bsc-leon-vatthauer/ElgotAlgebra.agda

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open import Level renaming (suc to -suc)
open import Function using (_$_) renaming (id to idf; _∘_ to _∘ᶠ_)
open import Data.Product using (_,_) renaming (_×_ to _∧_)
open import Categories.Category.Cocartesian.Bundle using (CocartesianCategory)
open import Categories.Functor renaming (id to idF)
open import Categories.Functor.Algebra
open import Categories.Object.Product
open import Categories.Object.Coproduct
open import Categories.Category
private
variable
o e : Level
module _ {C𝒞 : CocartesianCategory o e} where
open CocartesianCategory C𝒞 renaming (U to 𝒞; id to idC)
--*
-- F-guarded Elgot Algebra
--*
module _ {F : Endofunctor 𝒞} (FA : F-Algebra F) where
record Guarded-Elgot-Algebra : Set (o e) where
open Functor F public
open F-Algebra FA public
-- iteration operator
field
_# : {X} (X A + F₀ X) (X A)
-- _# properties
field
#-Fixpoint : {X} {f : X A + F₀ X }
f # [ idC , α F₁ (f #) ] f
#-Uniformity : {X Y} {f : X A + F₀ X} {g : Y A + F₀ Y} {h : X Y}
(idC +₁ F₁ h) f g h
f # g # h
#-Compositionality : {X Y} {f : X A + F₀ X} {h : Y X + F₀ Y}
(((f #) +₁ idC) h)# ([ (idC +₁ (F₁ i₁)) f , i₂ (F₁ i₂) ] [ i₁ , h ])# i₂
#-resp-≈ : {X} {f g : X A + F₀ X} f g (f #) (g #)
--*
-- (unguarded) Elgot-Algebra
--*
module _ where
record Elgot-Algebra : Set (o e) where
-- Object
field
A : Obj
-- iteration operator
field
_# : {X} (X A + X) (X A)
-- _# properties
field
#-Fixpoint : {X} {f : X A + X }
f # [ idC , f # ] f
#-Uniformity : {X Y} {f : X A + X} {g : Y A + Y} {h : X Y}
(idC +₁ h) f g h
f # g # h
#-Folding : {X Y} {f : X A + X} {h : Y X + Y}
((f #) +₁ h)# [ (idC +₁ i₁) f , i₂ h ] #
#-resp-≈ : {X} {f g : X A + X} f g (f #) (g #)
open HomReasoning
open Equiv
-- Compositionality is derivable
#-Compositionality : {X Y} {f : X A + X} {h : Y X + Y}
(((f #) +₁ idC) h)# ([ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ])# i₂
#-Compositionality {X} {Y} {f} {h} = begin
(((f #) +₁ idC) h)# ≈⟨ #-Uniformity {f = ((f #) +₁ idC) h} {g = (f #) +₁ h} {h = h} (
begin
((idC +₁ h) ((f #) +₁ idC) h) ≈⟨ sym-assoc
