# Contents

## Idea

In dependent type theory, the equivalence type is to types what the identity type is to terms: it represents the collection of “equalities” between types (equality of types being given by the notion of equivalence in type theory), in the same way that the identity type represents the collection of equalities between terms (equality of terms being given by the notion of identity/identification/path).

## Definition

In dependent type theory, the equivalence type between two types $A$ and $B$ is the type $A \simeq B$ whose terms are equivalences between $A$ and $B$. Like any other notion of type in dependent type theory, there are two different notions of equivalence types in type theory: strict and weak equivalence types. Strict equivalence types use judgmental equality in the conversion rules, while weak equivalence types use identity types in the conversion rules. Weak equivalence types could also be defined analytically from other type formers in the type theory.

### Strict equivalence types

#### Judgmentally strict equivalence types

Given types $A$ and $B$, one could define the type of strict equivalences betwen $A$ and $B$. These are given by the following rules:

Formation rule for judgmentally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma \vdash A \simeq B \; \mathrm{type}}$

Introduction rule for judgmentally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma \vdash \mathrm{tr}_B(a, b, p):B(a) \simeq B(b) \; \mathrm{type}}$

Elimination rules for judgmentally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B}{\Gamma, x:A \vdash f(x):B}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B}{\Gamma, y:B \vdash f^{-1}(x):B}$

Computation rules for judgmentally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash \mathrm{tr}_B(a, b, p)(w(a)) \equiv w(b):B(b)}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash w(a) \equiv \mathrm{tr}_B(a, b, p)^{-1}(w(b)):B(a)}$

#### Propositionally strict equivalence types

Given types $A$ and $B$, one could define the type of strict equivalences betwen $A$ and $B$. These are given by the following rules:

Formation rule for propositionally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma \vdash A \simeq B \; \mathrm{type}}$

Introduction rule for propositionally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma \vdash \mathrm{tr}_B(a, b, p):B(a) \simeq B(b) \; \mathrm{type}}$

Elimination rules for propositionally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B}{\Gamma, x:A \vdash f(x):B}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B}{\Gamma, y:B \vdash f^{-1}(x):B}$

Computation rules for propositionally strict equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash \mathrm{tr}_B(a, b, p)(w(a)) \equiv_{B(b)} w(b) \; \mathrm{true}}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash w(a) \equiv_{B(a)} \mathrm{tr}_B(a, b, p)^{-1}(w(b)) \; \mathrm{true}}$

### Weak equivalence types

#### As a dependent sum type of the isEquiv type family

Given a notion of the isEquiv type family on the function type $A \to B$, the equivalence type is defined by

$A \simeq B \coloneqq \sum_{f:A \to B} \mathrm{isEquiv}(f)$

#### Locally small equivalence types

Given a type universe $U$ and a notion of a $U$-small isEquiv type family for some type $F_U(A, B)$, the locally $U$-small equivalence type is defined by

$A \simeq_U B \coloneqq \sum_{f:F_U(A, B)} \mathrm{isEquiv}_U(f)$

$F_U(A, B)$ could be the type of $U$-small spans, the type of $U$-small multivalued partial functions, or the type of $U$-small correspondences.

#### As a type whose elements are encodings for one-to-one correspondence

Given types $A$ and $B$, we form the type $A \simeq B$ such that given element $R:A \simeq B$, there is a type family $x =_{A, B}^R y$ indexed by $x:A$ and $y:B$, with rules stating that $(-) =_{A, B}^R (-)$ is a one-to-one correspondence, and that given a type $A$, elements $a:A$ and $b:A$, identity $p:a =_A b$, a type family $B$ indexed by $A$ and a family of elements $w:B$ indexed by $A$, one could form the transport equivalence across $p$ for $B$, $\mathrm{tr}_B(p):B(a) \simeq B(b)$, and the dependent action on $p$ for $w$, $\mathrm{apd}_B(p, w):w(a) =_{B(a), B(b)}^{\mathrm{tr}_B(p)} w(b)$.

Rules for equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma \vdash A \simeq B \; \mathrm{type}} \qquad \frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, R:A \simeq B, x:A, y:B \vdash x =_{A, B}^R y \; \mathrm{type}}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma \vdash \mathrm{tr}_B(p):B(a) \simeq B(b) \; \mathrm{type}}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma, x:A, w:B \vdash \mathrm{apd}_B(p, w):w(a) =_{B(a), B(b)}^{\mathrm{tr}_B(p)} w(b)}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, f:A \simeq B, x:A \vdash \exists !y:B.x =_{A, B}^R y}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, f:A \simeq B, y:B \vdash \exists !x:A.x =_{A, B}^R y}$

