------------------------------------------------------------------------
-- The Agda standard library
--
-- Properties of functions, such as associativity and commutativity
------------------------------------------------------------------------

-- These properties can (for instance) be used to define algebraic
-- structures.

open import Level
open import Relation.Binary
open import Data.Sum

-- The properties are specified using the following relation as
-- "equality".

module Algebra.FunctionProperties
         {a } {A : Set a} (_≈_ : Rel A ) where

open import Data.Product

------------------------------------------------------------------------
-- Unary and binary operations

open import Algebra.FunctionProperties.Core public

------------------------------------------------------------------------
-- Properties of operations

Associative : Op₂ A  Set _
Associative _∙_ =  x y z  ((x  y)  z)  (x  (y  z))

Commutative : Op₂ A  Set _
Commutative _∙_ =  x y  (x  y)  (y  x)

LeftIdentity : A  Op₂ A  Set _
LeftIdentity e _∙_ =  x  (e  x)  x

RightIdentity : A  Op₂ A  Set _
RightIdentity e _∙_ =  x  (x  e)  x

Identity : A  Op₂ A  Set _
Identity e  = LeftIdentity e  × RightIdentity e 

LeftZero : A  Op₂ A  Set _
LeftZero z _∙_ =  x  (z  x)  z

RightZero : A  Op₂ A  Set _
RightZero z _∙_ =  x  (x  z)  z

Zero : A  Op₂ A  Set _
Zero z  = LeftZero z  × RightZero z 

LeftInverse : A  Op₁ A  Op₂ A  Set _
LeftInverse e _⁻¹ _∙_ =  x  ((x ⁻¹)  x)  e

RightInverse : A  Op₁ A  Op₂ A  Set _
RightInverse e _⁻¹ _∙_ =  x  (x  (x ⁻¹))  e

Inverse : A  Op₁ A  Op₂ A  Set _
Inverse e ⁻¹  = LeftInverse e ⁻¹  × RightInverse e ⁻¹ 

_DistributesOverˡ_ : Op₂ A  Op₂ A  Set _
_*_ DistributesOverˡ _+_ =
   x y z  (x * (y + z))  ((x * y) + (x * z))

_DistributesOverʳ_ : Op₂ A  Op₂ A  Set _
_*_ DistributesOverʳ _+_ =
   x y z  ((y + z) * x)  ((y * x) + (z * x))

_DistributesOver_ : Op₂ A  Op₂ A  Set _
* DistributesOver + = (* DistributesOverˡ +) × (* DistributesOverʳ +)

_IdempotentOn_ : Op₂ A  A  Set _
_∙_ IdempotentOn x = (x  x)  x

Idempotent : Op₂ A  Set _
Idempotent  =  x   IdempotentOn x

IdempotentFun : Op₁ A  Set _
IdempotentFun f =  x  f (f x)  f x

Selective : Op₂ A  Set _
Selective _∙_ =  x y  (x  y)  x  (x  y)  y

_Absorbs_ : Op₂ A  Op₂ A  Set _
_∙_ Absorbs _∘_ =  x y  (x  (x  y))  x

Absorptive : Op₂ A  Op₂ A  Set _
Absorptive   = ( Absorbs ) × ( Absorbs )

Involutive : Op₁ A  Set _
Involutive f =  x  f (f x)  x

Congruent₁ : Op₁ A  Set _
Congruent₁ f = f Preserves _≈_  _≈_

Congruent₂ : Op₂ A  Set _
Congruent₂  =  Preserves₂ _≈_  _≈_  _≈_