data.W.basic

# W types #

THIS FILE IS SYNCHRONIZED WITH MATHLIB4. Any changes to this file require a corresponding PR to mathlib4.

Given α : Type and β : α → Type, the W type determined by this data, W_type β, is the inductively defined type of trees where the nodes are labeled by elements of α and the children of a node labeled a are indexed by elements of β a.

This file is currently a stub, awaiting a full development of the theory. Currently, the main result is that if α is an encodable fintype and β a is encodable for every a : α, then W_type β is encodable. This can be used to show the encodability of other inductive types, such as those that are commonly used to formalize syntax, e.g. terms and expressions in a given language. The strategy is illustrated in the example found in the file prop_encodable in the archive/examples folder of mathlib.

## Implementation details #

While the name W_type is somewhat verbose, it is preferable to putting a single character identifier W in the root namespace.

inductive W_type {α : Type u_1} (β : α Type u_2) :
Type (max u_1 u_2)

Given β : α → Type*, W_type β is the type of finitely branching trees where nodes are labeled by elements of α and the children of a node labeled a are indexed by elements of β a.

Instances for W_type
@[protected, instance]
def W_type.inhabited  :
inhabited (W_type (λ (_x : unit), empty))
Equations
def W_type.to_sigma {α : Type u_1} {β : α Type u_2} :
(Σ (a : α), β a W_type β)

The canonical map to the corresponding sigma type, returning the label of a node as an element a of α, and the children of the node as a function β a → W_type β.

Equations
def W_type.of_sigma {α : Type u_1} {β : α Type u_2} :
(Σ (a : α), β a W_type β)

The canonical map from the sigma type into a W_type. Given a node a : α, and its children as a function β a → W_type β, return the corresponding tree.

Equations
@[simp]
theorem W_type.of_sigma_to_sigma {α : Type u_1} {β : α Type u_2} (w : W_type β) :
@[simp]
theorem W_type.to_sigma_of_sigma {α : Type u_1} {β : α Type u_2} (s : Σ (a : α), β a ) :
= s
def W_type.equiv_sigma {α : Type u_1} (β : α Type u_2) :
Σ (a : α), β a

The canonical bijection with the sigma type, showing that W_type is a fixed point of the polynomial functor X ↦ Σ a : α, β a → X.

Equations
@[simp]
theorem W_type.equiv_sigma_apply {α : Type u_1} (β : α Type u_2) (ᾰ : W_type β) :
= ᾰ.to_sigma
@[simp]
theorem W_type.equiv_sigma_symm_apply {α : Type u_1} (β : α Type u_2) (ᾰ : Σ (a : α), (λ (a : α), β a) a W_type (λ (a : α), β a)) :
.symm) =
def W_type.elim {α : Type u_1} {β : α Type u_2} (γ : Type u_3) (fγ : (Σ (a : α), β a γ) γ) :
γ

The canonical map from W_type β into any type γ given a map (Σ a : α, β a → γ) → γ.

Equations
• f) = a, λ (b : β a), (f b)
theorem W_type.elim_injective {α : Type u_1} {β : α Type u_2} (γ : Type u_3) (fγ : (Σ (a : α), β a γ) γ) (fγ_injective : function.injective fγ) :
@[protected, instance]
def W_type.is_empty {α : Type u_1} {β : α Type u_2} [hα : is_empty α] :
theorem W_type.infinite_of_nonempty_of_is_empty {α : Type u_1} {β : α Type u_2} (a b : α) [ha : nonempty (β a)] [he : is_empty (β b)] :
def W_type.depth {α : Type u_1} {β : α Type u_2} [Π (a : α), fintype (β a)] :

The depth of a finitely branching tree.

Equations
theorem W_type.depth_pos {α : Type u_1} {β : α Type u_2} [Π (a : α), fintype (β a)] (t : W_type β) :
0 < t.depth
theorem W_type.depth_lt_depth_mk {α : Type u_1} {β : α Type u_2} [Π (a : α), fintype (β a)] (a : α) (f : β a ) (i : β a) :
(f i).depth < f).depth
@[protected, instance]
def W_type.encodable {α : Type u_1} {β : α Type u_2} [Π (a : α), fintype (β a)] [Π (a : α), encodable (β a)] [encodable α] :

W_type is encodable when α is an encodable fintype and for every a : α, β a is encodable.

Equations