Polynomial functors

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

structure PFunctor : Type (u + 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 u

  The head type

• B : self.A → Type u

  The child family of types

instance PFunctor.instInhabited : Inhabited PFunctor.{u_1}

Equations

• PFunctor.instInhabited = { default := { A := default, B := default } }

def PFunctor.Obj (P : PFunctor.{u}) (α : Type v) : Type (max u u v)

Applying P to an object of Type

Equations

• ↑P α = ((x : P.A) × (P.B x → α))

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

Equations

• PFunctor.instCoeFunForallType = { coe := PFunctor.Obj }

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

Applying P to a morphism of Type

Equations

• P.map f x = match x with | ⟨a, g⟩ => ⟨a, f ∘ g⟩

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

Equations

• PFunctor.Obj.inhabited P = { default := ⟨default, default⟩ }

instance PFunctor.instFunctorObj (P : PFunctor.{u}) : Functor ↑P

Equations

• P.instFunctorObj = { map := @PFunctor.map P, mapConst := fun {α β : Type v} => P.map ∘ Function.const β }

@[simp]

theorem PFunctor.map_eq_map (P : PFunctor.{u}) {α : Type v} {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.{u}) {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.{u}) {α : Type v₁} (x : ↑P α) : P.map id x = x

@[simp]

theorem PFunctor.map_map (P : PFunctor.{u}) {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.{u}) : LawfulFunctor ↑P

Equations

• ⋯ = ⋯

def PFunctor.W (P : PFunctor.{u}) : Type u

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

Equations

• P.W = WType P.B

def PFunctor.W.head {P : PFunctor.{u}} : P.W → P.A

root element of a W tree

Equations

• x.head = match x with | WType.mk a _f => a

def PFunctor.W.children {P : PFunctor.{u}} (x : P.W) : P.B x.head → P.W

children of the root of a W tree

Equations

• x.children = match (motive := (x : P.W) → P.B x.head → P.W) x with | WType.mk _a f => f

def PFunctor.W.dest {P : PFunctor.{u}} : P.W → ↑P P.W

destructor for W-types

Equations

• x.dest = match x with | WType.mk a f => ⟨a, f⟩

def PFunctor.W.mk {P : PFunctor.{u}} : ↑P P.W → P.W

constructor for W-types

Equations

• PFunctor.W.mk x = match x with | ⟨a, f⟩ => WType.mk a f

@[simp]

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

@[simp]

theorem PFunctor.W.mk_dest {P : PFunctor.{u}} (p : P.W) : PFunctor.W.mk p.dest = p

def PFunctor.Idx (P : PFunctor.{u}) : Type u

Idx identifies a location inside the application of a pfunctor. 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

Equations

• P.Idx = ((x : P.A) × P.B x)

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

Equations

• PFunctor.Idx.inhabited P = { default := ⟨default, default⟩ }

def PFunctor.Obj.iget {P : PFunctor.{u}} [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

Equations

• x.iget i = if h : i.fst = x.fst then x.snd (cast ⋯ i.snd) else default

@[simp]

theorem PFunctor.fst_map {P : PFunctor.{u}} {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.{u}} {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.{u}) (P₁ : PFunctor.{u}) : PFunctor.{u}

functor composition for polynomial functors

Equations

• P₂.comp P₁ = { A := (a₂ : P₂.A) × (P₂.B a₂ → P₁.A), B := fun (a₂a₁ : (a₂ : P₂.A) × (P₂.B a₂ → P₁.A)) => (u : P₂.B a₂a₁.fst) × P₁.B (a₂a₁.snd u) }

def PFunctor.comp.mk (P₂ : PFunctor.{u}) (P₁ : PFunctor.{u}) {α : Type} (x : ↑P₂ (↑P₁ α)) : ↑(P₂.comp P₁) α

constructor for composition

Equations

• PFunctor.comp.mk P₂ P₁ x = ⟨⟨x.fst, Sigma.fst ∘ x.snd⟩, fun (a₂a₁ : (P₂.comp P₁).B ⟨x.fst, Sigma.fst ∘ x.snd⟩) => (x.snd a₂a₁.fst).snd a₂a₁.snd⟩

def PFunctor.comp.get (P₂ : PFunctor.{u}) (P₁ : PFunctor.{u}) {α : Type} (x : ↑(P₂.comp P₁) α) : ↑P₂ (↑P₁ α)

destructor for composition

Equations

• PFunctor.comp.get P₂ P₁ x = ⟨x.fst.fst, fun (a₂ : P₂.B x.fst.fst) => ⟨x.fst.snd a₂, fun (a₁ : P₁.B (x.fst.snd a₂)) => x.snd ⟨a₂, a₁⟩⟩⟩

theorem PFunctor.liftp_iff {P : PFunctor.{u}} {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.{u}} {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.{u}} {α : Type u} (r : α → α → Prop) (x : ↑P α) (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.{u}} {α : Type u} (a : P.A) (f : P.B a → α) : Functor.supp ⟨a, f⟩ = f '' Set.univ