title: Haskell check if a value in monad exists
body: <p>I'm wondering if there is an equivalent or something for <code>null</code> from lists generalized for monads. At the moment I'm trying to find erroneous values with <code>== mzero</code>. But this kind of check provokes false positives since <code>mzero /= Left "foo"</code> applies.</p>
  
  <p>Is there an elegant way to express what I want in standard libraries?</p>