# nLab transport

Contents

This page is about the notion in homotopy type theory. For parallel transport via connections in differential geometry see there. For the relation see below.

# Contents

## Idea

Gottfried Leibniz said (in translation from Lewis 1918, p. 373 with original Latin terms in parenthesis; see also Cartwright 1971, p. 119 and Gries & Schneider 1998)

Two terms are the same (eadem) if one can be substituted for the otherwithout altering the truth of any statement (salva veritate). If we have $P$ and $Q$, and $P$ enters into some true proposition, and the substitution of $Q$ for $P$ wherever it appears results in a new proposition that is likewise true, and if this can be done for every proposition, then $P$ and $Q$ are said to be the same; conversely, if $P$ and $Q$ are the same, they can be substituted for one another.

The converse mentioned at the end of the last sentence is known as the principle of substitution or the indiscernibility of identicals. In any simply typed first-order theory, given a predicate $P$ on $T$, this principle is formalized as the following

$\forall_{x,y\colon T} (x =_T y) \implies (P(x) \iff P(y))$

In the interpretation of propositions as types in type theory, propositions are interpreted as types, and the above statement has a generalization from the types which are propositions to all types: the universal quantifier becomes a dependent product type, the predicate becomes a type family, implication becomes an a function type, and logical equivalence of propositions becomes an equivalence type. This results in what is known as transport in type theory, which for the type $T$ and the type family $x:T \vdash P(x)$ results in the dependent function:

(1)$\mathrm{tr}_P \;\colon\; \prod_{x,y:T} (x =_T y) \to (P(x) \simeq P(y)) \,.$

## Definitions

### Using the uniqueness quantifier

Assuming that the type theory has uniqueness quantifiers, a type $A$, a family of types $x:A \vdash B(x)$, elements $a:A$ and $b:A$, and an identity $p:a =_A b$, there is a family of elements between the types $B(a)$ and $B(b)$ called transport

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

and a witness

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

that for all $y:B(b)$ there exists a unique elemnt $x:B(a)$ up to identity such that $\mathrm{tr}_B(a, b, p)(x) =_{B(b)} y$.

If one is inductively defining transport, then one additionally has to add that

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma \vdash a:A}{\Gamma, x:B(a) \vdash \beta_B^{\mathrm{tr}}(a):\mathrm{tr}_B(a, a, \mathrm{refl}_A(a), x) =_{B(a)} x}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma \vdash a:A}{\Gamma, x:B(a) \vdash \beta_B^{u}(x):u(\mathrm{tr}_B(a, a, \mathrm{refl}_A(a), x)) =_{\exists!y:B(a).(-) =_{B} y}^{\beta_B^{\mathrm{tr}}(a)} u(x)}$

which specifies how transport across reflexivity behaves.

Alternatively, one could specify that there is an equivalence between the dependent identity type $x =_B^p y$ and the identity type $\mathrm{tr}_B(a, b, p, x) =_{B(b)} y$

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

One then specifies how transport across reflexivity behaves by using the dependent function application to reflexivity.

### Using equivalence types

There are multiple notions of equivalence types in dependent type theory, which can be used for a definition of univalence for a type universe $U$; these include

Thus, given a notion of equivalence type $\simeq$, a type $A$, a family of types $x:A \vdash B(x)$, elements $a:A$ and $b:A$, and an identity $p:a =_A b$, there is an equivalence between the types $B(a)$ and $B(b)$ called transport

$\mathrm{tr}_B(a, b, p):B(a) \simeq B(b)$

The transport equivalence is called judgmentally strict transport if the equivalence type is a judgmentally strict equivalence type, and similarly it is called propositionally strict transport if the equivalence type is a propositionally strict equivalence type. It is called weak transport if the equivalence type is a weak equivalence type.

### As an introduction rule for the equivalence type

In some presentations of the equivalence type, transport is formed by one of the introduction rules for the equivalence type:

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

For the inductive definition of transport, there are also computation rules given by

$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma \vdash a:A}{\Gamma \vdash \mathrm{tr}_B(a, a, \mathrm{refl}_A(a)) \equiv \mathrm{id}_{B(a)}:B(a) \simeq B(a)}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma \vdash a:A}{\Gamma \vdash \mathrm{tr}_B(a, a, \mathrm{refl}_A(a)) \equiv_{B(a) \simeq B(a)} \mathrm{id}_{B(a)} \; \mathrm{true}}$
$\frac{\Gamma \vdash A \; \mathrm{type} \quad \Gamma, x:A \vdash B(x) \; \mathrm{type} \quad \Gamma \vdash a:A}{\Gamma \vdash \beta_B^{\mathrm{tr}}(a):\mathrm{tr}_B(a, a, \mathrm{refl}_A(a)) =_{B(a) \simeq B(a)} \mathrm{id}_{B(a)}}$

depending on whether the the computation rules for identity types use judgmental equality, propositional equality, or the identity type.