(((idC +₁ h) ((f #) +₁ idC)) h) ≈⟨ ∘-resp-≈ˡ +₁∘+₁
((((idC (f #)) +₁ (h idC))) h) ≈⟨ ∘-resp-≈ˡ (+₁-cong₂ identityˡ identityʳ)
((((f #) +₁ h)) h) )
((f # +₁ h)# h) ≈⟨ sym inject₂
(([ idC (f #) , (f # +₁ h)# h ] i₂)) ≈⟨ ∘-resp-≈ˡ (sym $ []∘+₁)
(([ idC , ((f # +₁ h)#) ] (f # +₁ h)) i₂) ≈⟨ (sym $ ∘-resp-≈ˡ (#-Fixpoint {f = (f # +₁ h) }))
(f # +₁ h)# i₂ ≈⟨ ∘-resp-≈ˡ #-Folding
([ (idC +₁ i₁) f , i₂ h ] # i₂) ≈⟨ ∘-resp-≈ˡ #-Fixpoint
([ idC , [ (idC +₁ i₁) f , i₂ h ] # ] [ (idC +₁ i₁) f , i₂ h ]) i₂ ≈⟨ assoc
[ idC , [ (idC +₁ i₁) f , i₂ h ] # ] ([ (idC +₁ i₁) f , i₂ h ] i₂) ≈⟨ ∘-resp-≈ʳ inject₂
[ idC , [ (idC +₁ i₁) f , i₂ h ] # ] (i₂ h) ≈⟨ sym-assoc
(([ idC , [ (idC +₁ i₁) f , i₂ h ] # ] i₂) h) ≈⟨ ∘-resp-≈ˡ inject₂
([ (idC +₁ i₁) f , i₂ h ] # h) ≈⟨ ∘-resp-≈ʳ $ sym (inject₂ {f = i₁} {g = h})
[ (idC +₁ i₁) f , i₂ h ] # ([ i₁ , h ] i₂) ≈⟨ sym-assoc
(([ (idC +₁ i₁) f , i₂ h ] # [ i₁ , h ]) i₂) ≈⟨ sym (∘-resp-≈ˡ (#-Uniformity {f = [ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ]} {g = [ (idC +₁ i₁) f , i₂ h ]} {h = [ i₁ , h ]} (
begin
(idC +₁ [ i₁ , h ]) [ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ] ≈⟨ ∘-resp-≈ʳ ∘[]
(idC +₁ [ i₁ , h ]) [ [ (idC +₁ i₁) f , i₂ i₂ ] i₁ , [ (idC +₁ i₁) f , i₂ i₂ ] h ] ≈⟨ ∘-resp-≈ʳ ([]-congʳ inject₁)
((idC +₁ [ i₁ , h ]) [ (idC +₁ i₁) f , [ (idC +₁ i₁) f , i₂ i₂ ] h ]) ≈⟨ ∘[]
[ (idC +₁ [ i₁ , h ]) ((idC +₁ i₁) f) , (idC +₁ [ i₁ , h ]) ([ (idC +₁ i₁) f , i₂ i₂ ] h) ] ≈⟨ []-cong₂ sym-assoc sym-assoc
[ ((idC +₁ [ i₁ , h ]) (idC +₁ i₁)) f , ((idC +₁ [ i₁ , h ]) [ (idC +₁ i₁) f , i₂ i₂ ]) h ] ≈⟨ []-cong₂ (∘-resp-≈ˡ +₁∘+₁) (∘-resp-≈ˡ ∘[])
[ ((idC idC) +₁ ([ i₁ , h ] i₁)) f , ([ (idC +₁ [ i₁ , h ]) ((idC +₁ i₁) f) , (idC +₁ [ i₁ , h ]) (i₂ i₂) ]) h ] ≈⟨ []-cong₂ (∘-resp-≈ˡ (+₁-cong₂ identity² (inject₁))) (∘-resp-≈ˡ ([]-cong₂ sym-assoc sym-assoc))
[ (idC +₁ i₁) f , ([ ((idC +₁ [ i₁ , h ]) (idC +₁ i₁)) f , ((idC +₁ [ i₁ , h ]) i₂) i₂ ]) h ] ≈⟨ []-congˡ (∘-resp-≈ˡ ([]-cong₂ (∘-resp-≈ˡ +₁∘+₁) (∘-resp-≈ˡ inject₂)))
[ (idC +₁ i₁) f , ([ ((idC idC) +₁ ([ i₁ , h ] i₁)) f , (i₂ [ i₁ , h ]) i₂ ]) h ] ≈⟨ []-congˡ (∘-resp-≈ˡ ([]-cong₂ (∘-resp-≈ˡ (+₁-cong₂ identity² inject₁)) assoc))
