diff --git a/nix/sources.nix b/nix/sources.nix
index c0941e9..051faaf 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -1,20 +1,58 @@
+# This file has been generated by Niv.
+
 # A record, from name to path, of the third-party packages
-with
+with rec
 {
+  pkgs =
+    if hasNixpkgsPath
+    then
+        if hasThisAsNixpkgsPath
+        then import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) {}
+        else import <nixpkgs> {}
+    else
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) {};
+
+  sources_nixpkgs =
+    if builtins.hasAttr "nixpkgs" sources
+    then sources.nixpkgs
+    else abort
+    ''
+        Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
+        add a package called "nixpkgs" to your sources.json.
+    '';
+
+  builtins_fetchTarball =
+      # fetchTarball version that is compatible between all the versions of
+      # Nix
+      { url, sha256 }@attrs:
+      let
+        inherit (builtins) lessThan nixVersion fetchTarball;
+      in
+        if lessThan nixVersion "1.12" then
+          fetchTarball { inherit url; }
+        else
+          fetchTarball attrs;
+
+  hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
+  hasThisAsNixpkgsPath =
+    (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
+
   sources = builtins.fromJSON (builtins.readFile ./sources.json);
 
-  # fetchTarball version that is compatible between all the sources of Nix
-  fetchTarball =
-    { url, sha256 }:
-      if builtins.lessThan builtins.nixVersion "1.12" then
-        builtins.fetchTarball { inherit url; }
-      else
-        builtins.fetchTarball { inherit url sha256; };
   mapAttrs = builtins.mapAttrs or
     (f: set: with builtins;
       listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
-};
 
+  getFetcher = spec:
+    let fetcherName =
+      if builtins.hasAttr "type" spec
+      then builtins.getAttr "type" spec
+      else "tarball";
+    in builtins.getAttr fetcherName {
+      "tarball" = pkgs.fetchzip;
+      "file" = pkgs.fetchurl;
+    };
+};
 # NOTE: spec must _not_ have an "outPath" attribute
 mapAttrs (_: spec:
   if builtins.hasAttr "outPath" spec
@@ -24,6 +62,6 @@ mapAttrs (_: spec:
     if builtins.hasAttr "url" spec && builtins.hasAttr "sha256" spec
     then
       spec //
-    { outPath = fetchTarball { inherit (spec) url sha256; } ; }
+      { outPath = getFetcher spec { inherit (spec) url sha256; } ; }
     else spec
   ) sources