{"title":"Formalising real numbers in homotopy type theory","authors":"Gaëtan Gilbert","doi":"10.1145/3018610.3018614","DOIUrl":"https://doi.org/10.1145/3018610.3018614","url":null,"abstract":"Cauchy reals can be defined as a quotient of Cauchy sequences of rationals. In this case, the limit of a Cauchy sequence of Cauchy reals is defined through lifting it to a sequence of Cauchy sequences of rationals. This lifting requires the axiom of countable choice or excluded middle, neither of which is available in homotopy type theory. To address this, the Univalent Foundations Program uses a higher inductive-inductive type to define the Cauchy reals as the free Cauchy complete metric space generated by the rationals. We generalize this construction to define the free Cauchy complete metric space generated by an arbitrary metric space. This forms a monad in the category of metric spaces with Lipschitz functions. When applied to the rationals it defines the Cauchy reals. Finally, we can use Altenkirch and Danielson (2016)'s partiality monad to define a semi-decision procedure comparing a real number and a rational number. The entire construction has been formalized in the Coq proof assistant. It is available at https://github.com/SkySkimmer/HoTTClasses/tree/CPP2017 .","PeriodicalId":262665,"journal":{"name":"Proceedings of the 6th ACM SIGPLAN Conference on Certified Programs and Proofs","volume":"101 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2016-10-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127157050","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Andrej Bauer, Jason Gross, P. Lumsdaine, Michael Shulman, Matthieu Sozeau, Bas Spitters
{"title":"The HoTT library: a formalization of homotopy type theory in Coq","authors":"Andrej Bauer, Jason Gross, P. Lumsdaine, Michael Shulman, Matthieu Sozeau, Bas Spitters","doi":"10.1145/3018610.3018615","DOIUrl":"https://doi.org/10.1145/3018610.3018615","url":null,"abstract":"We report on the development of the HoTT library, a formalization of homotopy type theory in the Coq proof assistant. It formalizes most of basic homotopy type theory, including univalence, higher inductive types, and significant amounts of synthetic homotopy theory, as well as category theory and modalities. The library has been used as a basis for several independent developments. We discuss the decisions that led to the design of the library, and we comment on the interaction of homotopy type theory with recently introduced features of Coq, such as universe polymorphism and private inductive types.","PeriodicalId":262665,"journal":{"name":"Proceedings of the 6th ACM SIGPLAN Conference on Certified Programs and Proofs","volume":"107 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2016-10-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126148369","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}