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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 or parent someSheep :: [Sheep] will use the list monad.

This next alternative grandparent function only works in the case of the List monad (see exercise 5.2 for why the Maybe monad does not work):

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