Whereas ordinary types approximate the results, session types approximate communication among computations. As a form of typestate, they describe not only what is communicated now but also what is to be communicated next. Writing session-typed programs in an ordinary programming language such an OCaml requires inordinary cleverness to simulate type-level computations and linear typing – meaning the implementation and the error messages are very hard to understand. One is constantly reminded of template metaprogramming in C++.

We present a system exploring a very different approach to session typing: lowering type-level sophistry to ordinary programming, while maintaining the static assurances. Error messages are detailed and customizable, and one can use an ordinary debugger to investigate session-type problems. Our system is a binary-session–typed DSL for service-oriented programming in OCaml, supporting multiple communication channels, internal and external choices, recursion, and also channel delegation.

The key idea is staging: ordinary run-time checks in the generator play the role of “type-checks” from the point of view of the generated program. What is a fancy type to the latter is ordinary data to the generator.