sources.nix

let

  #
  # The fetchers. fetch_<type> fetches specs of type <type>.
  #

  fetch_file = pkgs: spec:
    if spec.builtin or true then
      builtins_fetchurl { inherit (spec) url sha256; }
    else
      pkgs.fetchurl { inherit (spec) url sha256; };

  fetch_tarball = pkgs: spec:
    if spec.builtin or true then
      builtins_fetchTarball { inherit (spec) url sha256; }
    else
      pkgs.fetchzip { inherit (spec) url sha256; };

  fetch_git = spec:
    builtins.fetchGit {
      inherit (spec) rev ref;
      url = spec.repo;
    };

  #
  # Various helpers
  #

  # The set of packages used when specs are fetched using non-builtins.
  mkPkgs = sources:
    let
      sourcesNixpkgs =
        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; })
        { };
      hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
      hasThisAsNixpkgsPath = <nixpkgs> == ./.;
    in if builtins.hasAttr "nixpkgs" sources then
      sourcesNixpkgs
    else if hasNixpkgsPath && !hasThisAsNixpkgsPath then
      import <nixpkgs> { }
    else
      abort ''
        Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
        add a package called "nixpkgs" to your sources.json.
      '';

  # The actual fetching function.
  fetch = pkgs: name: spec:

    if !builtins.hasAttr "type" spec then
      abort "ERROR: niv spec ${name} does not have a 'type' attribute"
    else if spec.type == "file" then
      fetch_file pkgs spec
    else if spec.type == "tarball" then
      fetch_tarball pkgs spec
    else if spec.type == "git" then
      fetch_git spec
    else
      abort
      "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";

  # Ports of functions for older nix versions

  # a Nix version of mapAttrs if the built-in doesn't exist
  mapAttrs = builtins.mapAttrs or (f: set:
    with builtins;
    listToAttrs (map (attr: {
      name = attr;
      value = f attr set.${attr};
    }) (attrNames set)));

  # fetchTarball version that is compatible between all the versions of Nix
  builtins_fetchTarball = { url, sha256 }@attrs:
    let inherit (builtins) lessThan nixVersion fetchTarball;
    in if lessThan nixVersion "1.12" then
      fetchTarball { inherit url; }
    else
      fetchTarball attrs;

  # fetchurl version that is compatible between all the versions of Nix
  builtins_fetchurl = { url, sha256 }@attrs:
    let inherit (builtins) lessThan nixVersion fetchurl;
    in if lessThan nixVersion "1.12" then
      fetchurl { inherit url; }
    else
      fetchurl attrs;

  # Create the final "sources" from the config
  mkSources = config:
    mapAttrs (name: spec:
      if builtins.hasAttr "outPath" spec then
        abort
        "The values in sources.json should not have an 'outPath' attribute"
      else
        spec // { outPath = fetch config.pkgs name spec; }) config.sources;

  # The "config" used by the fetchers
  mkConfig = { sourcesFile ? ./sources.json
    , sources ? builtins.fromJSON (builtins.readFile sourcesFile)
    , pkgs ? mkPkgs sources }: rec {
      # The sources, i.e. the attribute set of spec name to spec
      inherit sources;

      # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
      inherit pkgs;
    };
in mkSources (mkConfig { }) // {
  __functor = _: settings: mkSources (mkConfig settings);
}