diff --git a/Main.hs b/Main.hs
index d9b4827..da8d8d6 100644
--- a/Main.hs
+++ b/Main.hs
@@ -623,8 +623,8 @@ pathShellNix = "shell.nix"
 -- | Simple shell that loads @niv@
 initShellNixContent :: String
 initShellNixContent = [s|
-let pkgs = import ./nix;
-in pkgs.mkShell
+with { pkgs = import ./nix {}; };
+pkgs.mkShell
   { buildInputs = [ pkgs.niv ];
   }
 |]
diff --git a/default.nix b/default.nix
index 765e32e..473778c 100644
--- a/default.nix
+++ b/default.nix
@@ -1,2 +1,2 @@
-with { pkgs = import ./nix {}; };
-{ inherit (pkgs) niv readme readme-test ; }
+{ pkgs ? import ./nix {} }:
+pkgs.callPackage ./nix/packages.nix {}
diff --git a/nix/default.nix b/nix/default.nix
index b6c5d55..df16d2c 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -1,3 +1,14 @@
 { sources ? import ./sources.nix }:
 import sources.nixpkgs
-  { overlays = import ./overlay.nix { inherit sources; } ; config = {}; }
+  { overlays =
+    [
+      # Snack
+      (self: super:
+        {
+          snack-exe = (import sources.snack).snack-exe;
+          snack-lib = (import sources.snack).snack-lib;
+        }
+      )
+    ];
+    config = {};
+  }
diff --git a/nix/overlay.nix b/nix/overlay.nix
deleted file mode 100644
index aba0b87..0000000
--- a/nix/overlay.nix
+++ /dev/null
@@ -1,53 +0,0 @@
-{ sources ? import ./sources.nix }:
-[
-
-  # Snack
-  (self: super:
-    {
-      snack-exe = (import sources.snack).snack-exe;
-      snack-lib = (import sources.snack).snack-lib;
-    }
-  )
-
-  # Built niv
-  (self: super:
-    { niv = super.snack-lib.executable ../package.yaml ; }
-  )
-
-  # README generation
-  (self: super:
-    rec { readme = self.writeText "README.md"
-        (with
-          { template = builtins.readFile ../README.tpl.md;
-            niv_help = builtins.readFile
-              (self.runCommand "niv_help" { buildInputs = [ self.niv ]; }
-                "niv --help > $out"
-              );
-            niv_add_help = builtins.readFile
-              (self.runCommand "niv_add_help" { buildInputs = [ self.niv ]; }
-                "niv add --help > $out"
-              );
-            niv_update_help = builtins.readFile
-              (self.runCommand "niv_update_help" { buildInputs = [ self.niv ]; }
-                "niv update --help > $out"
-              );
-            niv_drop_help = builtins.readFile
-              (self.runCommand "niv_drop_help" { buildInputs = [ self.niv ]; }
-                "niv drop --help > $out"
-              );
-          };
-        self.lib.replaceStrings
-          [
-            "replace_niv_help"
-            "replace_niv_add_help"
-            "replace_niv_update_help"
-            "replace_niv_drop_help"
-          ]
-          [ niv_help niv_add_help niv_update_help niv_drop_help ]
-          template
-        );
-      readme-test = self.runCommand "README-test" {}
-        "diff ${../README.md} ${readme} && echo dummy > $out";
-    }
-  )
-]
diff --git a/nix/packages.nix b/nix/packages.nix
new file mode 100644
index 0000000..d228e5e
--- /dev/null
+++ b/nix/packages.nix
@@ -0,0 +1,40 @@
+{ writeText
+, runCommand
+, lib
+, snack-lib
+}:
+rec
+{ niv = snack-lib.executable ../package.yaml;
+  readme = writeText "README.md"
+    (with
+      { template = builtins.readFile ../README.tpl.md;
+        niv_help = builtins.readFile
+          (runCommand "niv_help" { buildInputs = [ niv ]; }
+            "niv --help > $out"
+          );
+        niv_add_help = builtins.readFile
+          (runCommand "niv_add_help" { buildInputs = [ niv ]; }
+            "niv add --help > $out"
+          );
+        niv_update_help = builtins.readFile
+          (runCommand "niv_update_help" { buildInputs = [ niv ]; }
+            "niv update --help > $out"
+          );
+        niv_drop_help = builtins.readFile
+          (runCommand "niv_drop_help" { buildInputs = [ niv ]; }
+            "niv drop --help > $out"
+          );
+      };
+    lib.replaceStrings
+      [
+        "replace_niv_help"
+        "replace_niv_add_help"
+        "replace_niv_update_help"
+        "replace_niv_drop_help"
+      ]
+      [ niv_help niv_add_help niv_update_help niv_drop_help ]
+      template
+    );
+  readme-test = runCommand "README-test" {}
+    "diff ${../README.md} ${readme} && echo dummy > $out";
+}
diff --git a/nix/sources.nix b/nix/sources.nix
index 2ae3408..184d081 100644
--- a/nix/sources.nix
+++ b/nix/sources.nix
@@ -1,9 +1,9 @@
 # A record, from name to path, of the third-party packages
 with
 {
-  versions = builtins.fromJSON (builtins.readFile ./versions.json);
+  specs = builtins.fromJSON (builtins.readFile ./specs.json);
 
-  # fetchTarball version that is compatible between all the versions of Nix
+  # fetchTarball version that is compatible between all the specs of Nix
   fetchTarball =
     { url, sha256 }:
       if builtins.lessThan builtins.nixVersion "1.12" then
@@ -19,11 +19,11 @@ with
 mapAttrs (_: spec:
   if builtins.hasAttr "outPath" spec
   then abort
-    "The values in versions.json should not have an 'outPath' attribute"
+    "The values in specs.json should not have an 'outPath' attribute"
   else
     if builtins.hasAttr "url" spec && builtins.hasAttr "sha256" spec
     then
       spec //
     { outPath = fetchTarball { inherit (spec) url sha256; } ; }
     else spec
-  ) versions
+  ) specs
diff --git a/nix/versions.json b/nix/specs.json
similarity index 100%
rename from nix/versions.json
rename to nix/specs.json
diff --git a/script/test b/script/test
index 4c73862..2dfd93e 100755
--- a/script/test
+++ b/script/test
@@ -4,8 +4,16 @@
 #!nix-shell -p nix
 #!nix-shell --pure
 
+set -euo pipefail
+
+export NIX_PATH="nixpkgs=./nix"
+
 echo "Building"
 
 nix build --no-link
 
+echo "Testing shell"
+
+nix-shell --run "echo -n"
+
 echo "all good"
diff --git a/shell.nix b/shell.nix
index 834168e..fa416db 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,4 +1,4 @@
-with { pkgs = import ./nix; };
+with { pkgs = import ./nix {}; };
 pkgs.mkShell
   { buildInputs = [ pkgs.snack-exe ];
   }