[ (idC +₁ i₁) f , ([ (idC +₁ i₁) f , i₂ ([ i₁ , h ] i₂) ]) h ] ≈⟨ []-congˡ (∘-resp-≈ˡ ([]-congˡ (∘-resp-≈ʳ inject₂)))
[ (idC +₁ i₁) f , [ (idC +₁ i₁) f , i₂ h ] h ] ≈⟨ []-congʳ (sym (inject₁))
[ [ (idC +₁ i₁) f , i₂ h ] i₁ , [ (idC +₁ i₁) f , i₂ h ] h ] ≈⟨ sym ∘[]
[ (idC +₁ i₁) f , i₂ h ] [ i₁ , h ] )))
([ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ])# i₂
-- divergence constant
⊥ₑ : A
⊥ₑ = i₂ #
--*
-- let's define the category of elgot-algebras
--*
-- iteration preversing morphism between two elgot-algebras
module _ (E E₂ : Elgot-Algebra) where
open Elgot-Algebra E₁ renaming (_# to _#₁)
open Elgot-Algebra E₂ renaming (_# to _#₂; A to B)
record Elgot-Algebra-Morphism : Set (o e) where
field
h : A B
preserves : {X} {f : X A + X} h (f #₁) ((h +₁ idC) f)#₂
-- the category of elgot algebras for a given category
Elgot-Algebras : Category (o e) (o e) e
Elgot-Algebras = record
{ Obj = Elgot-Algebra
; _⇒_ = Elgot-Algebra-Morphism
; _≈_ = λ f g Elgot-Algebra-Morphism.h f Elgot-Algebra-Morphism.h g
; id = λ {EB} let open Elgot-Algebra EB in
record { h = idC; preserves = λ {X : Obj} {f : X A + X} begin
idC f # ≈⟨ identityˡ
(f #) ≈⟨ sym $ #-resp-≈ identityˡ
((idC f) #) ≈⟨ sym (#-resp-≈ (∘-resp-≈ˡ +-η))
(([ i₁ , i₂ ] f)#) ≈⟨ sym $ #-resp-≈ (∘-resp-≈ˡ ([]-cong₂ identityʳ identityʳ))
(([ i₁ idC , i₂ idC ] f)#) ≈⟨ sym $ #-resp-≈ (∘-resp-≈ˡ []∘+₁)
((([ i₁ , i₂ ] (idC +₁ idC)) f)#) ≈⟨ #-resp-≈ assoc
(([ i₁ , i₂ ] (idC +₁ idC) f)#) ≈⟨ #-resp-≈ (∘-resp-≈ˡ +-η)
((idC (idC +₁ idC) f)#) ≈⟨ #-resp-≈ identityˡ
((idC +₁ idC) f) # }
; _∘_ = λ {EA} {EB} {EC} f g let
open Elgot-Algebra-Morphism f renaming (h to hᶠ; preserves to preservesᶠ)
open Elgot-Algebra-Morphism g renaming (h to hᵍ; preserves to preservesᵍ)
open Elgot-Algebra EA using (A) renaming (_# to _#ᵃ)
open Elgot-Algebra EB using () renaming (_# to _#ᵇ; A to B)
open Elgot-Algebra EC using () renaming (_# to _#ᶜ; A to C; #-resp-≈ to #ᶜ-resp-≈)
in record { h = hᶠ hᵍ; preserves = λ {X} {f : X A + X} begin
(hᶠ hᵍ) (f #ᵃ) ≈⟨ assoc
(hᶠ hᵍ (f #ᵃ)) ≈⟨ ∘-resp-≈ʳ preservesᵍ
(hᶠ (((hᵍ +₁ idC) f) #ᵇ)) ≈⟨ preservesᶠ
