------------------------------------------------------------------------
-- The Agda standard library
--
-- Some algebraic structures (not packed up with sets, operations,
-- etc.)
------------------------------------------------------------------------

{-# OPTIONS --without-K --safe #-}

open import Relation.Binary using (Rel; Setoid; IsEquivalence)

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

-- All the structures are parameterised by the equivalence relation _≈_.
-- The file is divided into sections depending on the arities of the
-- components of the algebraic structure.

open import Algebra.FunctionProperties _≈_
import Algebra.FunctionProperties.Consequences as Consequences
open import Data.Product using (_,_; proj₁; proj₂)
open import Level using (_⊔_)

------------------------------------------------------------------------
-- Structures with 1 binary operation
------------------------------------------------------------------------

record IsMagma ( : Op₂ A) : Set (a  ) where
  field
    isEquivalence : IsEquivalence _≈_
    ∙-cong        : Congruent₂ 

  open IsEquivalence isEquivalence public

  setoid : Setoid a 
  setoid = record { isEquivalence = isEquivalence }

  ∙-congˡ : LeftCongruent 
  ∙-congˡ y≈z = ∙-cong refl y≈z

  ∙-congʳ : RightCongruent 
  ∙-congʳ y≈z = ∙-cong y≈z refl


record IsSemigroup ( : Op₂ A) : Set (a  ) where
  field
    isMagma : IsMagma 
    assoc   : Associative 

  open IsMagma isMagma public


record IsBand ( : Op₂ A) : Set (a  ) where
  field
    isSemigroup : IsSemigroup 
    idem        : Idempotent 

  open IsSemigroup isSemigroup public


record IsSemilattice ( : Op₂ A) : Set (a  ) where
  field
    isBand : IsBand 
    comm   : Commutative 

  open IsBand isBand public
    renaming (∙-cong to ∧-cong; ∙-congˡ to ∧-congˡ; ∙-congʳ to ∧-congʳ)


------------------------------------------------------------------------
-- Structures with 1 binary operation & 1 element
------------------------------------------------------------------------

record IsMonoid ( : Op₂ A) (ε : A) : Set (a  ) where
  field
    isSemigroup : IsSemigroup 
    identity    : Identity ε 

  open IsSemigroup isSemigroup public

  identityˡ : LeftIdentity ε 
  identityˡ = proj₁ identity

  identityʳ : RightIdentity ε 
  identityʳ = proj₂ identity


record IsCommutativeMonoid ( : Op₂ A) (ε : A) : Set (a  ) where
  field
    isSemigroup : IsSemigroup 
    identityˡ   : LeftIdentity ε 
    comm        : Commutative 

  open IsSemigroup isSemigroup public

  identityʳ : RightIdentity ε 
  identityʳ = Consequences.comm+idˡ⇒idʳ setoid comm identityˡ

  identity : Identity ε 
  identity = (identityˡ , identityʳ)

  isMonoid : IsMonoid  ε
  isMonoid = record
    { isSemigroup = isSemigroup
    ; identity    = identity
    }


record IsIdempotentCommutativeMonoid ( : Op₂ A)
                                     (ε : A) : Set (a  ) where
  field
    isCommutativeMonoid : IsCommutativeMonoid  ε
    idem                : Idempotent 

  open IsCommutativeMonoid isCommutativeMonoid public


------------------------------------------------------------------------
-- Structures with 1 binary operation, 1 unary operation & 1 element
------------------------------------------------------------------------

record IsGroup (_∙_ : Op₂ A) (ε : A) (_⁻¹ : Op₁ A) : Set (a  ) where
  field
    isMonoid  : IsMonoid _∙_ ε
    inverse   : Inverse ε _⁻¹ _∙_
    ⁻¹-cong   : Congruent₁ _⁻¹

  open IsMonoid isMonoid public

  infixl 7 _-_
  _-_ : Op₂ A
  x - y = x  (y ⁻¹)

  inverseˡ : LeftInverse ε _⁻¹ _∙_
  inverseˡ = proj₁ inverse

  inverseʳ : RightInverse ε _⁻¹ _∙_
  inverseʳ = proj₂ inverse

  uniqueˡ-⁻¹ :  x y  (x  y)  ε  x  (y ⁻¹)
  uniqueˡ-⁻¹ = Consequences.assoc+id+invʳ⇒invˡ-unique
                setoid ∙-cong assoc identity inverseʳ

  uniqueʳ-⁻¹ :  x y  (x  y)  ε  y  (x ⁻¹)
  uniqueʳ-⁻¹ = Consequences.assoc+id+invˡ⇒invʳ-unique
                setoid ∙-cong assoc identity inverseˡ


record IsAbelianGroup ( : Op₂ A)
                      (ε : A) (⁻¹ : Op₁ A) : Set (a  ) where
  field
    isGroup : IsGroup  ε ⁻¹
    comm    : Commutative 

  open IsGroup isGroup public

  isCommutativeMonoid : IsCommutativeMonoid  ε
  isCommutativeMonoid = record
    { isSemigroup = isSemigroup
    ; identityˡ   = identityˡ
    ; comm        = comm
    }


