Contents

# Contents

## Idea

The interval type is an axiomatization of the cellular interval object in the context of homotopy type theory.

## Definition

As a higher inductive type, the interval is given by

Inductive Interval : Type
| left : Interval
| right : Interval
| segment : Id Interval left right

This says that the type is inductive constructed from two terms in the interval, whose interpretation is as the endpoints of the interval, together with a term in the identity type of paths between these two terms, which interprets as the 1-cell of the interval

$left \stackrel{segment}{\to} right \,.$

### Induction principle

The induction principle for the interval $I$ says that for any $P:I\to Type$ equipped with point $left' : P(left)$ and $right' : P(right)$ and a dependent identification $segment':left'=_P^{segment} right'$, there is $f:\prod_{(x:I)} P(x)$ such that:

$f(left)=left' \qquad f(right)=right' \qquad apd_f(segment) = segment'$

and for every $y:I \vdash g:\prod_{(x:I)} P(x)$ such that

$y:I \vdash g(y)(left)=left' \qquad y:I \vdash g(y)(right)=right' \qquad y:I \vdash apd_{g(y)}(segment) = segment'$

there is an identification $y:I \vdash f = g(y)$.

As a special case, its recursion principle says that given any type $I$ with points $x:X$ and $y:X$ and an identification $p:x=y$, there is $f:I \to X$ with

$f(left)=x\qquad f(right)=y\qquad ap_f(segment)=p$

### Syntax

The interval type is defined by the following rules:

Formation rules for the interval type:

$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathbb{I} \; \mathrm{type}}$

Introduction rules for the interval type:

$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash 0:\mathbb{I}} \qquad \frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash 1:\mathbb{I}} \qquad \frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash p:0 =_\mathbb{I} 1}$

Elimination rules for the interval type:

$\frac{\Gamma, x:\mathbb{I} \vdash C(x) \; \mathrm{type} \quad \Gamma \vdash c_0:C(0) \quad \Gamma \vdash c_1:C(1) \quad \Gamma \vdash c_p:c_0 =_C^p c_1}{\Gamma, x:\mathbb{I} \vdash \mathrm{ind}_\mathbb{I}^C(c_0, c_1, c_p)(x):C(x)}$

Computation rules for the interval type:

$\frac{\Gamma, x:\mathbb{I} \vdash C(x) \; \mathrm{type} \quad \Gamma \vdash c_0:C(0) \quad \Gamma \vdash c_1:C(1) \quad \Gamma \vdash c_p:c_0 =_C^p c_1}{\Gamma \vdash \beta_\mathbb{I}^{0}: \mathrm{ind}_\mathbb{I}^{C}(c_0, c_1, c_p)(0) =_{C(0)} c_0}$
$\frac{\Gamma, x:\mathbb{I} \vdash C(x) \; \mathrm{type} \quad \Gamma \vdash c_0:C(0) \quad \Gamma \vdash c_1:C(1) \quad \Gamma \vdash c_p:c_0 =_C^p c_1}{\Gamma \vdash \beta_\mathbb{I}^{1}:\mathrm{ind}_\mathbb{I}^{C}(c_0, c_1, c_p)(1) =_{C(1)} c_1}$
$\frac{\Gamma, x:\mathbb{I} \vdash C(x) \; \mathrm{type} \quad \Gamma \vdash c_0:C(0) \quad \Gamma \vdash c_1:C(1) \quad \Gamma \vdash c_p:c_0 =_C^p c_1}{\Gamma \vdash \beta_\mathbb{I}^{p}:\mathrm{apd}_C(p, \mathrm{ind}_\mathbb{I}^{C}(c_0, c_1, c_p)) =_{c_0 =_C^p c_1} c_p}$

Uniqueness rules for the interval type:

$\frac{\Gamma, x:\mathbb{I} \vdash C(x) \; \mathrm{type} \quad \Gamma, x:\mathbb{I} \vdash c:C(x)}{\Gamma, x:\mathbb{I} \vdash \eta_\mathbb{I}(c):c =_{C(x)} \mathrm{ind}_\mathbb{I}^{C}(c(0), c(1), \mathrm{apd}_C(p, c))}$

### Using a function from the two-valued type

The interval type is defined by the following rules:

Formation rules for the interval type:

$\frac{\Gamma \; \mathrm{ctx}}{\Gamma \vdash \mathbb{I} \; \mathrm{type}}$

Introduction rules for the interval type:

