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parent :: MonadPlus m => Sheep -> m Sheep
parent s = (toMonad (father s)) `mplus` (toMonad (mother s))
grandparent :: MonadPlus m => Sheep -> m Sheep
grandparent s = (toMonad (parentalGrandfather s)) `mplus`
                (toMonad (parentalGrandmother s)) `mplus`
                (toMonad (maternalGrandfather s)) `mplus`
                (toMonad (maternalGrandmother s))
toMonad :: MonadPlus m => Maybe a -> m a
toMonad Nothing = mzero
toMonad (Just s) = return s
If the compiler cannot guess which MonadPlus to use you will need to specify it when the function is called. So,
parent someSheep :: Maybe Sheep
will use the Maybe monad and either
parent someSheep :: [] Sheep
parent someSheep :: [Sheep]
will use the list monad.

Alternative grandparent:

grandparent :: (MonadPlus m) => Sheep -> m Sheep
grandparent s = parent s >>= parent