9bedbbb739
They are classes, and mixing coercion with type class resolution is a recipe for disaster (aka counterintuitive behavior).
142 lines
3.7 KiB
Text
142 lines
3.7 KiB
Text
-- Copyright (c) 2014 Jeremy Avigad. All rights reserved.
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-- Released under Apache 2.0 license as described in the file LICENSE.
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-- Authors: Jeremy Avigad, Leonardo de Moura
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-- algebra.group
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-- =============
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-- Various structures with 1, *, inv, including groups.
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import logic.eq
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import data.unit data.sigma data.prod
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import algebra.binary
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open eq
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namespace algebra
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structure has_mul [class] (A : Type) :=
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(mul : A → A → A)
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structure has_one [class] (A : Type) :=
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(one : A)
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structure has_inv [class] (A : Type) :=
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(inv : A → A)
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infixl `*` := has_mul.mul
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postfix `⁻¹` := has_inv.inv
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notation 1 := !has_one.one
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structure semigroup [class] (A : Type) extends has_mul A :=
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(assoc : ∀ a b c, mul (mul a b) c = mul a (mul b c))
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set_option pp.notation false
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-- set_option pp.implicit true
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-- set_option pp.coercions true
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print instances has_mul
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section
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variables {A : Type} [s : semigroup A]
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include s
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variables a b : A
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example : a * b = semigroup.mul a b :=
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rfl
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theorem mul_assoc (a b c : A) : a * b * c = a * (b * c) :=
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semigroup.assoc a b c
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end
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structure comm_semigroup [class] (A : Type) extends semigroup A :=
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(comm : ∀a b, mul a b = mul b a)
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namespace comm_semigroup
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variables {A : Type} [s : comm_semigroup A]
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include s
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variables a b c : A
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theorem mul_comm : a * b = b * a := !comm_semigroup.comm
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theorem mul_left_comm : a * (b * c) = b * (a * c) :=
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binary.left_comm mul_comm mul_assoc a b c
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end comm_semigroup
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structure monoid [class] (A : Type) extends semigroup A, has_one A :=
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(right_id : ∀a, mul a one = a) (left_id : ∀a, mul one a = a)
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section
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variables {A : Type} [s : monoid A]
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variable a : A
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include s
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theorem mul_right_id : a * 1 = a := !monoid.right_id
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theorem mul_left_id : 1 * a = a := !monoid.left_id
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end
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structure comm_monoid [class] (A : Type) extends monoid A, comm_semigroup A
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structure Semigroup :=
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(carrier : Type) (struct : semigroup carrier)
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attribute Semigroup.carrier [coercion]
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attribute Semigroup.struct [instance]
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structure CommSemigroup :=
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(carrier : Type) (struct : comm_semigroup carrier)
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attribute CommSemigroup.carrier [coercion]
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attribute CommSemigroup.struct [instance]
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structure Monoid :=
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(carrier : Type) (struct : monoid carrier)
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attribute Monoid.carrier [coercion]
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attribute Monoid.struct [instance]
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structure CommMonoid :=
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(carrier : Type) (struct : comm_monoid carrier)
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attribute CommMonoid.carrier [coercion]
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attribute CommMonoid.struct [instance]
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end algebra
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open algebra
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section examples
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theorem test1 {S : Semigroup} (a b c d : S) : a * (b * c) * d = a * b * (c * d) :=
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calc
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a * (b * c) * d = a * b * c * d : {symm !mul_assoc}
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... = a * b * (c * d) : !mul_assoc
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theorem test2 {M : CommSemigroup} (a b : M) : a * b = a * b := rfl
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theorem test3 {M : Monoid} (a b c d : M) : a * (b * c) * d = a * b * (c * d) :=
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calc
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a * (b * c) * d = a * b * c * d : {symm !mul_assoc}
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... = a * b * (c * d) : !mul_assoc
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-- for test4b to work, we need instances at the level of the bundled structures as well
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definition Monoid_Semigroup [coercion] [reducible] (M : Monoid) : Semigroup :=
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Semigroup.mk (Monoid.carrier M) _
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theorem test4 {M : Monoid} (a b c d : M) : a * (b * c) * d = a * b * (c * d) :=
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test1 a b c d
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theorem test5 {M : Monoid} (a b c : M) : a * 1 * b * c = a * (b * c) :=
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calc
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a * 1 * b * c = a * b * c : {!mul_right_id}
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... = a * (b * c) : !mul_assoc
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theorem test5a {M : Monoid} (a b c : M) : a * 1 * b * c = a * (b * c) :=
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calc
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a * 1 * b * c = a * b * c : {!mul_right_id}
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... = a * (b * c) : !mul_assoc
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theorem test5b {A : Type} {M : monoid A} (a b c : A) : a * 1 * b * c = a * (b * c) :=
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calc
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a * 1 * b * c = a * b * c : {!mul_right_id}
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... = a * (b * c) : !mul_assoc
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end examples
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