nix

Purely functional package manager

Nix is a purely functional package manager. This means that it treats packages like values in purely functional programming languages such as Haskell -- they are built by functions that don't have side-effects, and they never change after they have been built. Nix stores packages in the Nix store, usually the directory /nix/store, where each package has its own unique subdirectory such as /nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1/ where b6gvzjyb2pg0... is a unique identifier for the package that captures all its dependencies (it's a cryptographic hash of the package's build dependency graph). This enables many powerful features.