All About Monads/Solutions

For exercise 1

maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = return s >>= \ms -> mother ms >>= \m -> father m fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandMother s = return s >>= \ms -> father ms >>= \f -> mother s >>= \gm -> mother gm mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = return s >>= \ms -> mother ms >>= \m -> father m >>= \gf -> father gf Note: the returns are not not necessary; they are only used for the sake of the exercise. An alternative solution without use of return: maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = mother s >>= \m -> father m fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandMother s = father s >>= \f -> mother s >>= \gm -> mother gm mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = mother s >>= \m -> father m >>= \gf -> father gf

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For exercise 2

parent :: Sheep -> Maybe Sheep parent s = father s `mplus` mother s grandparent :: Sheep -> Maybe Sheep grandparent s = paternalGrandfather s `mplus` paternalGrandmother s `mplus` maternalGrandfather s `mplus` maternalGrandmother s This next solution will not work. If the sheep has a father and only a maternal grandparent, this function will return Nothing: grandparent :: Sheep -> Maybe Sheep grandparent s = parent s >>= parent

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For exercise 3

parent :: Sheep -> [Sheep] parent s = (maybeToList (mother s)) ++ (maybeToList (father s)) grandparent :: Sheep -> [Sheep] grandparent s = (maybeToList (paternalGrandfather s)) ++ (maybeToList (paternalGrandmother s)) ++ (maybeToList (maternalGrandfather s)) ++ (maybeToList (maternalGrandmother s)) Alternate solution: parent :: Sheep -> [Sheep] parent s = (maybeToList $ mother s) `mplus` (maybeToList $ father s) grandparent :: Sheep -> [Sheep] grandparent s = parent s >>= parent

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For exercise 4

parent :: MonadPlus m => Sheep -> m Sheep parent s = (toMonad (father s)) `mplus` (toMonad (mother s)) grandparent :: MonadPlus m => Sheep -> m Sheep grandparent s = (toMonad (paternalGrandfather s)) `mplus` (toMonad (paternalGrandmother 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

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