Contents

Contents

Idea

Type theory and certain kinds of category theory are closely related. By a syntax-semantics duality one may view type theory as a formal syntactic language or calculus for category theory, and conversely one may think of category theory as providing semantics for type theory. The flavor of category theory used depends on the flavor of type theory; this also extends to homotopy type theory and certain kinds of (∞,1)-category theory.

Overview

Theorems

We discuss here formalizations and proofs of the relation/equivalence between various flavors of type theories and the corresponding flavors of categories.

First-order logic and hyperdoctrines

(Seely, 1984a)

Dependent type theory and locally cartesian closed categories

We discuss here how dependent type theory is the syntax of which locally cartesian closed categories provide the semantics. For a dedicated discussion of this (and the subtle coherence issues involved) see also at categorical model of dependent types.

This was originally claimed as an equivalence of categories (Seely, theorem 6.3). However, that argument did not properly treat a subtlety central to the whole subject: that substitution of terms for variables composes strictly, while its categorical semantics by pullback is by the very nature of pullbacks only defined up to isomorphism. This problem was pointed out and ways to fix it were given in (Curien) and (Hofmann); see categorical model of dependent types for the latter. However, the full equivalence of categories was not recovered until (Clairambault-Dybjer) solved both problems by promoting the statement to an equivalence of 2-categories, see also (Curien-Garner-Hofmann). Another approach to this which also works with intensional identity types and hence with homotopy type theory is in (Lumsdaine-Warren 13).

We now indicate some of the details.

Type theories

For definiteness, self-containedness and for references below, we say what a dependent type theory is, following (Seely, def. 1.1).

Definition A Martin-Löf dependent type theory T T is a theory with some signature of dependent function symbols with values in types and in terms (…) subject to the following rules type formation rules 1 1 is a type (the unit type); if a , b a, b are terms of type A A , then ( a = b ) (a = b) is a type (the equality type); if A A and B [ x ] B[x] are types, B B depending on a free variable of type A A , then the following symbols are types ∏ a : A B [ a ] \prod_{a : A} B[a] (dependent product), written also ( A → B ) (A \to B) if B [ x ] B[x] in fact does not depend on x x ; ∑ a : A B [ a ] \sum_{a : A} B[a] (dependent sum), written also A × B A \times B if B [ x ] B[x] in fact does not depend on x x ; term formation rules * ∈ 1 * \in 1 is a term of the unit type; (…) equality rules (…)

Category of contexts

Definition Given a dependent type theory T T , its category of contexts Con ( T ) Con(T) is the category whose objects are the types of T T ;

morphisms f : A → B f : A \to B are the terms f f of function type A → B A \to B . Composition is given in the evident way.

Proposition Con ( T ) Con(T) has finite limits and is a cartesian closed category.

(Seely, prop. 3.1)

Proof Constructions are straightforward. We indicated some of them. Notice that all finite limits (as discussed there) are induced as soon as there are all pullbacks and equalizers. A pullback in Con ( T ) Con(T) P → A ↓ ↓ f B → g C \array{ P &\to& A \\ \downarrow && \downarrow^{\mathrlap{f}} \\ B &\stackrel{g}{\to}& C } is given by P ≃ ∑ a : A ∑ b ∈ B ( f ( a ) = g ( b ) ) . P \simeq \sum_{a : A} \sum_{b \in B} (f(a) = g(b)) \,. The equalizer P → A → g → f B P \to A \stackrel{\overset{f}{\to}}{\underset{g}{\to}} B is given by P = ∑ a : A ( f ( a ) = g ( a ) ) . P = \sum_{a : A} (f(a) = g(a)) \,. Next, the internal hom/exponential object is given by function type [ A , B ] ≃ ( A → B ) . [A,B] \simeq (A \to B) \,.

Proposition Con ( T ) Con(T) is a locally cartesian closed category.

(Seely, theorem 3.2)

Proof Define the Con ( T ) Con(T) -indexed hyperdoctrine P ( T ) P(T) by taking for A ∈ Con ( T ) A \in Con(T) the category P ( T ) ( A ) P(T)(A) to have as objects the A A -dependent types and as morphisms ( a : A ⊢ X ( a ) : type ) → ( a : A ⊢ Y ( a ) : type ) (a : A \vdash X(a) : type) \to (a : A \vdash Y(a) : type) the terms of dependent function type ( a : A ⊢ t : ( X ( a ) → Y ( a ) ) ) (a : A \vdash t : (X(a) \to Y(a))) . This is cartesian closed by the same kind of argument as in the previous proof. It is now sufficient to exhibit a compatible equivalence of categories with the slice category Con ( T ) / A Con(T)_{/A} . Con ( T ) / A ≃ P ( T ) ( A ) . Con(T)_{/A} \simeq P(T)(A) \,. In one direction, send a morphism f : X → A f : X \to A to the dependent type a : A ⊢ f − 1 ( a ) ≔ ∑ x : X ( a = f ( x ) ) . a : A \vdash f^{-1}(a) \coloneqq \sum_{x : X} (a = f(x)) \,. Conversely, for a : A ⊢ X ( a ) a : A \vdash X(a) a dependent type, send it to the projection ∑ a : A X ( a ) → A \sum_{a : A} X(a) \to A . One shows that this indeed gives an equivalence of categories which is compatible with base change (Seely, prop. 3.2.4).

