diff --git a/nix/sources.nix b/nix/sources.nix
index 9520f31..1aaa5db 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -1,6 +1,57 @@
 # This file has been generated by Niv.
 
 let
+
+  #
+  # The fetchers. fetch_<type> fetches specs of type <type>.
+  #
+
+  fetch_file = spec:
+    if spec.builtin or true then
+      builtins_fetchurl { inherit (spec) url sha256; }
+    else
+      pkgs.fetchurl { inherit (spec) url sha256; };
+
+  fetch_tarball = spec:
+    if spec.builtin or true then
+      builtins_fetchTarball { inherit (spec) url sha256; }
+    else
+      pkgs.fetchzip { inherit (spec) url sha256; };
+
+  fetch_builtin-tarball = spec:
+    builtins.trace
+      ''
+        WARNING:
+          The niv type "builtin-tarball" will soon be deprecated. You should
+          instead use `builtin = true`.
+
+          $ niv modify <package> -a type=tarball -a builtins=true
+      '' # TODO: attribute as JSON
+      builtins_fetchTarball { inherit (spec) url sha256; };
+
+  fetch_builtin-url = spec:
+    builtins.trace
+      ''
+        WARNING:
+          The niv type "builtin-url" will soon be deprecated. You should
+          instead use `builtin = true`.
+
+          $ niv modify <package> -a type=file -a builtins=true
+      '' # TODO: attribute as JSON
+
+      (builtins_fetchurl { inherit (spec) url sha256; });
+
+  #
+  # The sources to fetch.
+  #
+
+  sources = builtins.fromJSON (builtins.readFile ./sources.json);
+
+  #
+  # Various helpers
+  #
+
+  # The set of packages used when specs are fetched using non-builtins.
   pkgs =
     if hasNixpkgsPath
     then
@@ -19,6 +70,30 @@ let
         add a package called "nixpkgs" to your sources.json.
       '';
 
+  hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
+  hasThisAsNixpkgsPath =
+    (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
+
+  # The actual fetching function.
+  fetch = 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 spec
+    else if spec.type == "tarball" then fetch_tarball spec
+    else if spec.type == "builtin-tarball" then fetch_builtin-tarball spec
+    else if spec.type == "builtin-url" then fetch_builtin-url spec
+    else
+      abort "ERROR: niv spec ${name} has unknown type ${builtins.fromJSON spec.type}";
+
+  # Ports of functions for previous nix versions
+
+  # a Nix version of mapAttrs if the built-in doesn't exist
+  mapAttrs = builtins.mapAttrs or (
+    f: set: with builtins;
+    listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
+  );
+
   # fetchTarball version that is compatible between all the versions of Nix
   builtins_fetchTarball = { url, sha256 }@attrs:
     let
@@ -39,56 +114,12 @@ let
       else
         fetchurl attrs;
 
-  # A wrapper around pkgs.fetchzip that has inspectable arguments,
-  # annoyingly this means we have to specify them
-  fetchzip = { url, sha256 }@attrs: pkgs.fetchzip attrs;
-
-  # A wrapper around pkgs.fetchurl that has inspectable arguments,
-  # annoyingly this means we have to specify them
-  fetchurl = { url, sha256 }@attrs: pkgs.fetchurl attrs;
-
-  hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
-  hasThisAsNixpkgsPath =
-    (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
-
-  sources = builtins.fromJSON (builtins.readFile ./sources.json);
-
-  mapAttrs = builtins.mapAttrs or (
-    f: set: with builtins;
-    listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
-  );
-
-  # borrowed from nixpkgs
-  functionArgs = f: f.__functionArgs or (builtins.functionArgs f);
-  callFunctionWith = autoArgs: f: args:
-    let
-      auto = builtins.intersectAttrs (functionArgs f) autoArgs;
-    in
-      f (auto // args);
-
-  getFetcher = spec:
-    let
-      fetcherName =
-        if builtins.hasAttr "type" spec
-        then builtins.getAttr "type" spec
-        else "builtin-tarball";
-    in
-      builtins.getAttr fetcherName {
-        "tarball" = fetchzip;
-        "builtin-tarball" = builtins_fetchTarball;
-        "file" = fetchurl;
-        "builtin-url" = builtins_fetchurl;
-      };
 in
-# NOTE: spec must _not_ have an "outPath" attribute
 mapAttrs (
-  _: spec:
+  name: spec:
     if builtins.hasAttr "outPath" spec
     then abort
       "The values in sources.json should not have an 'outPath' attribute"
     else
-      if builtins.hasAttr "url" spec && builtins.hasAttr "sha256" spec
-      then
-        spec // { outPath = callFunctionWith spec (getFetcher spec) {}; }
-      else spec
+      spec // { outPath = fetch name spec; }
 ) sources