Cubical Higher Type Theory as a Programming Language

I gave a presentation at the Workshop on Categorical Logic and Univalent Foundations held in Leeds, UK July 27-29th 2016. My talk, entitled Computational Higher Type Theory, concerns a formulation of higher-dimensional type theory in which terms are interpreted directly as programs and types as programs that express specifications of program behavior. This approach to type theory, first suggested by Per Martin-Löf in his famous paper Constructive Mathematics and Computer Programming and developed more fully by The NuPRL Project, emphasizes constructive mathematics in the Brouwerian sense: proofs are programs, propositions are types.

The now more popular accounts of type theory emphasize the axiomatic freedom given by making fewer foundational commitments, such as not asserting the decidability of every type, but give only an indirect account of their computational content, and then only in some cases. In particular, the computational content of Voevodsky’s Univalence Axiom in Homotopy Type Theory remains unclear, though the Bezem-Coquand-Huber model in cubical sets carried out in constructive set theory gives justification for its constructivity.

To elicit the computational meaning of higher type theory more clearly, emphasis has shifted to cubical type theory (in at least two distinct forms) in which the higher-dimensional structure of types is judgmentally explicit as the higher cells of a type, which are interpreted as identifications. In the above-linked talk I explain how to construe a cubical higher type theory directly as a programming language. Other efforts, notably by Cohen-Coquand-Huber-Mörtberg, have similar goals, but using somewhat different methods.

For more information, please see my home page on which are linked two arXiv papers providing the mathematical details, and a 12-page paper summarizing the approach and the major results obtained so far. These papers represent joint work with Carlo Angiuli and Todd Wilson.

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