Definition For T T a dependent type theory and C C a locally cartesian closed category, an interpretation of T T in C C is a morphism of locally cartesian closed categories Con ( T ) → C . Con(T) \to C \,. An interpretation of T T in another dependent type theory T ′ T' is a morphism of locally cartesian closed categories Con ( T ) → Con ( T ′ ) . Con(T) \to Con(T') \,.

Internal language

Proposition Given a locally cartesian closed category C C , define the corresponding dependent type theory Lang ( C ) Lang(C) as follows the non-dependent types of Lang ( C ) Lang(C) are the objects of C C ;

the A A -dependent types are the morphisms B → A B \to A ;

a context x 1 : X 1 , x 2 : X 2 , ⋯ , x n : X n x_1 : X_1 , x_2 : X_2, \cdots , x_n : X_n is a tower of morphisms X n ↓ ⋯ ↓ X 2 ↓ X 1 \array{ X_n \\ \downarrow \\ \cdots \\ \downarrow \\ X_2 \\ \downarrow \\ X_1 }

the terms t [ x A ] : B [ x A ] t[x_A] : B[x_A] are the sections A → B A \to B in C / A C_{/A}

the equality type ( x A = y A ) (x_A = y_A) is the diagonal A → A × A A \to A \times A

…

Homotopy type theory and locally cartesian closed ( ∞ , 1 ) (\infty,1) -categories

All of the above has an analog in (∞,1)-category theory and homotopy type theory.

Some form of this statement was originally formally conjectured in (Joyal 11), following (Awodey 10). For more details see at locally cartesian closed (∞,1)-category.

Univalent homotopy type theory and elementary ( ∞ , 1 ) (\infty,1) -toposes

More precise information can be found on the homotopytypetheory wiki.

A (locally presentable) locally Cartesian closed (∞,1)-category (as above) which in addition has a system of object classifiers is an ((∞,1)-sheaf-)(∞,1)-topos.

It has been conjectured in (Awodey 10) that this object classifier is the categorical semantics of a univalent type universe (type of types), hence that homotopy type theory with univalence has categorical semantics in (∞,1)-toposes. This statement was proven for the canonical ( ∞ , 1 ) (\infty,1) -topos ∞Grpd in (Kapulkin-Lumsdaine-Voevodsky 12), and more generally for (∞,1)-presheaf ( ∞ , 1 ) (\infty,1) -toposes over elegant Reedy categories in (Shulman 13).

In these proofs the type-theoretic model categories which interpret the homotopy type theory syntax are required to provide type universes that behave strictly under pullback. This matches the usual syntactically convenient universes in type theory (either a la Russell or a la Tarski), but more difficult to implement in the categorical semantics. More flexibly, one may consider syntactic type universes weakly à la Tarski (Luo 12, Gallozzi 14). These are more complicated to work with syntactically, but should have interpretations in a (type-theoretic model categories presenting) any (∞,1)-topos. Discussion of univalence in this general flexible sense is in (Gepner-Kock 12). For the general syntactic issue see at

While (∞,1)-sheaf (∞,1)-toposes are those currently understood, the basic type theory with univalent universes does not see or care about their local presentability as such (although it is used in other places, such as the construction of higher inductive types). It is to be expected that there is a decent concept of elementary (∞,1)-topos such that homotopy type theory with univalent type universes and some supply of higher inductive types has categorical semantics precisely in elementary (∞,1)-toposes (as conjectured in Awodey 10). But the fine-tuning of this statement is currently still under investigation.

Notice that this statement, once realized, makes (or would make) Univalent HoTT+HITs a sort of homotopy theoretic refinement of foundations of mathematics in topos theory as proposed by William Lawvere. It could be compared to his elementary theory of the category of sets, although being a type theory rather than a theory in first-order logic, it is more analogous to the internal type theory of an elementary topos.

Related concepts

References

An elementary exposition of in terms of the Haskell programming language is in

The equivalence of categories between first order theories and hyperdoctrines is discussed in

R. A. G. Seely, Hyperdoctrines, natural deduction, and the Beck condition, Zeitschrift für Math. Logik und Grundlagen der Math. (1984) (pdf)

The categorical model of dependent types and initiality is discussed in

Simon Castellan, Dependent type theory as the initial category with families, 2014 (pdf)

which was formalized inside type theory with set quotients of higher inductive types in:

Thorsten Altenkirch, Ambrus Kaposi, Type Theory in Type Theory using Quotient Inductive Types, (2015) (pdf), (formalisation in Agda).

