bracket type in nLab

Contents

1. Idea

There are various different paradigms for the interpretation of predicate logic in type theory. In “logic-enriched type theory”, there is a separate class of “propositions” from the class of “types”. But we can also identify propositions with particular types. In the propositions as types-paradigm, every proposition is a type, and also every type is identified with a proposition (the proposition that it is an inhabited type).

By contrast, in the paradigm that may be called propositions as some types, every proposition is a type, but not every type is a proposition. The types which are propositions are generally those which “have at most one inhabitant” — in homotopy type theory this is called being of h-level 1 or being a (-1)-type. This paradigm is often used in the categorical semantics of type theory, such as the internal logic of various kinds of categories.

Under “propositions as types”, all type-theoretic operations represent corresponding logical operations (dependent sum is the existential quantifier, dependent product the universal quantifier, and so on). However, under “propositions as some types”, not every such operation preserves the class of propositions; this is particularly the case for dependent sum and disjunction(or). Thus, in order to obtain the correct logical operations, we need to reflect these constructions back into propositions somehow, finding the “underlying proposition”, corresponding to the (-1)-truncation/h-level 1-projection. This operation in type theory is called the bracket type (when denoted [A]); in homotopy type theory it can be identified with the higher inductive type isInhab.

2. Definition

In type theory

In homotopy type theory

We discuss the definition in homotopy type theory.

For A a type, the support of A denoted supp(A) or isInhab(A) or τ−1A or ‖A‖−1 or ‖A‖ or, lastly, [A], is the higher inductive type defined by the two constructors

where in the last sequent on the right we have the identity type. (Voevodsky, HoTTLibrary)

This says that supp(A) is the type which is universal with the property that the terms of A map to it and that any two term of A become equivalent in supp(A).

In Agda syntax this is

data isinhab {i : Level} (A : Set i) : Set i where
  inhab : A → isinhab A
  inhab-path : (x y : isinhab A) → x ≡ y

The recursion principle for supp(A) says that if B is a mere proposition and we have f:A→B, then there is an induced g:supp(A)→B such that g(isinhab(a))≡f(a) for all a:A. In other words, any mere proposition which follows from (the inhabitedness of) A already follows from supp(A). Thus, supp(A), as a mere proposition, contains no more information than the inhabitedness of A.

For more see at n-truncation modality.

3. Semantics

One presentation of the internal type theory of regular categories consists of dependent type theory with the unit type, strong extensional equality types?, strong dependent sums, and bracket types. (The internal logic of a regular category can alternatively be presented as a logic-enriched type theory?.)

The semantics of bracket types in a regular category C is as follows.

A dependent type (a type in context X)

is interpreted in C as an arbitrary morphism

The corresponding bracket type

is interpreted then as the image-factorization

Therefore [A]→X is a monomorphism, and hence the interpretation of a proposition about the elements of X.

4. Related concepts

5. References

The original articles are

• M.E. Maietti, The Type Theory of Categorical Universes PhD thesis, Università Delgi Studi di Padova, 1998

(which speaks of “mono types”) and

• Frank Pfenning, Intensionality, extensionality, and proof irrelevance in modal type theory, In Proceedings of the 16th Annual Symposium on Logic in Computer Science (LICS’01), June 2001.

• Steve Awodey, Andrej Bauer, Propositions as [Types], Journal of Logic and Computation. Volume 14, Issue 4, August 2004, pp. 447-471 (pdf)

Exposition in the context of homotopy type theory is in

• Mike Shulman, Minicourse on homotopy type theory (2012) (web)

Formalization in the context of homotopy type theory is in

• Vladimir Voevodsky, The hProp version of the “inhabited” construction. (web)

Discussion of this in the more general context of truncations is in

• Guillaume Brunerie, Truncations and truncated higher inductive types (web)