(((hᶠ +₁ idC) (hᵍ +₁ idC) f) #ᶜ) ≈⟨ #ᶜ-resp-≈ sym-assoc
((((hᶠ +₁ idC) (hᵍ +₁ idC)) f) #ᶜ) ≈⟨ #ᶜ-resp-≈ (∘-resp-≈ˡ +₁∘+₁)
((((hᶠ hᵍ) +₁ (idC idC)) f) #ᶜ) ≈⟨ #ᶜ-resp-≈ (∘-resp-≈ˡ (+₁-cong₂ refl (identity²)))
((hᶠ hᵍ +₁ idC) f) #ᶜ }
; identityˡ = identityˡ
; identityʳ = identityʳ
; identity² = identity²
; assoc = assoc
; sym-assoc = sym-assoc
; equiv = record
{ refl = refl
; sym = sym
; trans = trans}
; ∘-resp-≈ = ∘-resp-≈
}
where
open Elgot-Algebra-Morphism
open HomReasoning
open Equiv
--*
-- products and exponentials of elgot-algebras
--*
-- product elgot-algebras
Product-Elgot-Algebra : {EA EB : Elgot-Algebra} Product 𝒞 (Elgot-Algebra.A EA) (Elgot-Algebra.A EB) Elgot-Algebra
Product-Elgot-Algebra {EA} {EB} p = record
{ A = A×B
; _# = λ {X : Obj} (h : X A×B + X) ((π₁ +₁ idC) h)#ᵃ , ((π₂ +₁ idC) h)#ᵇ
; #-Fixpoint = λ {X} {f} begin
((π₁ +₁ idC) f)#ᵃ , ((π₂ +₁ idC) f)#ᵇ ≈⟨ {! !} -- maybe use fixpoint on each side of pair..
([ idC , ((π₁ +₁ idC) f)#ᵃ , ((π₂ +₁ idC) f)#ᵇ ] f) -- TODO
; #-Uniformity = _ -- TODO
; #-Folding = _ -- TODO
; #-resp-≈ = _ -- TODO
}
where
open Elgot-Algebra EA using (A) renaming (_# to _#ᵃ)
open Elgot-Algebra EB using () renaming (A to B; _# to _#ᵇ)
open Product 𝒞 p
open HomReasoning
--*
-- Here follows the proof of equivalence for unguarded and Id-guarded Elgot-Algebras
--*
private
-- identity functor on 𝒞
Id : Functor 𝒞 𝒞
Id = idF {C = 𝒞}
-- identity algebra
Id-Algebra : Obj F-Algebra Id
Id-Algebra A = record
{ A = A
; α = idC
}
where open Functor Id
-- constructing an Id-Guarded Elgot-Algebra from an unguarded one
Unguarded→Id-Guarded : (EA : Elgot-Algebra) Guarded-Elgot-Algebra (Id-Algebra (Elgot-Algebra.A EA))
Unguarded→Id-Guarded ea = record
{ _# = _#
; #-Fixpoint = λ {X} {f} begin
f # ≈⟨ #-Fixpoint
[ idC , f # ] f ≈⟨ sym $ ∘-resp-≈ˡ ([]-congˡ identityˡ)
[ idC , idC f # ] f
; #-Uniformity = #-Uniformity
; #-Compositionality = #-Compositionality
; #-resp-≈ = #-resp-≈
}
where
open Elgot-Algebra ea
open HomReasoning
open Equiv
-- constructing an unguarded Elgot-Algebra from an Id-Guarded one
Id-Guarded→Unguarded : {A : Obj} Guarded-Elgot-Algebra (Id-Algebra A) Elgot-Algebra