#### As a type whose elements are encodings for spans with contractible fibers

Given types $A$ and $B$, we form the type $A \simeq B$ such that given element $R:A \simeq B$, there is a type $C(R)$ and families of elements $x:C(R) \vdash g(R, x):A$ and $x:C(R) \vdash h(R, x):B$, with rules stating that the above structures have contractible fibers, and that given a type $A$, elements $a:A$ and $b:A$, identity $p:a =_A b$, a type family $B$ indexed by $A$ and a family of elements $w:B$ indexed by $A$, one could form the transport equivalence across $p$ for $B$, $\mathrm{tr}_B(p):B(a) \simeq B(b)$, and the components for the dependent action on $p$ for $w$, $\mathrm{apd}_B(p, w) \coloneqq (\alpha_B(p, w), \gamma_B(p, w), \eta_B(p, w))$ with $\alpha_B(p, w):C(\mathrm{tr}_B(p))$, $\gamma_B(p, w):g(\mathrm{tr}_B(p), \alpha_B(p, w)) =_{B(a)} w(a)$, and $\eta_B(p, w):h(\mathrm{tr}_B(p), \alpha_B(p, w)) =_{B(b)} w(b)$.

Rules for equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma \vdash A \simeq B \; \mathrm{type}} \qquad \frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, R:A \simeq B \vdash C_{A, B}(R) \; \mathrm{type}}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, R:A \simeq B, z:C_{A, B}(R) \vdash g(R, z):A} \qquad \frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, R:A \simeq B, z:C_{A, B}(R) \vdash h(R, z):B}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma \vdash \mathrm{tr}_B(p):B(a) \simeq B(b) \; \mathrm{type}}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma, x:A, w:B \vdash \alpha_B(p, w):C_{B(a), B(b)}(\mathrm{tr}_B(p))}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma, x:A, w:B \vdash \gamma_B(p, w):g(\mathrm{tr}_B(p), \alpha_B(p, w)) =_{B(a)} w(a)}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma, x:A, w:B \vdash \eta_B(p, w):h(\mathrm{tr}_B(p), \alpha_B(p, w)) =_{B(b)} w(b)}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, R:A \simeq B, x:A \vdash \eta_g(R, x):\exists!z:C_{A, B}(R).g(R, z) =_A x}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma, R:A \simeq B, y:B \vdash \eta_h(R, y):\exists!z:C_{A, B}(R).h(R, z) =_B y}$

#### Coinductive definition

Given two types $A$ and $B$ and two functions $f:A \to B$ and $g:B \to A$, $f$ and $g$ are inverses of each other if for every element $a:A$ and $b:A$, there is an equivalence of types between $f(a) =_B b$ and $a =_A g(b)$:

$\mathrm{isInv}(f, g) \coloneqq \prod_{a:A} \prod_{b:B} (f(a) =_B b) \simeq (a =_A g(b))$

This leads to a coinductive definition of the type of equivalences

$A \simeq B \coloneqq \sum_{f:A \to B} \sum_{g:B \to A} \prod_{a:A} \prod_{b:B} (f(a) =_B b) \simeq (a =_A g(b))$

Formation rule for equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type}}{\Gamma \vdash A \simeq B \; \mathrm{type}}$

Introduction rule for equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type}}{\Gamma \vdash \mathrm{tr}_B(a, b, p):B(a) \simeq B(b) \; \mathrm{type}}$

Elimination rules for equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B \quad \Gamma \vdash x:A}{\Gamma \vdash f(x):B} \qquad \frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B \quad \Gamma \vdash y:B}{\Gamma \vdash f^{-1}(y):A}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash B \; \mathrm{type} \quad \Gamma \vdash f:A \simeq B \quad \Gamma \vdash x:A \quad \Gamma \vdash y:B}{\Gamma\vdash \mathrm{coh}(f, x, y):(f(x) =_B y) \simeq (x =_A f^{-1}(y))}$

Computation rules for equivalence types:

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash \mathrm{apdl}_B(a, b, p, w):w(a) =_{B(a)} \mathrm{tr}_B(a, b, p)^{-1}(w(b))}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash \mathrm{apdr}_B(a, b, p, w):\mathrm{tr}_B(a, b, p)(w(a)) =_{B(b)} w(b)}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma, x:A \vdash w:B(x)}{\Gamma \vdash \mathrm{cohapd}_B(a, b, p, w):\left(\mathrm{coh}(\mathrm{tr}_B(a, b, p), w(a), w(b))(\mathrm{apdr}_B(a, b, p, w)) =_{w(a) =_{B(a)} \mathrm{tr}_B(a, b, p)^{-1}(w(b))} \mathrm{apdl}_B(a, b, p, w)\right) \simeq \left(\mathrm{apdr}_B(a, b, p, w) =_{\mathrm{tr}_B(a, b, p)(w(a)) =_{B(b)} w(b)} \mathrm{coh}(\mathrm{tr}_B(a, b, p), w(a), w(b))^{-1}(\mathrm{apdl}_B(a, b, p, w))\right)}$

## Properties

### Relation to interval types

Given types $A$ and $B$ and an equivalence $f:A \simeq B$, one could define the dependent type $x:\mathbb{I} \vdash C(x)$ indexed by the interval type $\mathbb{I}$ as $C(0) \equiv A$, $C(1) \equiv B$, and $\mathrm{tr}_C(0, 1, p) \equiv f$.

