Sebastian Ullrich (KIT, Germany), gives the fourth talk in the second panel, Dependently Typed Programming, on the 3rd day of the ICFP conference. Co-written by Gabriel Exner (Vienna University of Technology, Austria), Jared Roesch (University of Washington, USA), Jeremy Avigad (Carnegie Mellon University, USA), Leonardo De Moura (Microsoft Research).

Dependent type theory is a powerful framework for interactive theorem proving and automated reasoning, allowing us to encode mathematical objects, data type specifications, assertions, proofs, and programs, all in the same language.

Here we show that dependent type theory can also serve as its own metaprogramming language, that is, a language in which one can write programs that assist in the construction and manipulation of terms in dependent type theory itself. Specifically, we describe the metaprogramming language currently in use in the Lean theorem prover, which extends Lean's object language with an API for accessing natively implemented procedures and provides ways of reflecting object-level expressions into the metalanguage. We provide evidence to show that our language is performant, and that it provides a convenient and flexible way of writing not only small-scale interactive tactics, but also more substantial kinds of automation.