Guarded Cubical Type Theory

  • Lars Birkedal
  • Aleš Bizjak
  • Ranald Clouston
  • Hans Bugge Grathwohl
  • Bas Spitters
  • Andrea Vezzosi


This paper improves the treatment of equality in guarded dependent type theory (\(\mathsf {GDTT}\)), by combining it with cubical type theory (\(\mathsf {CTT}\)). \(\mathsf {GDTT}\) is an extensional type theory with guarded recursive types, which are useful for building models of program logics, and for programming and reasoning with coinductive types. We wish to implement \(\mathsf {GDTT}\) with decidable type checking, while still supporting non-trivial equality proofs that reason about the extensions of guarded recursive constructions. \(\mathsf {CTT}\) is a variation of Martin–Löf type theory in which the identity type is replaced by abstract paths between terms. \(\mathsf {CTT}\) provides a computational interpretation of functional extensionality, enjoys canonicity for the natural numbers type, and is conjectured to support decidable type-checking. Our new type theory, guarded cubical type theory (\(\mathsf {GCTT}\)), provides a computational interpretation of extensionality for guarded recursive types. This further expands the foundations of \(\mathsf {CTT}\) as a basis for formalisation in mathematics and computer science. We present examples to demonstrate the expressivity of our type theory, all of which have been checked using a prototype type-checker implementation. We show that \(\mathsf {CTT}\) can be given semantics in presheaves on \(\mathcal {C}\times \mathbb {D}\), where \(\mathcal {C}\) is the cube category, and \(\mathbb {D}\) is any small category with an initial object. We then show that the category of presheaves on \(\mathcal {C}\times \omega \) provides semantics for \(\mathsf {GCTT}\).


Homotopy type theory Cubical type theory Guarded recursion 



We gratefully acknowledge our discussions with Thierry Coquand, and the comments of our reviewers of the conference version of this article [7], and of this article. This research was supported in part by the ModuRes Sapere Aude Advanced Grant from The Danish Council for Independent Research for the Natural Sciences (FNU), and in part by the Guarded homotopy type theory project, funded by the Villum Foundation (Grant No. 00012386). Aleš Bizjak was supported in part by a Microsoft Research Ph.D. Grant.

Supplementary material


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© Springer Nature B.V. 2018

Authors and Affiliations

  1. 1.Department of Computer ScienceAarhus UniversityAarhusDenmark
  2. 2.Department of Computer Science and EngineeringChalmers University of TechnologyGothenburgSweden

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