------------------------------------------------------------------------
-- Structures with 2 binary operations
------------------------------------------------------------------------

-- Note that `IsLattice` is not defined in terms of `IsSemilattice`
-- because the idempotence laws of ∨ and ∧ can be derived from the
-- absorption laws, which makes the corresponding "idem" fields
-- redundant.  The derived idempotence laws are stated and proved in
-- `Algebra.Properties.Lattice` along with the fact that every lattice
-- consists of two semilattices.

record IsLattice (  : Op₂ A) : Set (a  ) where
  field
    isEquivalence : IsEquivalence _≈_
    ∨-comm        : Commutative 
    ∨-assoc       : Associative 
    ∨-cong        : Congruent₂ 
    ∧-comm        : Commutative 
    ∧-assoc       : Associative 
    ∧-cong        : Congruent₂ 
    absorptive    : Absorptive  

  open IsEquivalence isEquivalence public

  ∨-absorbs-∧ :  Absorbs 
  ∨-absorbs-∧ = proj₁ absorptive

  ∧-absorbs-∨ :  Absorbs 
  ∧-absorbs-∨ = proj₂ absorptive

  ∧-congˡ : LeftCongruent 
  ∧-congˡ y≈z = ∧-cong refl y≈z

  ∧-congʳ : RightCongruent 
  ∧-congʳ y≈z = ∧-cong y≈z refl

  ∨-congˡ : LeftCongruent 
  ∨-congˡ y≈z = ∨-cong refl y≈z

  ∨-congʳ : RightCongruent 
  ∨-congʳ y≈z = ∨-cong y≈z refl


record IsDistributiveLattice (  : Op₂ A) : Set (a  ) where
  field
    isLattice    : IsLattice  
    ∨-∧-distribʳ :  DistributesOverʳ 

  open IsLattice isLattice public


------------------------------------------------------------------------
-- Structures with 2 binary operations & 1 element
------------------------------------------------------------------------

record IsNearSemiring (+ * : Op₂ A) (0# : A) : Set (a  ) where
  field
    +-isMonoid    : IsMonoid + 0#
    *-isSemigroup : IsSemigroup *
    distribʳ      : * DistributesOverʳ +
    zeroˡ         : LeftZero 0# *

  open IsMonoid +-isMonoid public
    renaming
    ( assoc       to +-assoc
    ; ∙-cong      to +-cong
    ; ∙-congˡ     to +-congˡ
    ; ∙-congʳ     to +-congʳ
    ; identity    to +-identity
    ; identityˡ   to +-identityˡ
    ; identityʳ   to +-identityʳ
    ; isMagma     to +-isMagma
    ; isSemigroup to +-isSemigroup
    )

  open IsSemigroup *-isSemigroup public
    using ()
    renaming
    ( assoc    to *-assoc
    ; ∙-cong   to *-cong
    ; ∙-congˡ  to *-congˡ
    ; ∙-congʳ  to *-congʳ
    ; isMagma  to *-isMagma
    )


record IsSemiringWithoutOne (+ * : Op₂ A) (0# : A) : Set (a  ) where
  field
    +-isCommutativeMonoid : IsCommutativeMonoid + 0#
    *-isSemigroup         : IsSemigroup *
    distrib               : * DistributesOver +
    zero                  : Zero 0# *

  open IsCommutativeMonoid +-isCommutativeMonoid public
    using ()
    renaming
    ( isMonoid    to +-isMonoid
    ; comm        to +-comm
    )

  zeroˡ : LeftZero 0# *
  zeroˡ = proj₁ zero

  zeroʳ : RightZero 0# *
  zeroʳ = proj₂ zero

  isNearSemiring : IsNearSemiring + * 0#
  isNearSemiring = record
    { +-isMonoid    = +-isMonoid
    ; *-isSemigroup = *-isSemigroup
    ; distribʳ      = proj₂ distrib
    ; zeroˡ         = zeroˡ
    }

  open IsNearSemiring isNearSemiring public
    hiding (+-isMonoid; zeroˡ)