Id-Guarded→Unguarded gea = record
{ _# = _#
; #-Fixpoint = λ {X} {f} begin
f # ≈⟨ #-Fixpoint
[ idC , idC f # ] f ≈⟨ ∘-resp-≈ˡ ([]-congˡ identityˡ)
[ idC , f # ] f
; #-Uniformity = #-Uniformity
; #-Folding = λ {X} {Y} {f} {h} begin
((f #) +₁ h) # ≈⟨ sym +-g-η
[ (f # +₁ h)# i₁ , (f # +₁ h)# i₂ ] ≈⟨ []-cong₂ left right
[ [ (idC +₁ i₁) f , i₂ h ] # i₁ , [ (idC +₁ i₁) f , i₂ h ] # i₂ ] ≈⟨ +-g-η
([ (idC +₁ i₁) f , i₂ h ] #)
; #-resp-≈ = #-resp-≈
}
where
open Guarded-Elgot-Algebra gea
open HomReasoning
open Equiv
left : {X Y} {f : X A + X} {h : Y X + Y}
(f # +₁ h)# i₁ [ (idC +₁ i₁) f , i₂ h ] # i₁
left {X} {Y} {f} {h} = begin
(f # +₁ h)# i₁ ≈⟨ ∘-resp-≈ˡ #-Fixpoint
(([ idC , idC (((f #) +₁ h) #) ] ((f #) +₁ h)) i₁) ≈⟨ assoc
([ idC , idC (((f #) +₁ h) #) ] (((f #) +₁ h) i₁)) ≈⟨ ∘-resp-≈ ([]-congˡ identityˡ) +₁∘i₁
([ idC , ((f #) +₁ h) # ] (i₁ (f #))) ≈⟨ sym-assoc
(([ idC , ((f #) +₁ h) # ] i₁) (f #)) ≈⟨ ∘-resp-≈ˡ inject₁
idC (f #) ≈⟨ identityˡ
(f #) ≈⟨ #-Uniformity {f = f} {g = [ (idC +₁ i₁) f , i₂ h ]} {h = i₁} (sym inject₁)
([ (idC +₁ i₁) f , i₂ h ] # i₁)
right : {X Y} {f : X A + X} {h : Y X + Y}
(f # +₁ h)# i₂ [ (idC +₁ i₁) f , i₂ h ] # i₂
right {X} {Y} {f} {h} = begin
(f # +₁ h)# i₂ ≈⟨ ∘-resp-≈ˡ #-Fixpoint
(([ idC , idC (((f #) +₁ h) #) ] ((f #) +₁ h)) i₂) ≈⟨ assoc
([ idC , idC (((f #) +₁ h) #) ] ((f #) +₁ h) i₂) ≈⟨ ∘-resp-≈ ([]-congˡ identityˡ) +₁∘i₂
([ idC , ((f #) +₁ h) # ] (i₂ h)) ≈⟨ sym-assoc
([ idC , ((f #) +₁ h) # ] i₂) h ≈⟨ ∘-resp-≈ˡ inject₂
((f #) +₁ h) # h ≈⟨ sym (#-Uniformity {f = ((f #) +₁ idC) h} {g = (f #) +₁ h} {h = h} (
begin
(idC +₁ h) ((f #) +₁ idC) h ≈⟨ sym-assoc
(((idC +₁ h) ((f #) +₁ idC)) h) ≈⟨ ∘-resp-≈ˡ +₁∘+₁
(((idC (f #)) +₁ (h idC)) h) ≈⟨ ∘-resp-≈ˡ (+₁-cong₂ identityˡ identityʳ)
(f # +₁ h) h ))
((((f #) +₁ idC) h) #) ≈⟨ #-Compositionality
(([ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ])# i₂) ≈⟨ ∘-resp-≈ˡ (#-Uniformity {f = [ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ]} {g = [ (idC +₁ i₁) f , i₂ h ]} {h = [ i₁ , h ]} (
begin
(idC +₁ [ i₁ , h ]) [ (idC +₁ i₁) f , i₂ i₂ ] [ i₁ , h ] ≈⟨ ∘-resp-≈ʳ ∘[]
