anchor.diff:        # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
         inherit pkgs;
       };
  +
   in
   mkSources (mkConfig {}) // { __functor = _: settings: mkSources (mkConfig settings); }
  
positive.code:         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
  
negative.code:     else
        pkgs.fetchurl { inherit (spec) url sha256; };
  
    fetch_tarball = pkgs: name: spec:
      let
        ok = str: ! builtins.isNull (builtins.match "[a-zA-Z0-9+-._?=]" str);
        # sanitize the name, though nix will still fail if name starts with period
        name' = stringAsChars (x: if ! ok x then "-" else x) "${name}-src";
      in
        if spec.builtin or true then
          builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
        else
          pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
  
    fetch_git = spec:
      builtins.fetchGit { url = spec.repo; inherit (spec) rev ref; };
  
    fetch_local = spec: spec.path;
  
    fetch_builtin-tarball = name: throw
      ''[${name}] The niv type "builtin-tarball" is deprecated. You should instead use `builtin = true`.
          $ niv modify ${name} -a type=tarball -a builtin=true'';
  
    fetch_builtin-url = name: throw
      ''[${name}] The niv type "builtin-url" will soon be deprecated. You should instead use `builtin = true`.
          $ niv modify ${name} -a type=file -a builtin=true'';
  
    #
    # Various helpers
  