Alternatively, one has the dependent function application to identities for the computation rules for transport:

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

depending on whether the equivalence type uses judgmental equality, propositional equality, or the identity type in its computation rules.

## Examples and applications

### Univalent Tarski universes

Tarski universes are types which behave like internal models of dependent type theory inside of the type theory itself. We use a very general notion of Tarski universe: a Tarski universe merely consists of a type of encodings for types $\mathcal{U}$ and a universal type family $\mathcal{T}_\mathcal{U}$. Given an encoding $A:\mathcal{U}$, an internal type family indexed by $A$ is a function $B:\mathcal{T}_\mathcal{U}(A) \to \mathcal{U}$.

There are two ways to say that $A:\mathcal{U}$ and $B:\mathcal{U}$ are the same, by way of the identity type of the Tarski universe $A =_\mathcal{U} B$, and by way of the type of equivalences between the type reflections of $A:\mathcal{U}$ and $B:\mathcal{U}$, $\mathcal{T}_\mathcal{U}(A) \simeq \mathcal{T}_\mathcal{U}(B)$. By transport, there is a canonical function

$\mathrm{trans}^{\mathcal{T}_\mathcal{U}}(A, B):(A =_\mathcal{U} B) \to (\mathcal{T}_\mathcal{U}(A) \simeq \mathcal{T}_\mathcal{U}(B))$

The Tarski universe $\mathcal{U}$ is then said to be a univalent universe if the transport function $\mathrm{trans}^{\mathcal{T}_\mathcal{U}}(A, B)$ is an equivalence of types

$\mathrm{trans}^{\mathcal{T}_\mathcal{U}}(A, B):(A =_\mathcal{U} B) \simeq (\mathcal{T}_\mathcal{U}(A) \simeq \mathcal{T}_\mathcal{U}(B))$

for all encodings $A:\mathcal{U}$ and $B:\mathcal{U}$.

One could also internalize the type of equivalences inside of the Tarski universe $\mathcal{U}$, but that requires the Tarski universe be closed under identity types, dependent product types, and dependent sum types.

A Tarski universe $\mathcal{U}$ is weakly closed under identity types if it has an internal encoding of identity types in the Tarski universe: Given an element $A:\mathcal{U}$, there is a function $\mathrm{Id}_A^\mathcal{U}:\mathcal{T}_\mathcal{U}(A) \times \mathcal{T}_\mathcal{U}(A) \to \mathcal{U}$ and for all elemments $a:\mathcal{T}_\mathcal{U}(A)$ and $b:\mathcal{T}_\mathcal{U}(A)$ an equivalence

$\mathrm{canonical}_{\mathrm{Id}_A}(a, b):\mathcal{T}_\mathcal{U}(\mathrm{Id}_A^\mathcal{U}(a, b)) \simeq (a =_{\mathcal{T}_\mathcal{U}(A)} b)$

A Tarski universe $\mathcal{U}$ is weakly closed under dependent product types if it has an internal encoding of dependent product types in the Tarski universe: given an element $A:\mathcal{U}$ and a function $B:\mathcal{T}_\mathcal{U}(A) \to \mathcal{U}$, there is an element $\Pi_\mathcal{U}(x:A).B(x):\mathcal{U}$, and an equivalence

$\mathrm{canonical}_\Pi(A, B):\mathcal{T}_\mathcal{U}(\Pi_\mathcal{U}(x:A).B(x)) \simeq \prod_{x:\mathcal{T}_\mathcal{U}(A)} \mathcal{T}_\mathcal{U}(B(x))$

A Tarski universe $\mathcal{U}$ is weakly closed under dependent sum types if it has an internal encoding of dependent sum types in the Tarski universe: given an element $A:\mathcal{U}$ and a function $B:\mathcal{T}_\mathcal{U}(A) \to \mathcal{U}$, there is an element $\Sigma_\mathcal{U}(x:A).B(x):\mathcal{U}$, and an equivalence

$\mathrm{canonical}_\Sigma(A, B):\mathcal{T}_\mathcal{U}(\Sigma_\mathcal{U}(x:A).B(x)) \simeq \sum_{x:\mathcal{T}_\mathcal{U}(A)} \mathcal{T}_\mathcal{U}(B(x))$

Every Tarski universe $\mathcal{U}$ which is weakly closed under identity types, dependent product types, and dependent sum types types has an internal encoding of type of equivalences $A \simeq_\mathcal{U} B$, defined for all $A:\mathcal{U}$ and $B:\mathcal{U}$ as

$A \simeq_\mathcal{U} B \coloneqq \Sigma_\mathcal{U}(f:A \to_\mathcal{U} B).\mathrm{isEquiv}_\mathcal{U}(f)$

with $\mathrm{isEquiv}_\mathcal{U}$ defined by whatever suitable definition of equivalence in homotopy type theory.

