Identity types

Content created by Egbert Rijke, Fredrik Bakke, Raymond Baker and Jonathan Prieto-Cubides.

Created on 2022-02-04.
Last modified on 2023-09-24.

module foundation-core.identity-types where

open import foundation.universe-levels

Idea

The equality relation on a type is a reflexive relation, with the universal property that it maps uniquely into any other reflexive relation. In type theory, we introduce the identity type as an inductive family of types, where the induction principle can be understood as expressing that the identity type is the least reflexive relation.

Notation of the identity type

We include two notations for the identity type. First, we introduce the identity type using Martin-Löf's original notation Id. Then we introduce as a secondary option the infix notation _＝_.

Note: The equals sign in the infix notation is not the standard equals sign on your keyboard, but it is the full width equals sign. Note that the full width equals sign is slightly wider, and it is highlighted like all the other defined constructions in Agda. In order to type the full width equals sign in Agda's Emacs Mode, you need to add it to your agda input method as follows:

• Type M-x customize-variable and press enter.
• Type agda-input-user-translations and press enter.
• Click the INS button
• Type the regular equals sign = in the Key sequence field.
• Click the INS button
• Type the full width equals sign ＝ in the translations field.
• Click the Apply and save button.

After completing these steps, you can type \= in order to obtain the full width equals sign ＝.

List of files directly related to identity types

The following table lists files that are about identity types and operations on identifications in arbitrary types.

Definition

module _
  {l : Level} {A : UU l}
  where

  data Id (x : A) : A → UU l where
    refl : Id x x

  infix  _＝_
  _＝_ : A → A → UU l
  (a ＝ b) = Id a b

{-# BUILTIN EQUALITY Id #-}

The induction principle

The induction principle of identity types states that given a base point x : A and a family of types over the identity types based at x, B : (y : A) (p : x ＝ y) → UU l2, then to construct a dependent function f : (y : A) (p : x ＝ y) → B y p it suffices to define it at f x refl.

Note that Agda's pattern matching machinery allows us to define many operations on the identity type directly. However, sometimes it is useful to explicitly have the induction principle of the identity type.

ind-Id :
  {l1 l2 : Level} {A : UU l1}
  (x : A) (B : (y : A) (p : x ＝ y) → UU l2) →
  (B x refl) → (y : A) (p : x ＝ y) → B y p
ind-Id x B b y refl = b

Structure

The identity types form a weak groupoidal structure on types.

Concatenation of identifications

module _
  {l : Level} {A : UU l}
  where

  infixl  _∙_
  _∙_ : {x y z : A} → x ＝ y → y ＝ z → x ＝ z
  refl ∙ q = q

  concat : {x y : A} → x ＝ y → (z : A) → y ＝ z → x ＝ z
  concat p z q = p ∙ q

  concat' : (x : A) {y z : A} → y ＝ z → x ＝ y → x ＝ z
  concat' x q p = p ∙ q

Inverting identifications

module _
  {l : Level} {A : UU l}
  where

  inv : {x y : A} → x ＝ y → y ＝ x
  inv refl = refl

The groupoidal laws for types

module _
  {l : Level} {A : UU l}
  where

  assoc :
    {x y z w : A} (p : x ＝ y) (q : y ＝ z) (r : z ＝ w) →
    ((p ∙ q) ∙ r) ＝ (p ∙ (q ∙ r))
  assoc refl q r = refl

  left-unit : {x y : A} {p : x ＝ y} → (refl ∙ p) ＝ p
  left-unit = refl

  right-unit : {x y : A} {p : x ＝ y} → (p ∙ refl) ＝ p
  right-unit {p = refl} = refl

  left-inv : {x y : A} (p : x ＝ y) → ((inv p) ∙ p) ＝ refl
  left-inv refl = refl

  right-inv : {x y : A} (p : x ＝ y) → (p ∙ (inv p)) ＝ refl
  right-inv refl = refl

  inv-inv : {x y : A} (p : x ＝ y) → (inv (inv p)) ＝ p
  inv-inv refl = refl

  distributive-inv-concat :
    {x y : A} (p : x ＝ y) {z : A} (q : y ＝ z) →
    (inv (p ∙ q)) ＝ ((inv q) ∙ (inv p))
  distributive-inv-concat refl refl = refl

Transposing inverses

left-transpose-eq-concat :
  {l : Level} {A : UU l} {x y : A} (p : x ＝ y) {z : A} (q : y ＝ z)
  (r : x ＝ z) → ((p ∙ q) ＝ r) → q ＝ ((inv p) ∙ r)
left-transpose-eq-concat refl q r s = s

right-transpose-eq-concat :
  {l : Level} {A : UU l} {x y : A} (p : x ＝ y) {z : A} (q : y ＝ z)
  (r : x ＝ z) → ((p ∙ q) ＝ r) → p ＝ (r ∙ (inv q))
right-transpose-eq-concat p refl r s = ((inv right-unit) ∙ s) ∙ (inv right-unit)

The fact that left-transpose-eq-concat and right-transpose-eq-concat are equivalences is recorded in foundation.identity-types.

Concatenation is injective

module _
  {l1 : Level} {A : UU l1}
  where

  is-injective-concat :
    {x y z : A} (p : x ＝ y) {q r : y ＝ z} → (p ∙ q) ＝ (p ∙ r) → q ＝ r
  is-injective-concat refl s = s

  is-injective-concat' :
    {x y z : A} (r : y ＝ z) {p q : x ＝ y} → (p ∙ r) ＝ (q ∙ r) → p ＝ q
  is-injective-concat' refl s = (inv right-unit) ∙ (s ∙ right-unit)

Equational reasoning

Identifications can be constructed by equational reasoning in the following way:

equational-reasoning
  x ＝ y by eq-1
    ＝ z by eq-2
    ＝ v by eq-3

The resulting identification of this computaion is eq-1 ∙ (eq-2 ∙ eq-3), i.e., the identification is associated fully to the right. For examples of the use of equational reasoning, see addition-integers.

infixl  equational-reasoning_
infixl  step-equational-reasoning

equational-reasoning_ :
  {l : Level} {X : UU l} (x : X) → x ＝ x
equational-reasoning x = refl

step-equational-reasoning :
  {l : Level} {X : UU l} {x y : X} →
  (x ＝ y) → (u : X) → (y ＝ u) → (x ＝ u)
step-equational-reasoning p z q = p ∙ q

syntax step-equational-reasoning p z q = p ＝ z by q

References

Our setup of equational reasoning is derived from the following sources:

1. Martín Escardó. https://github.com/martinescardo/TypeTopology/blob/master/source/Id.lagda

2. Martín Escardó. https://github.com/martinescardo/TypeTopology/blob/master/source/UF-Equiv.lagda

3. The Agda standard library. https://github.com/agda/agda-stdlib/blob/master/src/Relation/Binary/PropositionalEquality/Core.agda

Recent changes

• 2023-09-24. Raymond Baker. Refactor Eckmann-Hilton (#788).
• 2023-09-19. Egbert Rijke. Add the universal property of identity systems to the overview tables (#789).
• 2023-09-17. Egbert Rijke. Moving file about hexagons of identifications (#782).
• 2023-09-11. Fredrik Bakke. Transport along and action on equivalences (#706).
• 2023-09-10. Fredrik Bakke. Rename inv-con and con-inv to left/right-transpose-eq-concat (#730).