record IsCommutativeSemiringWithoutOne
         (+ * : Op₂ A) (0# : A) : Set (a  ) where
  field
    isSemiringWithoutOne : IsSemiringWithoutOne + * 0#
    *-comm               : Commutative *

  open IsSemiringWithoutOne isSemiringWithoutOne public


------------------------------------------------------------------------
-- Structures with 2 binary operations & 2 elements
------------------------------------------------------------------------

record IsSemiringWithoutAnnihilatingZero (+ * : Op₂ A)
                                         (0# 1# : A) : Set (a  ) where
  field
    -- Note that these structures do have an additive unit, but this
    -- unit does not necessarily annihilate multiplication.
    +-isCommutativeMonoid : IsCommutativeMonoid + 0#
    *-isMonoid            : IsMonoid * 1#
    distrib               : * DistributesOver +

  distribˡ : * DistributesOverˡ +
  distribˡ = proj₁ distrib

  distribʳ : * DistributesOverʳ +
  distribʳ = proj₂ distrib

  open IsCommutativeMonoid +-isCommutativeMonoid public
    renaming
    ( assoc       to +-assoc
    ; ∙-cong      to +-cong
    ; ∙-congˡ     to +-congˡ
    ; ∙-congʳ     to +-congʳ
    ; identity    to +-identity
    ; identityˡ   to +-identityˡ
    ; identityʳ   to +-identityʳ
    ; comm        to +-comm
    ; isMagma     to +-isMagma
    ; isSemigroup to +-isSemigroup
    ; isMonoid    to +-isMonoid
    )

  open IsMonoid *-isMonoid public
    using ()
    renaming
    ( assoc       to *-assoc
    ; ∙-cong      to *-cong
    ; ∙-congˡ     to *-congˡ
    ; ∙-congʳ     to *-congʳ
    ; identity    to *-identity
    ; identityˡ   to *-identityˡ
    ; identityʳ   to *-identityʳ
    ; isMagma     to *-isMagma
    ; isSemigroup to *-isSemigroup
    )


record IsSemiring (+ * : Op₂ A) (0# 1# : A) : Set (a  ) where
  field
    isSemiringWithoutAnnihilatingZero :
      IsSemiringWithoutAnnihilatingZero + * 0# 1#
    zero : Zero 0# *

  open IsSemiringWithoutAnnihilatingZero
         isSemiringWithoutAnnihilatingZero public

  isSemiringWithoutOne : IsSemiringWithoutOne + * 0#
  isSemiringWithoutOne = record
    { +-isCommutativeMonoid = +-isCommutativeMonoid
    ; *-isSemigroup         = *-isSemigroup
    ; distrib               = distrib
    ; zero                  = zero
    }

  open IsSemiringWithoutOne isSemiringWithoutOne public
    using
    ( isNearSemiring
    ; zeroˡ
    ; zeroʳ
    )


record IsCommutativeSemiring (+ * : Op₂ A) (0# 1# : A) : Set (a  ) where
  field
    +-isCommutativeMonoid : IsCommutativeMonoid + 0#
    *-isCommutativeMonoid : IsCommutativeMonoid * 1#
    distribʳ              : * DistributesOverʳ +
    zeroˡ                 : LeftZero 0# *

  private
    module +-CM = IsCommutativeMonoid +-isCommutativeMonoid
    open module *-CM = IsCommutativeMonoid *-isCommutativeMonoid public
           using () renaming (comm to *-comm)

  distribˡ : * DistributesOverˡ +
  distribˡ = Consequences.comm+distrʳ⇒distrˡ
              +-CM.setoid +-CM.∙-cong *-comm distribʳ

  distrib : * DistributesOver +
  distrib = (distribˡ , distribʳ)

  zeroʳ : RightZero 0# *
  zeroʳ = Consequences.comm+zeˡ⇒zeʳ +-CM.setoid *-comm zeroˡ

  zero : Zero 0# *
  zero = (zeroˡ , zeroʳ)

  isSemiring : IsSemiring + * 0# 1#
  isSemiring = record
    { isSemiringWithoutAnnihilatingZero = record
      { +-isCommutativeMonoid = +-isCommutativeMonoid
      ; *-isMonoid            = *-CM.isMonoid
      ; distrib               = distrib
      }
    ; zero                    = zero
    }

  open IsSemiring isSemiring public
    hiding
    ( distrib; distribʳ; distribˡ
    ; zero; zeroˡ; zeroʳ
    ; +-isCommutativeMonoid
    )

  isCommutativeSemiringWithoutOne :
    IsCommutativeSemiringWithoutOne + * 0#
  isCommutativeSemiringWithoutOne = record
    { isSemiringWithoutOne = isSemiringWithoutOne
    ; *-comm               = *-CM.comm
    }