Thus, there is a third way to say that $A:\mathcal{U}$ and $B:\mathcal{U}$ are the same, by way of the type reflection of the internal encoding of the type of equivalences: $\mathcal{T}_\mathcal{U}(A \simeq_\mathcal{U} B)$. It could be proven, from the closure of the universe under identity types, dependent product types, and dependent sum types, that there is an equivalence

$\mathrm{canonical}_\simeq(A, B):\mathcal{T}_\mathcal{U}(A \simeq_\mathcal{U} B) \simeq (\mathcal{T}_\mathcal{U}(A) \simeq \mathcal{T}_\mathcal{U}(B))$

There is a more common definition of a univalent Tarski universe: a Tarski universe $\mathcal{U}$ is a univalent universe if the canonical function

$\mathrm{idtoequiv}(A, B):(A =_\mathcal{U} B) \to \mathcal{T}_\mathcal{U}(A \simeq_\mathcal{U} B)$

is an equivalence

$\mathrm{idtoequiv}(A, B):(A =_\mathcal{U} B) \simeq \mathcal{T}_\mathcal{U}(A \simeq_\mathcal{U} B)$

These two definitions of univalent universes are the same. In order to show that the two definitions are the same, we need to show that there is an identification

$i(p):\mathrm{canonical}_\simeq(A, B)(\mathrm{idtoequiv}(A, B)(p)) =_{\mathcal{T}_\mathcal{U}(A) \simeq \mathcal{T}_\mathcal{U}(B)} \mathrm{trans}^{\mathcal{T}_\mathcal{U}}(A, B)(p)$

for all identifications $p:A =_\mathcal{U} B$. By the J rule it is enough to show that $\mathrm{canonical}_\simeq(A, A)$ maps the identity function of $\mathcal{T}_\mathcal{U}(A \to_\mathcal{U} A)$ to the identity function of $\mathcal{T}_\mathcal{U}(A) \to \mathcal{T}_\mathcal{U}(A)$. Since the identity function in $\mathcal{T}_\mathcal{U}(A \to_\mathcal{U} A)$ is just $\mathrm{canonical}_\simeq^{-1}(A, A)(id_A)$ the above statement is always true. Thus, the second definition of a univalent universe is equivalent to the definition by transport.

These results apply to strictly Tarski universes as well, which are strictly closed rather than weakly closed (the equivalences become definitional equality): since $A \equiv B$ implies that $A \simeq B$ for types $A$ and $B$, every strictly Tarski universe is weakly Tarski.

### Relation to parallel transport

###### Remark

(relation to parallel transport – dcct §3.8.5, ScSh12 §3.1.2)
In cohesive homotopy type theory the shape modality $\esh$ has the interpretation of turning any cohesive type $X$ into its path $\infty$-groupoid $\esh X$: The 1-morphisms $p \colon (x =_{\esh X} y)$ of $\esh X$ have the interpretation of being (whatever identities existed in $X$ composed with) cohesive (e.g. continuous or smooth) paths in $X$, and similarly for the higher order paths-of-paths.

Accordingly, an $\esh X$-dependent type $B$ has the interpretation of being a “local system” of $B$-coefficients over $X$, namely a $B(x)$-fiber $\infty$-bundle equipped with a flat $\infty$-connection.

In this case, the identity transport (1) along paths in $\esh X$ has the interpretation of being the parallel transport (in the original sense of differential geometry) with respect to this flat $\infty$-connection (and the higher parallel transport when applied to paths-of-paths).

The traditional notion of the “transport” in algebraic topology (via the fundamental theorem of covering spaces and more generally via the notion of Serre fibrations):

The notion in homotopy type theory:

Implementation in Agda:

Leibniz’s original paragraph (from an unpublished manuscript probably written after 1683) is reproduced in the Latin Original in

• K. Gerhard (ed.), Section XIX, p. 228 in: Die philosophischen Schriften von Gottfried Wilhelm Leibniz, Siebenter Band, Weidmannsche Buchhandlung (1890) [archive.org]

and in English translation in:

• Clarence I. Lewis, Appendix (p. 373) of: A Survey of Symbolic Logic, University of California (1918) [pdf]

and further discussed in:

• Richard Cartwright, Identity and Substitutivity, p. 119-133 of: Milton Munitz (ed.) Identity and Individuation, New York University Press (1971) [pdf]

• David Gries, Fred Schneider, Formalizations Of Substitution Of Equals For Equals (1998) [pdf, ecommons:1813/7340

The understanding of transport in HoTT as expressing Leibniz‘s principle of “indiscernibility of identicals” (aka “substitution of equals”, “substitutivity”) has been made explicit in:

The converse implication of path induction from transport (together with the uniqueness principle for id-types) is made explicit in:

For the role of transport in defining an equivalent notion of univalence in weakly Tarski universes, see:

• Madeleine Birchfield, Valery Isaev, Univalence for weakly Tarski universes, MathOverflow, (web)