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nixpkgs/pkgs/top-level/impure.nix

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Nix

/* Impure default args for `pkgs/top-level/default.nix`. See that file
for the meaning of each argument. */
{ # Fallback: Assume we are building packages for the current (host, in GNU
# Autotools parlance) system.
system ? builtins.currentSystem
, # Fallback: The contents of the configuration file found at $NIXPKGS_CONFIG or
# $HOME/.nixpkgs/config.nix.
config ? let
inherit (builtins) getEnv pathExists;
configFile = getEnv "NIXPKGS_CONFIG";
homeDir = getEnv "HOME";
configFile2 = homeDir + "/.nixpkgs/config.nix";
in
if configFile != "" && pathExists configFile then import configFile
else if homeDir != "" && pathExists configFile2 then import configFile2
else {}
, # Overlays are used to extend Nixpkgs collection with additional
# collections of packages. These collection of packages are part of the
# fix-point made by Nixpkgs.
overlays ? let
inherit (builtins) getEnv pathExists readDir attrNames map sort
lessThan;
dirEnv = getEnv "NIXPKGS_OVERLAYS";
dirHome = (getEnv "HOME") + "/.nixpkgs/overlays";
dirCheck = dir: dir != "" && pathExists (dir + "/.");
overlays = dir:
let content = readDir dir; in
map (n: import "${dir}/${n}") (sort lessThan (attrNames content));
in
if dirEnv != "" then
if dirCheck dirEnv then overlays dirEnv
else throw "The environment variable NIXPKGS_OVERLAYS does not name a valid directory."
else if dirCheck dirHome then overlays dirHome
else []
, ...
} @ args:
import ./. (args // { inherit system config overlays; })