Homotopical Patch Theory Carlo Angiuli ∗ Ed Morehouse∗ Daniel R. Licata Carnegie Mellon University Carnegie Mellon University Wesleyan University
[email protected] [email protected] [email protected] Robert Harper∗ Carnegie Mellon University
[email protected] Abstract identity type p =a=Ab q. The rules for the identity type allow one Homotopy type theory is an extension of Martin-Löf type the- to define the operations on paths that are considered in homotopy ory, based on a correspondence with homotopy theory and higher theory. These include identity paths refl : a = a (reflexivity of category theory. The propositional equality type becomes proof- equality), inverse paths ! p : b = a when p : a = b (symme- relevant, and acts like paths in a space. Higher inductive types are try of equality), and composition of paths q ◦ p : a = c when a new class of datatypes which are specified by constructors not p : a = b and q : b = c (transitivity of equality), as well as homo- only for points but also for paths. In this paper, we show how patch topies relating these operations (for example, refl ◦ p = p), and ho- theory in the style of the Darcs version control system can be de- motopies relating these homotopies, etc. This correspondence has veloped in homotopy type theory. We reformulate patch theory us- suggested several extensions to type theory. One is Voevodsky’s ing the tools of homotopy type theory, and clearly separate formal univalence axiom [16, 34], which describes the path structure of theories of patches from their interpretation in terms of basic revi- the universe (the type of small types).