For those interested in contributing to this new branch of mathematics, it may be encouraging to know that there are many interesting open questions.
Perhaps the most pressing of them is the “constructivity” of the Univalence Axiom, posed by Voevodsky in . The basic system of type theory follows the structure of Gentzen’s natural deduction. Logical connectives are defined by their introduction rules, and have elimination rules justified by computation rules. Following this pattern, and using Tait’s computability method, originally designed to analyse Gödel’s Dialectica interpretation, one can show the property of normalization for type theory. This in turn implies important properties such as decidability of type-checking (a crucial property since type-checking corresponds to proof-checking, and one can argue that we should be able to “recognize a proof when we see one”), and the so-called “canonicity property” that any closed term of the type of natural numbers reduces to a numeral. This last property, and the uniform structure of introduction/elimination rules, are lost when one extends type theory with an axiom, such as the axiom of function extensionality, or the univalence axiom. Voevodsky has formulated a precise mathematical conjecture connected to this question of canonicity for type theory extended with the axiom of Univalence: given a closed term of the type of natural numbers, is it always possible to find a numeral and a proof that this term is equal to this numeral, where this proof of equality may itself use the univalence axiom? More generally, an important issue is whether it is possible to provide a constructive justification of the univalence axiom. What about if one adds other homotopically motivated constructions, like higher inductive types? These questions remain open at the present time, although methods are currently being developed to try to find answers.
Another basic issue is the difficulty of working with types, such as the natural numbers, that are essentially sets (i.e., discrete spaces), containing only trivial paths. At present, homotopy type theory can really only characterize spaces up to homotopy equivalence, which means that these “discrete spaces” may only be homotopy equivalent to discrete spaces. Type-theoretically, this means there are many paths that are equal to reflexivity, but not judgmentally equal to it (see §1.1 (http://planetmath.org/11typetheoryversussettheory) for the meaning of “judgmentally”). While this homotopy-invariance has advantages, these “meaningless” identity terms do introduce needless complications into arguments and constructions, so it would be convenient to have a systematic way of eliminating or collapsing them.
A more specialized, but no less important, problem is the relation between homotopy type theory and the research on higher toposes currently happening at the intersection of higher category theory and homotopy theory. There is a growing conviction among those familiar with both subjects that they are intimately connected. For instance, the notion of a univalent universe should coincide with that of an object classifier, while higher inductive types should be an “elementary” reflection of local presentability. More generally, homotopy type theory should be the “internal language” of -toposes, just as intuitionistic higher-order logic is the internal language of ordinary 1-toposes. Despite this general consensus, however, details remain to be worked out — in particular, questions of coherence and strictness remain to be addressed — and doing so will undoubtedly lead to further insights into both concepts.
But by far the largest field of work to be done is in the ongoing formalization of everyday mathematics in this new system. Recent successes in formalizing some facts from basic homotopy theory and category theory have been encouraging; some of these are described in http://planetmath.org/node/87582Chapter 8,http://planetmath.org/node/87583Chapter 9. Obviously, however, much work remains to be done.
The homotopy type theory community maintains a web site and group blog at \urlhttp://homotopytypetheory.org, as well as a discussion email list. Newcomers are always welcome!
- 1 Vladimir Voevodsky. A universe polymorphic type system. \urlhttp://uf-ias-2012.wikispaces.com/file/view/Universe+polymorphic+type+sytem.pdf, 2012.