Calculating the Fundamental Group of the Circle in Homotopy Type Theory

Recent work on homotopy type theory exploits an exciting new correspondence between Martin-Lof's dependent type theory and the mathematical disciplines of category theory and homotopy theory. The category theory and homotopy theory suggest new principles to add to type theory, and type theory can be used in novel ways to formalize these areas of mathematics. In this paper, we formalize a basic result in algebraic topology, that the fundamental group of the circle is the integers. Though simple, this example is interesting for several reasons: it illustrates the new principles in homotopy type theory; it mixes ideas from traditional homotopy-theoretic proofs of the result with type-theoretic inductive reasoning; and it provides a context for understanding an existing puzzle in type theory---that a universe (type of types) is necessary to prove that the constructors of inductive types are disjoint and injective.