Code staging in GNU Guix.

GNU Guix is a "functional" package manager that borrows from earlier work on Nix by Dolstra et al.. Guix implements high-level abstractions such as packages and operating system services as domain-specific languages (DSL) embedded in Scheme, and it also implements build actions and operating system orchestration in Scheme. This leads to a multi-tier programming environment where embedded code snippets are staged for eventual execution. In this paper we present G-expressions or "gexps". We explain our journey from traditional Lisp S-expressions to G-expressions, which augment the former with contextual information, and we discuss the implementation of gexps. We report on our experience using gexps in a variety of operating system use cases-from package build processes to system services. Gexps provide a novel way to cover many aspects of OS configuration in a single, multi-tier language while facilitating code reuse and code sharing.