(idC +₁ [ i₁ , h ]) [ [ (idC +₁ i₁) f , i₂ i₂ ] i₁ , [ (idC +₁ i₁) f , i₂ i₂ ] h ] ≈⟨ ∘-resp-≈ʳ ([]-congʳ inject₁)
((idC +₁ [ i₁ , h ]) [ (idC +₁ i₁) f , [ (idC +₁ i₁) f , i₂ i₂ ] h ]) ≈⟨ ∘[]
[ (idC +₁ [ i₁ , h ]) ((idC +₁ i₁) f) , (idC +₁ [ i₁ , h ]) ([ (idC +₁ i₁) f , i₂ i₂ ] h) ] ≈⟨ []-cong₂ sym-assoc sym-assoc
[ ((idC +₁ [ i₁ , h ]) (idC +₁ i₁)) f , ((idC +₁ [ i₁ , h ]) [ (idC +₁ i₁) f , i₂ i₂ ]) h ] ≈⟨ []-cong₂ (∘-resp-≈ˡ +₁∘+₁) (∘-resp-≈ˡ ∘[])
[ ((idC idC) +₁ ([ i₁ , h ] i₁)) f , ([ (idC +₁ [ i₁ , h ]) ((idC +₁ i₁) f) , (idC +₁ [ i₁ , h ]) (i₂ i₂) ]) h ] ≈⟨ []-cong₂ (∘-resp-≈ˡ (+₁-cong₂ identity² (inject₁))) (∘-resp-≈ˡ ([]-cong₂ sym-assoc sym-assoc))
[ (idC +₁ i₁) f , ([ ((idC +₁ [ i₁ , h ]) (idC +₁ i₁)) f , ((idC +₁ [ i₁ , h ]) i₂) i₂ ]) h ] ≈⟨ []-congˡ (∘-resp-≈ˡ ([]-cong₂ (∘-resp-≈ˡ +₁∘+₁) (∘-resp-≈ˡ inject₂)))
[ (idC +₁ i₁) f , ([ ((idC idC) +₁ ([ i₁ , h ] i₁)) f , (i₂ [ i₁ , h ]) i₂ ]) h ] ≈⟨ []-congˡ (∘-resp-≈ˡ ([]-cong₂ (∘-resp-≈ˡ (+₁-cong₂ identity² inject₁)) assoc))
[ (idC +₁ i₁) f , ([ (idC +₁ i₁) f , i₂ ([ i₁ , h ] i₂) ]) h ] ≈⟨ []-congˡ (∘-resp-≈ˡ ([]-congˡ (∘-resp-≈ʳ inject₂)))
[ (idC +₁ i₁) f , [ (idC +₁ i₁) f , i₂ h ] h ] ≈⟨ []-congʳ (sym (inject₁))
[ [ (idC +₁ i₁) f , i₂ h ] i₁ , [ (idC +₁ i₁) f , i₂ h ] h ] ≈⟨ sym ∘[]
[ (idC +₁ i₁) f , i₂ h ] [ i₁ , h ] ))
(([ (idC +₁ i₁) f , i₂ h ] # [ i₁ , h ]) i₂) ≈⟨ assoc
([ (idC +₁ i₁) f , i₂ h ] # ([ i₁ , h ] i₂)) ≈⟨ (∘-resp-≈ʳ $ inject₂)
([ (idC +₁ i₁) f , i₂ h ] # h) ≈⟨ sym $ ∘-resp-≈ˡ inject₂
(([ idC , [ (idC +₁ i₁) f , i₂ h ] # ] i₂) h) ≈⟨ assoc
([ idC , [ (idC +₁ i₁) f , i₂ h ] # ] i₂ h) ≈⟨ sym (∘-resp-≈ ([]-congˡ identityˡ) inject₂)
([ idC , idC [ (idC +₁ i₁) f , i₂ h ] # ] ([ (idC +₁ i₁) f , i₂ h ] i₂)) ≈⟨ sym-assoc
(([ idC , idC [ (idC +₁ i₁) f , i₂ h ] # ] [ (idC +₁ i₁) f , i₂ h ]) i₂) ≈⟨ ∘-resp-≈ˡ (sym #-Fixpoint)
([ (idC +₁ i₁) f , i₂ h ] # i₂)
-- unguarded elgot-algebras are just Id-guarded Elgot-Algebras
Unguarded↔Id-Guarded : ((ea : Elgot-Algebra) Guarded-Elgot-Algebra (Id-Algebra (Elgot-Algebra.A ea))) ( {A : Obj} Guarded-Elgot-Algebra (Id-Algebra A) Elgot-Algebra)
Unguarded↔Id-Guarded = Unguarded→Id-Guarded , Id-Guarded→Unguarded