[Haskell-cafe] Missing MaybeT MonadFix instance

[Job Vranish]

Is there a particular reason that the
transformers<http://hackage.haskell.org/packages/archive/transformers/latest/doc/html/Control-Monad-Trans-Maybe.html#v:MaybeT>version
of the MaybeT monad does not have a MonadFix instance?

It seems like the following instance would be suitable:

instance (MonadFix m) => MonadFix (MaybeT m) where
  mfix f = MaybeT $ mfix $ \a -> runMaybeT $ f $ case a of
     Just a -> a
     Nothing -> error "mfix MaybeT: Nothing"

I don't think it's possible to hit the error case:
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're done. If f returns a Just, then we don't hit the error
case.

- Job
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