$\frac{\Gamma \vdash a:\mathbb{2}}{\Gamma \vdash j(a):\mathbb{I}} \qquad \frac{\Gamma \vdash 0:\mathbb{2} \quad \Gamma \vdash 1:\mathbb{2}}{\Gamma \vdash p:j(0) =_\mathbb{I} j(1)}$

Elimination rules for the interval type:

$\frac{\Gamma, x:\mathbb{I} \vdash C \; \mathrm{type} \quad \Gamma, a:\mathbb{2} \vdash c:C[j(a)/x] \quad \Gamma \vdash 0:\mathbb{2} \quad \Gamma \vdash 1:\mathbb{2} \quad \Gamma \vdash c_p:c[j(0)/x] =_C^p c[j(1)/x]}{\Gamma, x:\mathbb{I}, a:\mathbb{2} \vdash \mathrm{ind}_\mathbb{I}^C(c[j(a)/x], c_p):C}$

Computation rules for the interval type:

$\frac{\Gamma, x:\mathbb{I} \vdash C \; \mathrm{type} \quad \Gamma, a:\mathbb{2} \vdash c:C[j(a)/x] \quad \Gamma \vdash 0:\mathbb{2} \quad \Gamma \vdash 1:\mathbb{2} \quad \Gamma \vdash c_p:c[j(0)/x] =_C^p c[j(1)/x]}{\Gamma, a:\mathbb{2} \vdash \beta_\mathbb{I}^{j}:\mathrm{ind}_\mathbb{I}^{C}(c[j(a)/x], c_p)[j(a)/x] =_{C[j(a)/x]} c[j(a)/x]}$
$\frac{\Gamma, x:\mathbb{I} \vdash C \; \mathrm{type} \quad \Gamma, a:\mathbb{2} \vdash c:C[j(a)/x] \quad \Gamma \vdash 0:\mathbb{2} \quad \Gamma \vdash 1:\mathbb{2} \quad \Gamma \vdash c_p:c[j(0)/x] =_C^p c[j(1)/x]}{\Gamma, a:\mathbb{2}, \vdash \beta_\mathbb{I}^{p}:\mathrm{apd}_C^p(\mathrm{ind}_\mathbb{I}^{C}(c[j(a)/x], c_p)) =_{c[j(0)/x] =_C^p c[j(1)/x]} c_p}$

Uniqueness rules for the interval type:

## Properties

• An interval type is provably contractible. Conversely, any contractible type satisfies the rules of an interval type up to typal equality.

• An interval type is a suspension type of the unit type, and the suspension of an interval type is a 2-globe type.

• An interval type is a cone type of the unit type.

• An interval type is a cubical type? $\Box^1$.

### Relation to identity types

Every identity $q:a =_A b$ between two terms $a:A$ and $b:A$ of a type $A$ has an associated term in context $x:\mathbb{I} \vdash f(x):A$, inductively defined by

• $\beta_f^0:f(0) =_A a$
• $\beta_f^1:f(1) =_A b$
• $\beta_f^p:\mathrm{ap}_f(p) =_{f(0) =_A f(1)} \mathrm{concat}_{f(0), b, f(1)}(\mathrm{concat}_{f(0), a, b}(\beta_f^0, q), \mathrm{inv}_{f(1), b}(\beta_f^1))$

where

$\mathrm{ap}_f:(0 =_\mathbb{I} 1) \to (f(0) =_A f(1))$
$\mathrm{concat}_{a, b, c}:(a =_A b) \times (b =_A c) \to (a =_A c)$

is concatenation of identities (i.e. transitivity), and

$\mathrm{inv}_{a, b}:(a =_A b) \to (b =_A a)$

is the inverse of identities (i.e. symmetry).

Conversely, given a function $f:\mathbb{I} \to A$ and terms $a:A$ and $b:A$ with identities $\delta_a:f(0) =_A a$ and $\delta_b:f(1) =_A b$, there is an identity

$\mathrm{concat}_{a, f(1), b}(\mathrm{concat}_{a, f(0), f(1)}(\mathrm{inv}_{f(0), a}(\delta_a), \mathrm{ap}_{f}(p)), \delta_b):a =_{A} b$

### Relation to dependent identity types

Given a type $A$, a dependent type $x:A \vdash B$, terms $a_0:A$ and $a_1:A$, identity $q:a_0 =_A a_1$, terms $b_0:B[a_0/x]$ and $b_1:B[a_1/x]$, and dependent identity $r:b_0 =_B^q b_1$, let us inductively define the family of elements $x:\mathbb{I} \vdash f(x):B(x)$ by

