Is there a particular reason that the <a href="http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Control-Monad-Trans-Maybe.html#v:MaybeT">transformers</a> version of the MaybeT monad does not have a MonadFix instance?<div>
<br></div><div>It seems like the following instance would be suitable:</div><div><br></div><div><div>instance (MonadFix m) =&gt; MonadFix (MaybeT m) where</div><div>  mfix f = MaybeT $ mfix $ \a -&gt; runMaybeT $ f $ case a of</div>
<div>     Just a -&gt; a</div><div>     Nothing -&gt; error &quot;mfix MaybeT: Nothing&quot;</div></div><div><br></div><div>I don&#39;t think it&#39;s possible to hit the error case:</div><div>In order to terminate f cannot be strict so it must return a value without evaluating its input. If f returns a Nothing, then you have your return value and you&#39;re done. If f returns a Just, then we don&#39;t hit the error case.</div>
<div><br></div><div>- Job</div>