Clean up nix files

default.nix

@@ -1,17 +1,2 @@
-with { fetch = import ./nix/fetch.nix; };
-let
-
-  # The (pinned) Nixpkgs where the original packages are sourced from
-  pkgs = import fetch.nixpkgs {};
-
-  # The list of packages to be installed
-  shellPackages = [ snack-exe ];
-
-  snack-exe = (import fetch.snack).snack-exe;
-  snack-lib = (import fetch.snack).snack-lib;
-in
-  if pkgs.lib.inNixShell
-  then pkgs.mkShell
-    { buildInputs = shellPackages;
-    }
-  else snack-lib.inferHPackBuild ./package.yaml
+with { pkgs = import ./nix; };
+pkgs.snack-lib.inferHPackBuild ./package.yaml

nix/default.nix

@@ -0,0 +1,12 @@
+with { fetch = import ./fetch.nix; };
+import fetch.nixpkgs
+  { overlays =
+      [ (self: super:
+          {
+            snack-exe = (import fetch.snack).snack-exe;
+            snack-lib = (import fetch.snack).snack-lib;
+          }
+        )
+      ] ;
+    config = { } ;
+  }

shell.nix

@@ -0,0 +1,4 @@
+with { pkgs = import ./nix; };
+pkgs.mkShell
+  { buildInputs = [ pkgs.snack-exe ];
+  }