• $\beta_f^0:f(0) =_{B[a_0/x]} b_0$
• $\beta_f^1:f(1) =_{B[a_1/x]} b_1$
• $\beta_f^p:\mathrm{apd}_f(p) =_{f(0) =_B^q f(1)} \mathrm{concat}_{\mathrm{trans}_B^q(f(0)), b_1, f(1)}(\mathrm{concat}_{\mathrm{trans}_B^q(f(0)), \mathrm{trans}_B^q(b_0), b_1}(\mathrm{apd}_{\mathrm{trans}_B^q}(\beta_f^0), r), \mathrm{inv}_{f(1), b}(\beta_f^1))$

where $\mathrm{trans}_B^q:B[a_0/x] \to B[a_1/x]$ is transport, $\mathrm{ap}_f:(0 =_\mathbb{I} 1) \to (f(0) =_A f(1))$ is the function application to identities, $\mathrm{concat}_{a, b, c}:(a =_A b) \times (b =_A c) \to (a =_A c)$ is concatenation of identities (i.e. transitivity), and $\mathrm{inv}_{a, b}:(a =_A b) \to (b =_A a)$ is the inverse of identities (i.e. symmetry).

### Relation to function extensionality

Postulating an interval type with judgmental computation rules for the point constructors of the interval type implies function extensionality. (Shulman).

The proof assumes a typal uniqueness rule for function types. First it constructs a function $k:A \to (\mathbb{I} \to B)$ from a dependent function $h:\prod_{x:A} f(x) =_B g(x)$, inductively defined by

• $\beta_{k(x)}^0:k(x)(0) =_B f(x)$
• $\beta_{k(x)}^1:k(x)(1) =_B g(x)$
• $\beta_{k(x)}^p:\mathrm{ap}_{k(x)}(p) =_{k(x)(0) =_B k(x)(1)} \mathrm{concat}_{k(x)(0), f(x), k(x)(1)}(\mathrm{concat}_{k(x)(0), f(x), g(x)}(\beta_{k(x)}^0, h(x)), \mathrm{inv}_{k(x)(1), g(x)}(\beta_{k(x)}^1))$

Then it uses the properties of function types, product types, currying, uncurrying, and the symmetry of products $A \times B \simeq B \times A$, to construct a function $k':\mathbb{I} \to (A \to B)$, inductively defined by

• $\beta_{k'}^0(x):k'(0)(x) =_B f(x)$
• $\beta_{k'}^1(x):k'(1)(x) =_B g(x)$
• $\beta_{k'}^p(x):\mathrm{ap}_{k'}(p)(x) =_{k'(0)(x) =_B k'(1)(x)} \mathrm{concat}_{k'(0)(x), f(x), k'(1)(x)}(\mathrm{concat}_{k'(0)(x), f(x), g(x)}(\beta_{k'}^0(x), h(x)), \mathrm{inv}_{k'(1)(x), g(x)}(\beta_{k'}^1(x)))$

If the interval type has judgmental computation rules for the point constructors, then $k'(x)(0) \equiv f(x)$ and $k'(x)(1) \equiv g(x)$ for all $x:A$, which implies that $k'(0)(x) \equiv f(x)$ and $k'(1)(x) \equiv g(x)$ for all $x:A$, and subsequently that $k'(0) \equiv f$ and $k'(1) \equiv g$. This means that there are identities $\beta_{k'}^0:k'(0) =_{A \to B} f$ and $\beta_{k'}^1:k'(1) =_{A \to B} g$, and an identity

$\mathrm{concat}_{f, k'(1), g}(\mathrm{concat}_{f, k'(0), k'(1)}(\mathrm{inv}_{k'(0), f}(\beta_{k'}^0), \mathrm{ap}_{k'}(p)), \beta_{k'}^1):f =_{A \to B} g$

thus proving function extensionality.

An interval type with only typal computation rules for the point constructors does not imply function extensionality. This is because the proof with the judgmental computation rules uses the fact that $k'(0)(x) \equiv f(x)$ and $k'(1)(x) \equiv g(x)$ for all $x:A$ implies that $k'(0) \equiv f$ and $k'(1) \equiv g$. However, if the computation rules are typal, then the equivalent statement is that having identities $\beta_{k'}^0(x):k'(0)(x) =_B f(x)$ and $\beta_{k'}^1(x):k'(1)(x) =_B g(x)$ for all $x:A$ implies that there are identities $\beta_{k'}^0:k'(0) =_{A \to B} f$ and $\beta_{k'}^1:k'(1) =_{A \to B} g$, which is precisely function extensionality, and so cannot be used to prove function extensionality.