Surveys inclue

Tom Hirschowitz, Introduction to categorical logic (2010) (pdf) (see the discussion building up to the theorem on slide 96)

Roy Crole, Deriving category theory from type theory, Theory and Formal Methods 1993 Workshops in Computing 1993, pp 15-26

Maria Maietti, Modular correspondence between dependent type theories and categories including pretopoi and topoi, Mathematical Structures in Computer Science archive Volume 15 Issue 6, December 2005 Pages 1089 - 1149 (pdf)

The equivalence between linear logic and star-autonomous categories is due to

and reviews/further developments are in

G. M. Bierman, What is a Categorical Model of Intuitionistic Linear Logic? (web)

Andrew Graham Barber, Linear Type Theories, Semantics and Action Calculi, 1997 (web, pdf)

Paul-André Melliès , Categorial Semantics of Linear Logic, in Interactive models of computation and program behaviour, Panoramas et synthèses 27, 2009 (pdf)

For dependent linear type theory see

Matthijs Vákár, Syntax and Semantics of Linear Dependent Types (arXiv:1405.0033)

An adjunction between the category of type theories with product types and toposes is discussed in chapter II of

Joachim Lambek, P. Scott, Introduction to higher order categorical logic, Cambridge University Press (1986) .

The equivalence of categories between locally cartesian closed categories and dependent type theories was originally claimed in

R. A. G. Seely, Locally cartesian closed categories and type theory, Math. Proc. Camb. Phil. Soc. (1984) 95 (pdf)

following a statement earlier conjectured in

Per Martin-Löf, An intuitionistic theory of types: predicative part, In Logic Colloquium (1973), ed. H. E. Rose and J. C. Shepherdson (North-Holland, 1974), 73-118. (web)

The problem with strict substitution compared to weak pullback in this argument was discussed and fixed in

Pierre-Louis Curien, Substitution up to isomorphism, Fundamenta Informaticae, 19(1,2):51–86 (1993)

Martin Hofmann, On the interpretation of type theory in locally cartesian closed categories, Proc. CSL ‘94, Kazimierz, Poland. Jerzy Tiuryn and Leszek Pacholski, eds. Springer LNCS, Vol. 933 (CiteSeer)

but in the process the equivalence of categories was lost. This was finally all rectified in

Pierre Clairambault, Peter Dybjer, The Biequivalence of Locally Cartesian Closed Categories and Martin-Löf Type Theories, in Typed lambda calculi and applications, Lecture Notes in Comput. Sci. 6690, Springer 2011 (arXiv:1112.3456)

and

Another version of this which also applies to intensional identity types and hence to homotopy type theory is in

The analogous statement relating homotopy type theory and locally cartesian closed (infinity,1)-categories was formally conjectured around

André Joyal, Remarks on homotopical logic, Oberwolfach (2011) (pdf)

following earlier suggestions by Steve Awodey. Explicitly, the suggestion that with the univalence axiom added this is refined to (∞,1)-topos theory appears around

Steve Awodey, Type theory and homotopy (pdf)

Details on this higher categorical semantics of homotopy type theory are in

Michael Shulman, Univalence for inverse diagrams and homotopy canonicity, Mathematical Structures in Computer Science, Volume 25, Issue 5 ( From type theory and homotopy theory to Univalent Foundations of Mathematics ) June 2015 (arXiv:1203.3253, doi:/10.1017/S0960129514000565)

with lecture notes in

Mike Shulman, Categorical models of homotopy type theory, April 13, 2012 (pdf)

André Joyal, Remarks on homotopical logic, Oberwolfach (2011) (pdf)

André Joyal, Categorical homotopy type theory, March 17, 2014 (pdf)

See also

Models specifically in (constructive) cubical sets are discussed in

Marc Bezem, Thierry Coquand, Simon Huber, A model of type theory in cubical sets, 2013 (web, pdf)

Ambrus Kaposi, Thorsten Altenkirch, A syntax for cubical type theory (pdf)

Simon Docherty, A model of type theory in cubical sets with connection, 2014 (pdf)

A precise definition of elementary (infinity,1)-topos inspired by giving a natural equivalence to homotopy type theory with univalence was then proposed in

Mike Shulman, Inductive and higher inductive types (2012) (pdf)

Categorical semantics of univalent type universes is discussed in

Proof that all ∞-stack (∞,1)-topos have presentations by model categories which interpret (provide categorical semantics) for homotopy type theory with univalent type universes:

Michael Shulman, All ( ∞ , 1 ) (\infty,1) -toposes have strict univalent universes (arXiv:1904.07004).

Discussion of weak Tarskian homotopy type universes is in

Zhaohui Luo, Notes on Universes in Type Theory, 2012 (pdf)

Cesare Gallozzi, Constructive Set Theory from a Weak Tarski Universe, MSc thesis (2014) (pdf)

A discussion of the correspondence between type theories and categories of various sorts, from lex categories to toposes is in