don't run dhall-nix on osx

nixpkgs.nix

@@ -0,0 +1,6 @@
+import (
+  builtins.fetchTarball {
+    url = "https://github.com/nixos/nixpkgs/archive/a7bf8161fa834a602278c15fcbdd955656b3aed8.tar.gz";
+    sha256 = "0lzd35niw0j8qy4mhglvqwv5qwyvdb9mhv8hpmn73ym57r92i4p1";
+  }
+)

shell.nix

@@ -1,9 +1,17 @@
 let
-  pkgs = import <nixpkgs> {};
-
-  default = import ./default.nix {};
+  pkgs = import ./nixpkgs.nix {};
+  default = import ./default.nix { inherit pkgs; };
 
 in
-pkgs.mkShell {
-  buildInputs = builtins.attrValues default;
-}
+  with default;
+  pkgs.mkShell {
+    buildInputs = [
+      dhall-simple
+      dhall-json-simple
+      dhall-bash-simple
+    ] ++ (
+      if pkgs.stdenv.isDarwin then [] else [
+        dhall-nix-simple
+      ]
+    );
+  }

test.bash

@@ -17,6 +17,10 @@ function test_exe () {
 test_exe dhall;
 test_exe dhall-to-json;
 test_exe dhall-to-bash;
-test_exe dhall-to-nix;
+
+if [[ "$OSTYPE" != "darwin"* ]]
+then
+  test_exe dhall-to-nix;
+fi
 
 exit $ERRORS;