Coalgebras of polynomial endofunctors

Content created by Egbert Rijke and Fredrik Bakke.

Created on 2023-05-03.
Last modified on 2023-05-16.

module trees.coalgebras-polynomial-endofunctors where
Imports 
open import foundation.dependent-pair-types
open import foundation.universe-levels

open import trees.polynomial-endofunctors

Idea

Coalgebras for polynomial endofunctors are types X equipped with a function

  X → Σ (a : A), B a → X

Definitions

module _
  {l1 l2 : Level} (l : Level) (A : UU l1) (B : A → UU l2)
  where

  coalgebra-polynomial-endofunctor : UU (l1 ⊔ l2 ⊔ lsuc l)
  coalgebra-polynomial-endofunctor =
    Σ (UU l) (λ X → X → type-polynomial-endofunctor A B X)

module _
  {l1 l2 l3 : Level} {A : UU l1} {B : A → UU l2}
  (X : coalgebra-polynomial-endofunctor l3 A B)
  where

  type-coalgebra-polynomial-endofunctor : UU l3
  type-coalgebra-polynomial-endofunctor = pr1 X

  structure-coalgebra-polynomial-endofunctor :
    type-coalgebra-polynomial-endofunctor →
    type-polynomial-endofunctor A B type-coalgebra-polynomial-endofunctor
  structure-coalgebra-polynomial-endofunctor = pr2 X

  shape-coalgebra-polynomial-endofunctor :
    type-coalgebra-polynomial-endofunctor → A
  shape-coalgebra-polynomial-endofunctor x =
    pr1 (structure-coalgebra-polynomial-endofunctor x)

  component-coalgebra-polynomial-endofunctor :
    (x : type-coalgebra-polynomial-endofunctor) →
    B (shape-coalgebra-polynomial-endofunctor x) →
    type-coalgebra-polynomial-endofunctor
  component-coalgebra-polynomial-endofunctor x =
    pr2 (structure-coalgebra-polynomial-endofunctor x)

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