Polynomial Functors

This file defines polynomial functors and the W-type construction as a polynomial functor. (For the M-type construction, see Mathlib/Data/PFunctor/Univariate/M.lean.)

structure PFunctor : Type (max (uA + 1) (uB + 1))

A polynomial functor P is given by a type A and a family B of types over A. P maps any type α to a new type P α, which is defined as the sigma type Σ x, P.B x → α.

An element of P α is a pair ⟨a, f⟩, where a is an element of a type A and f : B a → α. Think of a as the shape of the object and f as an index to the relevant elements of α.

• A : Type uA

  The head type

• B : self.A → Type uB

  The child family of types

instance PFunctor.instInhabited : Inhabited PFunctor.{u_2, u_1}

def PFunctor.Obj (P : PFunctor.{uA, uB}) (α : Type v) : Type (max v uA uB)

Applying P to an object of Type

instance PFunctor.instCoeFunForallType : CoeFun PFunctor.{uA, uB} fun (x : PFunctor.{uA, uB}) => Type v → Type (max v uA uB)

def PFunctor.map (P : PFunctor.{uA, uB}) {α : Type v₁} {β : Type v₂} (f : α → β) : ↑P α → ↑P β

Applying P to a morphism of Type

instance PFunctor.Obj.inhabited (P : PFunctor.{uA, uB}) {α : Type v₁} [Inhabited P.A] [Inhabited α] : Inhabited (↑P α)

instance PFunctor.instFunctorObj (P : PFunctor.{uA, uB}) : Functor ↑P

@[simp]

theorem PFunctor.map_eq_map (P : PFunctor.{uA, uB}) {α β : Type v} (f : α → β) (x : ↑P α) : f <$> x = P.map f x

We prefer PFunctor.map to Functor.map because it is universe-polymorphic.

@[simp]

theorem PFunctor.map_eq (P : PFunctor.{uA, uB}) {α : Type v₁} {β : Type v₂} (f : α → β) (a : P.A) (g : P.B a → α) : P.map f ⟨a, g⟩ = ⟨a, f ∘ g⟩

@[simp]

theorem PFunctor.id_map (P : PFunctor.{uA, uB}) {α : Type v₁} (x : ↑P α) : P.map id x = x

@[simp]

theorem PFunctor.map_map (P : PFunctor.{uA, uB}) {α : Type v₁} {β : Type v₂} {γ : Type v₃} (f : α → β) (g : β → γ) (x : ↑P α) : P.map g (P.map f x) = P.map (g ∘ f) x

instance PFunctor.instLawfulFunctorObj (P : PFunctor.{uA, uB}) : LawfulFunctor ↑P

def PFunctor.W (P : PFunctor.{uA, uB}) : Type (max uA uB)

Re-export existing definition of W-types and adapt it to a packaged definition of polynomial functor.

def PFunctor.W.head {P : PFunctor.{uA, uB}} : P.W → P.A

The root element of a W tree

def PFunctor.W.children {P : PFunctor.{uA, uB}} (x : P.W) : P.B x.head → P.W

The children of the root of a W tree

def PFunctor.W.dest {P : PFunctor.{uA, uB}} : P.W → ↑P P.W

The destructor for W-types

def PFunctor.W.mk {P : PFunctor.{uA, uB}} : ↑P P.W → P.W

The constructor for W-types

@[simp]

theorem PFunctor.W.dest_mk {P : PFunctor.{uA, uB}} (p : ↑P P.W) : (mk p).dest = p

@[simp]

theorem PFunctor.W.mk_dest {P : PFunctor.{uA, uB}} (p : P.W) : mk p.dest = p

def PFunctor.Idx (P : PFunctor.{uA, uB}) : Type (max uA uB)

Idx identifies a location inside the application of a polynomial functor. For F : PFunctor, x : F α and i : F.Idx, i can designate one part of x or is invalid, if i.1 ≠ x.1.

instance PFunctor.Idx.inhabited (P : PFunctor.{uA, uB}) [Inhabited P.A] [Inhabited (P.B default)] : Inhabited P.Idx

def PFunctor.Obj.iget {P : PFunctor.{uA, uB}} [DecidableEq P.A] {α : Type u_1} [Inhabited α] (x : ↑P α) (i : P.Idx) : α

x.iget i takes the component of x designated by i if any is or returns a default value

@[simp]

theorem PFunctor.fst_map {P : PFunctor.{uA, uB}} {α : Type v₁} {β : Type v₂} (x : ↑P α) (f : α → β) : (P.map f x).fst = x.fst

@[simp]

theorem PFunctor.iget_map {P : PFunctor.{uA, uB}} {α : Type v₁} {β : Type v₂} [DecidableEq P.A] [Inhabited α] [Inhabited β] (x : ↑P α) (f : α → β) (i : P.Idx) (h : i.fst = x.fst) : (P.map f x).iget i = f (x.iget i)

def PFunctor.comp (P₂ : PFunctor.{uA₂, uB₂}) (P₁ : PFunctor.{uA₁, uB₁}) : PFunctor.{max uA₁ uA₂ uB₂, max uB₁ uB₂}

Composition for polynomial functors

def PFunctor.comp.mk (P₂ : PFunctor.{uA₂, uB₂}) (P₁ : PFunctor.{uA₁, uB₁}) {α : Type v} (x : ↑P₂ (↑P₁ α)) : ↑(P₂.comp P₁) α

Constructor for composition

def PFunctor.comp.get (P₂ : PFunctor.{uA₂, uB₂}) (P₁ : PFunctor.{uA₁, uB₁}) {α : Type v} (x : ↑(P₂.comp P₁) α) : ↑P₂ (↑P₁ α)

Destructor for composition

theorem PFunctor.liftp_iff {P : PFunctor.{uA, uB}} {α : Type u} (p : α → Prop) (x : ↑P α) : Functor.Liftp p x ↔ ∃ (a : P.A) (f : P.B a → α), x = ⟨a, f⟩ ∧ ∀ (i : P.B a), p (f i)

theorem PFunctor.liftp_iff' {P : PFunctor.{uA, uB}} {α : Type u} (p : α → Prop) (a : P.A) (f : P.B a → α) : Functor.Liftp p ⟨a, f⟩ ↔ ∀ (i : P.B a), p (f i)

theorem PFunctor.liftr_iff {P : PFunctor.{uA, uB}} {α : Type u} (r : α → α → Prop) (x y : ↑P α) : Functor.Liftr r x y ↔ ∃ (a : P.A) (f₀ : P.B a → α) (f₁ : P.B a → α), x = ⟨a, f₀⟩ ∧ y = ⟨a, f₁⟩ ∧ ∀ (i : P.B a), r (f₀ i) (f₁ i)

theorem PFunctor.supp_eq {P : PFunctor.{uA, uB}} {α : Type u} (a : P.A) (f : P.B a → α) : Functor.supp ⟨a, f⟩ = f '' Set.univ