### Relation to propositional truncations

An interval type is the propositional truncation of the two-valued type $\mathbb{2}$. We use the definition of an interval type using a function from $\mathbb{2}$. Since the interval type has identities

• $\mathrm{refl}_\mathbb{I}(j(0)):j(0) =_\mathbb{I} j(0)$,
• $p:j(0) =_\mathbb{I} j(1)$,
• $\mathrm{inv}_{j(0), j(1)}(p):j(1) =_\mathbb{I} j(0)$,
• $\mathrm{refl}_\mathbb{I}(j(1)):j(1) =_\mathbb{I} j(1)$,

there is a dependent function

$f:\prod_{a:\mathbb{2}} \prod_{b:\mathbb{2}} j(a) =_\mathbb{I} j(b)$

inductively defined by the identities

$\beta_{f}(0, 0):f(0)(0) =_{j(0) =_\mathbb{I} j(0)} \mathrm{refl}_\mathbb{I}(j(0))$
$\beta_{f}(0, 1):f(0)(1) =_{j(0) =_\mathbb{I} j(1)} p$
$\beta_{f}(1, 0):f(1)(0) =_{j(1) =_\mathbb{I} j(0)} \mathrm{inv}_{j(0), j(1)}(p)$
$\beta_{f}(1, 1):f(1)(1) =_{j(1) =_\mathbb{I} j(1)} \mathrm{refl}_\mathbb{I}(j(1))$

If dependent product types have judgmental computation rules, then the above becomes

$f(0)(0) \equiv \mathrm{refl}_\mathbb{I}(j(0)):j(0) =_\mathbb{I} j(0)$
$f(0)(1) \equiv p:j(0) =_\mathbb{I} j(1)$
$f(1)(0) \equiv \mathrm{inv}_{j(0), j(1)}(p):j(1) =_\mathbb{I} j(0)$
$f(1)(1) \equiv \mathrm{refl}_\mathbb{I}(j(1)):j(1) =_\mathbb{I} j(1)$

Both show that the interval type is the propositional truncation of the two-valued type.

The converse is true as well: the propositional truncation of the two-valued type is the interval type. Recall that $\left[\mathbb{2}\right]$ is inductively generated by a function $j:\mathbb{2} \to \left[\mathbb{2}\right]$ and a dependent function

$f:\prod_{a:\mathbb{2}} \prod_{b:\mathbb{2}} j(a) =_{\left[\mathbb{2}\right]} j(b)$

which makes $\left[\mathbb{2}\right]$ into an h-proposition. By definition of an h-proposition, for each element $a:\mathbb{2}$ and $b:\mathbb{2}$, the identity type $j(a) =_{\left[\mathbb{2}\right]} j(b)$ is a contractible type. In particular, by induction on $\mathbb{2}$, this means that there are identities

$\beta_{f}(0, 0):f(0)(0) =_{j(0) =_{\left[\mathbb{2}\right]} j(0)} \mathrm{refl}_{\left[\mathbb{2}\right]}(j(0))$
$\beta_{f}(1, 1):f(1)(1) =_{j(1) =_{\left[\mathbb{2}\right]} j(1)} \mathrm{refl}_{\left[\mathbb{2}\right]}(j(1))$

and for every identity $p:0 =_{\left[\mathbb{2}\right]} 1$, there are identities

$\beta_{f}(0, 1):f(0)(1) =_{j(0) =_{\left[\mathbb{2}\right]} j(1)} p$
$\beta_{f}(1, 0):f(1)(0) =_{j(1) =_{\left[\mathbb{2}\right]} j(0)} \mathrm{inv}_{j(0), j(1)}(p)$

Thus, one could simply take $j(0)$ and $j(1)$ as the term constructors and $f(0)(1)$ as the identity constructor of the interval type.

If both propositional truncations and the two-valued type have judgmental computation rules, the the interval type also has judgmental computation rules. See (this file)

Proofs of function extensionality using an interval type with judgmental computation rules for point constructors could be found here

• Carlo Angiuli, Univalence implies function extensionality (blog, pdf)