anchor.diff:      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 == "tarball" then fetch_tarball pkgs name spec
       else if spec.type == "git" then fetch_git spec
  -    else if spec.type == "builtin-tarball" then fetch_builtin-tarball spec
  -    else if spec.type == "builtin-url" then fetch_builtin-url spec
  +    else if spec.type == "local" then fetch_local spec
  +    else if spec.type == "builtin-tarball" then fetch_builtin-tarball name
  +    else if spec.type == "builtin-url" then fetch_builtin-url name
       else
         abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
  +
  +  # If the environment variable NIV_OVERRIDE_${name} is set, then use
  +  # the path directly as opposed to the fetched source.
  +  replace = name: drv:
  +    let
  +      saneName = stringAsChars (c: if isNull (builtins.match "[a-zA-Z0-9]" c) then "_" else c) name;
  +      ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
  +    in
  +      if ersatz == "" then drv else ersatz;
   
     # Ports of functions for older nix versions
   
  
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
  