Merge pull request tweag#28 from philderbeast/patch-1

Mark repository revision as required, fixes tweag#26.

README.md

@@ -69,7 +69,7 @@ nixpkgs_git_repository(name, revision, sha256)
     <tr>
       <td><code>revision</code></td>
       <td>
-        <p><code>String; optional</code></p>
+        <p><code>String; required</code></p>
         <p>Git commit hash or tag identifying the version of Nixpkgs
            to use.</p>
       </td>
@@ -79,14 +79,16 @@ nixpkgs_git_repository(name, revision, sha256)
       <td>
         <p><code>String; optional</code></p>
         <p>The URI of the remote Git repository. This must be a HTTP
-           URL. There is currently no support for authentication.</p>
+           URL. There is currently no support for authentication.
+           Defaults to <a href="https://github.com/NixOS/nixpkgs">
+           upstream nixpkgs.</a></p>
       </td>
     </tr>
     <tr>
       <td><code>sha256</code></td>
       <td>
         <p><code>String; optional</code></p>
-        <p>The SHA256 used to verify the integrity of the repository</p>
+        <p>The SHA256 used to verify the integrity of the repository.</p>
       </td>
     </tr>
   </tbody>

nixpkgs/nixpkgs.bzl

@@ -13,7 +13,7 @@ def _nixpkgs_git_repository_impl(ctx):
 nixpkgs_git_repository = repository_rule(
   implementation = _nixpkgs_git_repository_impl,
   attrs = {
-    "revision": attr.string(),
+    "revision": attr.string(mandatory = True),
     "remote": attr.string(default = "https://github.com/NixOS/nixpkgs"),
     "sha256": attr.string(),
   },