Nix is a system for controlling the automatic creation and distribution of data, such as computer programs and other software artifacts. This is a very general problem, and there are many applications that fall under this description. Build management tools are used to perform software builds, that is, the construction of derived products such as executable programs from source code. A commonly used build tool is Make, which is a standard tool on Unix systems. These tools have to deal with several issues: After software has been built, is must also be deployed in the intended target environment, e.g., the user's workstation. Examples include the Red Hat package manager (RPM), Microsoft's MSI, and so on. Here also we have to deal with several issues: The creation of packages from some formal description of what artifacts should be distributed in the package. The deployment of packages, that is, the mechanism by which we get them onto the intended target environment. This can be as simple as copying a file, but complexity comes from the wide range of possible installation media (such as a network install), and the scalability of the process (if a program must be installed on a thousand systems, we do not want to visit each system and perform some manual steps to install the program on that system; that is, the complexity for the system administrator should be constant, not linear). ... Existing tools in this field generally both a underlying model (such as the derivation graph of build tools, or the versioning scheme that determines when two packages are compatible in a package management system) and a formalism that allows ... Following the principle of separation of mechanism and policy, the Nix system separates the low-level aspect of file system object management form the high-level aspect of the ...