### One-to-one correspondences

Given types $A$ and $B$ and an equivalence $f:A \simeq B$, one could define a correspondence $x:A, y:B \vdash R(x, y)$ as the dependent identity type

$R(x, y) \coloneqq x =_C^p y$

where $x:\mathbb{I} \vdash C(x)$ is defined as in the previous section. By the properties of dependent identity types, the correspondence is always a one-to-one correspondence.

### Quasi-inverse functions with contractible fibers

By the rules for function types, given an equivalence $R:A \simeq B$, one could derive functions $\rho(R):A \to B$ and $\lambda(R):B \to A$. One could show that these functions are quasi-inverse functions of each other: for all $x:A$ and $y:B$ and equivalences $R:A \simeq B$, there are identities

$\rho_\kappa(R, \lambda(R, y), y, \lambda_\tau(R, y)):\rho(R)(\lambda(R)(y)) =_B y$
$\lambda_\kappa(R, x, \rho(R)(x), \rho_\tau(R, x)^{-1}):\lambda(R)(\rho(R)(x)) =_A x$

where $p^{-1}:b =_A a$ is the inverse identity of $p:a =_A b$. By the introduction rule for dependent product types, there are homotopies

$\lambda y.\rho_\kappa(R, \lambda(R, y), y, \lambda_\tau(R, y)):\prod_{y:B} \rho(R)(\lambda(R)(y)) =_B y$
$\lambda x.\lambda_\kappa(R, x, \rho(R)(x), \rho_\tau(R, x)^{-1}):\prod_{x:A} \lambda(R)(\rho(R)(x)) =_A x$

which indicate that $\rho(R)$ and $\lambda(R)$ are quasi-inverse functions of each other.

By the rules for dependent sum types and dependent product types, one could show that the above functions each have contractible fibers, making both of them coherent inverse functions of each other.

### Heterogeneous identity types

Given the definition of the equivalence type as the type of encodings for one-to-one correspondences, the heterogeneous identity type is defined by the rule

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma \vdash a:A \quad \Gamma \vdash b:A \quad \Gamma \vdash p:a =_A b \quad \Gamma, x:A \vdash B \; \mathrm{type}}{\Gamma \vdash (x =_B^p y) \equiv (x =_{B(a), B(b)}^{\mathrm{tr}_B(p)} y) \; \mathrm{type}}$

### Identity equivalences, inverse equivalences, and composition of equivalences

The identity equivalence on a type $A$ is defined as an equivalence $\mathrm{id}_A:A \simeq A$ such that for all elements $a:A$,

$\lambda(\mathrm{id}_A, a) \coloneqq a$
$\rho(\mathrm{id}_A, a) \coloneqq a$
$(a =_{A, A}^{\mathrm{id}_A} b) \coloneqq (a =_A b)$

Given an equivalence $R:A \simeq B$, the inverse equivalence of $R$ is an equivalence $R^{-1}:B \simeq A$ such that for all elements $a:A$ and $b:B$,

$\rho(R^{-1}, a) \coloneqq \lambda(R, a)$
$\lambda(R^{-1}, b) \coloneqq \rho(R, b)$
$b =_{B, A}^{R^{-1}} a \coloneqq a =_{A, B}^R b$

Given equivalences $R:A \simeq B$ and $S:B \simeq C$, the composite of $R$ and $S$ is an equivalence $S \circ R:A \simeq C$ such that for all elements $a:A$ and $c:C$,

$\lambda(S \circ R, a) \coloneqq \lambda(R, \lambda(S, a))$
$\rho(S \circ R, c) \coloneqq \rho(S, \rho(R, c))$
$a =_{A, C}^{S \circ R} c \coloneqq \sum_{b:B} (a =_{A, B}^R b) \times (b =_{B, C}^R c)$

### Relation to universes and univalence

Given a Russell universe $U$, there are two ways to say that types $A:U$ and $B:U$ are equal: by the identity type $A =_U B$, and the equivalence type $A \simeq B$. The univalence axiom says that these two types $A =_U B$ and $A \simeq B$ are the same, which is represented by an equivalence between the two types

$\mathrm{ua}(A, B):(A =_U B) \simeq (A \simeq B)$

For Tarski universes $(U, \mathrm{El})$, one instead says that $A =_U B$ is the same as $\mathrm{El}(A) \simeq \mathrm{El}(B)$, represented as

$\mathrm{ua}(A, B):(A =_U B) \simeq (\mathrm{El}(A) \simeq \mathrm{El}(B))$

### Action on equivalences

We introduce a modal operator $L$ to the type theory, which we assume in general not to be idempotent or monadic; this is given by the formation rule

$\frac{\Gamma \vdash A \; \mathrm{type}}{\Gamma \vdash L(A) \; \mathrm{type}}$

$L$ preserves equivalences: given types $A$ and $B$, there is a function $\mathrm{ae}_L:(A \simeq B) \to L(A) \simeq L(B)$, called the action on equivalences for $L$.

### Categorical semantics

The categorical semantics of an equivalence type is an object of isomorphisms.