/* Impure default args for `pkgs/top-level/default.nix`. See that file
   for the meaning of each argument. */

with builtins;

let

  homeDir = builtins.getEnv "HOME";

  # Return ‘x’ if it evaluates, or ‘def’ if it throws an exception.
  try = x: def: let res = tryEval x; in if res.success then res.value else def;

in

{ # We put legacy `system` into `localSystem`, if `localSystem` was not passed.
  # If neither is passed, assume we are building packages on the current
  # (build, in GNU Autotools parlance) platform.
  localSystem ? { system = args.system or builtins.currentSystem; }

# These are needed only because nix's `--arg` command-line logic doesn't work
# with unnamed parameters allowed by ...
, system ? localSystem.system
, crossSystem ? localSystem

, # Fallback: The contents of the configuration file found at $NIXPKGS_CONFIG or
  # $HOME/.config/nixpkgs/config.nix.
  config ? let
      configFile = getEnv "NIXPKGS_CONFIG";
      configFile2 = homeDir + "/.config/nixpkgs/config.nix";
      configFile3 = homeDir + "/.nixpkgs/config.nix"; # obsolete
    in
      if configFile != "" && pathExists configFile then import configFile
      else if homeDir != "" && pathExists configFile2 then import configFile2
      else if homeDir != "" && pathExists configFile3 then import configFile3
      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
      isDir = path: pathExists (path + "/.");
      pathOverlays = try (toString <nixpkgs-overlays>) "";
      homeOverlaysFile = homeDir + "/.config/nixpkgs/overlays.nix";
      homeOverlaysDir = homeDir + "/.config/nixpkgs/overlays";
      overlays = path:
        # check if the path is a directory or a file
        if isDir path then
          # it's a directory, so the set of overlays from the directory, ordered lexicographically
          let content = readDir path; in
          map (n: import (path + ("/" + n)))
            (builtins.filter (n: builtins.match ".*\\.nix" n != null || pathExists (path + ("/" + n + "/default.nix")))
              (attrNames content))
        else
          # it's a file, so the result is the contents of the file itself
          import path;
    in
      if pathOverlays != "" && pathExists pathOverlays then overlays pathOverlays
      else if pathExists homeOverlaysFile && pathExists homeOverlaysDir then
        throw ''
          Nixpkgs overlays can be specified with ${homeOverlaysFile} or ${homeOverlaysDir}, but not both.
          Please remove one of them and try again.
        ''
      else if pathExists homeOverlaysFile then
        if isDir homeOverlaysFile then
          throw (homeOverlaysFile + " should be a file")
        else overlays homeOverlaysFile
      else if pathExists homeOverlaysDir then
        if !(isDir homeOverlaysDir) then
          throw (homeOverlaysDir + " should be a directory")
        else overlays homeOverlaysDir
      else []

, crossOverlays ? []

, ...
} @ args:

# If `localSystem` was explicitly passed, legacy `system` should
# not be passed, and vice-versa.
assert args ? localSystem -> !(args ? system);
assert args ? system -> !(args ? localSystem);

import ./. (builtins.removeAttrs args [ "system" ] // {
  inherit config overlays localSystem;
})