
namespace algebra
structure has_mul [class] (A : Type) :=
(mul : A → A → A)
structure has_one [class] (A : Type) :=
(one : A)
structure has_inv [class] (A : Type) :=
(inv : A → A)
infixl `*` := has_mul.mul
postfix `⁻¹` := has_inv.inv
notation 1 := !has_one.one
structure semigroup [class] (A : Type) extends has_mul A :=
(assoc : ∀ a b c, mul (mul a b) c = mul a (mul b c))
set_option pp.notation false
-- set_option pp.implicit true
-- set_option pp.coercions true
print instances has_mul
section
variables {A : Type} [s : semigroup A]
include s
variables a b : A
example : a * b = semigroup.mul a b :=
rfl
theorem mul_assoc (a b c : A) : a * b * c = a * (b * c) :=
semigroup.assoc a b c
end
structure comm_semigroup [class] (A : Type) extends semigroup A :=
(comm : ∀a b, mul a b = mul b a)
namespace comm_semigroup
variables {A : Type} [s : comm_semigroup A]
include s
variables a b c : A
theorem mul_comm : a * b = b * a := !comm_semigroup.comm
theorem mul_left_comm : a * (b * c) = b * (a * c) :=
binary.left_comm mul_comm mul_assoc a b c
end comm_semigroup
structure monoid [class] (A : Type) extends semigroup A, has_one A :=
(right_id : ∀a, mul a one = a) (left_id : ∀a, mul one a = a)
section
variables {A : Type} [s : monoid A]
variable a : A
include s
theorem mul_right_id : a * 1 = a := !monoid.right_id
theorem mul_left_id : 1 * a = a := !monoid.left_id
end
structure comm_monoid [class] (A : Type) extends monoid A, comm_semigroup A
structure Semigroup :=
(carrier : Type) (struct : semigroup carrier)
attribute Semigroup.carrier [coercion]
attribute Semigroup.struct [instance]
structure CommSemigroup :=
(carrier : Type) (struct : comm_semigroup carrier)
attribute CommSemigroup.carrier [coercion]
attribute CommSemigroup.struct [instance]
structure Monoid :=
(carrier : Type) (struct : monoid carrier)
attribute Monoid.carrier [coercion]
attribute Monoid.struct [instance]
structure CommMonoid :=
(carrier : Type) (struct : comm_monoid carrier)
attribute CommMonoid.carrier [coercion]
attribute CommMonoid.struct [instance]
end algebra
open algebra
section examples
theorem test1 {S : Semigroup} (a b c d : S) : a * (b * c) * d = a * b * (c * d) :=
calc
a * (b * c) * d = a * b * c * d : {symm !mul_assoc}
... = a * b * (c * d) : !mul_assoc
theorem test2 {M : CommSemigroup} (a b : M) : a * b = a * b := rfl
theorem test3 {M : Monoid} (a b c d : M) : a * (b * c) * d = a * b * (c * d) :=
calc
a * (b * c) * d = a * b * c * d : {symm !mul_assoc}
... = a * b * (c * d) : !mul_assoc
-- for test4b to work, we need instances at the level of the bundled structures as well
definition Monoid_Semigroup [coercion] [reducible] (M : Monoid) : Semigroup :=
Semigroup.mk (Monoid.carrier M) _
theorem test4 {M : Monoid} (a b c d : M) : a * (b * c) * d = a * b * (c * d) :=
test1 a b c d
theorem test5 {M : Monoid} (a b c : M) : a * 1 * b * c = a * (b * c) :=
calc
a * 1 * b * c = a * b * c : {!mul_right_id}
... = a * (b * c) : !mul_assoc
theorem test5a {M : Monoid} (a b c : M) : a * 1 * b * c = a * (b * c) :=
calc
a * 1 * b * c = a * b * c : {!mul_right_id}
... = a * (b * c) : !mul_assoc
theorem test5b {A : Type} {M : monoid A} (a b c : A) : a * 1 * b * c = a * (b * c) :=
calc
a * 1 * b * c = a * b * c : {!mul_right_id}
... = a * (b * c) : !mul_assoc
end examples