------------------------------------------------------------------------
-- Structures with 2 binary operations, 1 unary operation & 2 elements
------------------------------------------------------------------------

record IsRing (+ * : Op₂ A) (-_ : Op₁ A) (0# 1# : A) : Set (a  ) where
  field
    +-isAbelianGroup : IsAbelianGroup + 0# -_
    *-isMonoid       : IsMonoid * 1#
    distrib          : * DistributesOver +

  open IsAbelianGroup +-isAbelianGroup public
    renaming
    ( assoc               to +-assoc
    ; ∙-cong              to +-cong
    ; ∙-congˡ             to +-congˡ
    ; ∙-congʳ             to +-congʳ
    ; identity            to +-identity
    ; identityˡ           to +-identityˡ
    ; identityʳ           to +-identityʳ
    ; inverse             to -‿inverse
    ; inverseˡ            to -‿inverseˡ
    ; inverseʳ            to -‿inverseʳ
    ; ⁻¹-cong             to -‿cong
    ; comm                to +-comm
    ; isMagma             to +-isMagma
    ; isSemigroup         to +-isSemigroup
    ; isMonoid            to +-isMonoid
    ; isCommutativeMonoid to +-isCommutativeMonoid
    ; isGroup             to +-isGroup
    )

  open IsMonoid *-isMonoid public
    using ()
    renaming
    ( assoc       to *-assoc
    ; ∙-cong      to *-cong
    ; ∙-congˡ     to *-congˡ
    ; ∙-congʳ     to *-congʳ
    ; identity    to *-identity
    ; identityˡ   to *-identityˡ
    ; identityʳ   to *-identityʳ
    ; isMagma     to *-isMagma
    ; isSemigroup to *-isSemigroup
    )

  zeroˡ : LeftZero 0# *
  zeroˡ = Consequences.assoc+distribʳ+idʳ+invʳ⇒zeˡ setoid
           +-cong *-cong +-assoc (proj₂ distrib) +-identityʳ -‿inverseʳ

  zeroʳ : RightZero 0# *
  zeroʳ = Consequences.assoc+distribˡ+idʳ+invʳ⇒zeʳ setoid
           +-cong *-cong +-assoc (proj₁ distrib) +-identityʳ -‿inverseʳ

  zero : Zero 0# *
  zero = (zeroˡ , zeroʳ)

  isSemiringWithoutAnnihilatingZero
    : IsSemiringWithoutAnnihilatingZero + * 0# 1#
  isSemiringWithoutAnnihilatingZero = record
    { +-isCommutativeMonoid = +-isCommutativeMonoid
    ; *-isMonoid            = *-isMonoid
    ; distrib               = distrib
    }

  isSemiring : IsSemiring + * 0# 1#
  isSemiring = record
    { isSemiringWithoutAnnihilatingZero =
        isSemiringWithoutAnnihilatingZero
    ; zero = zero
    }

  open IsSemiring isSemiring public
    using (distribˡ; distribʳ; isNearSemiring; isSemiringWithoutOne)


record IsCommutativeRing
         (+ * : Op₂ A) (- : Op₁ A) (0# 1# : A) : Set (a  ) where
  field
    isRing : IsRing + * - 0# 1#
    *-comm : Commutative *

  open IsRing isRing public

  *-isCommutativeMonoid : IsCommutativeMonoid * 1#
  *-isCommutativeMonoid =  record
    { isSemigroup = *-isSemigroup
    ; identityˡ   = *-identityˡ
    ; comm        = *-comm
    }

  isCommutativeSemiring : IsCommutativeSemiring + * 0# 1#
  isCommutativeSemiring = record
    { +-isCommutativeMonoid = +-isCommutativeMonoid
    ; *-isCommutativeMonoid = *-isCommutativeMonoid
    ; distribʳ              = distribʳ
    ; zeroˡ                 = zeroˡ
    }

  open IsCommutativeSemiring isCommutativeSemiring public
    using ( isCommutativeSemiringWithoutOne )


record IsBooleanAlgebra
         (  : Op₂ A) (¬ : Op₁ A) (  : A) : Set (a  ) where
  field
    isDistributiveLattice : IsDistributiveLattice  
    ∨-complementʳ         : RightInverse  ¬ 
    ∧-complementʳ         : RightInverse  ¬ 
    ¬-cong                : Congruent₁ ¬

  open IsDistributiveLattice isDistributiveLattice public