Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/4696#discussion_r25598561
--- Diff: docs/programming-guide.md ---
@@ -1005,7 +1068,8 @@ for details.
</tr>
<tr>
<td> <b>foreach</b>(<i>func</i>) </td>
- <td> Run a function <i>func</i> on each element of the dataset. This is
usually done for side effects such as updating an accumulator variable (see
below) or interacting with external storage systems. </td>
+ <td> Run a function <i>func</i> on each element of the dataset. This is
usually done for side effects such as updating an accumulator variable (see
below) or interacting with external storage systems.
+ <br /><b>Note</b>: modifying variables outside of the
<code>foreach()</code> will only have the desired effect when running locally
(e.g. the spark shell) or when updating an accumulator. See <a
href="#ClosuresLink">Understanding closures </a> for more details.</td>
--- End diff --
Same, I'd describe this as undefined instead. It will not necessarily
update local variables in the driver and should be avoided.
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