Functional package management is a discipline that transcribes the functional programming paradigm to software deployment: build and installation processes are viewed as pure functions in the mathematical sense—whose result depends exclusively on the inputs—, and their result is a value—that is, an immutable directory. Since build and installation processes are pure functions, their results can effectively be “cached” on disk. Likewise, two independent runs of a given build process for a given set of inputs should return the same value—i.e., bit-identical files. This approach was first described and implemented in the Nix package manager. Guix reuses low-level mechanisms from Nix to implement the same paradigm, but offers a unified interface for package definitions and their implementations, all embedded in a single programming language.