Exercises.hs

import Control.Monad
import Data.Maybe

maybeToMonad :: (MonadPlus m) => Maybe a -> m a
maybeToMonad Nothing  = mzero
maybeToMonad (Just x) = return x

type Sheep = Int

father :: Sheep -> Maybe Sheep
father 1 = Just 2
father 2 = Just 4
father 3 = Just 11
father 5 = Just 12
father _ = Nothing

mother :: Sheep -> Maybe Sheep
mother 1 = Just 3
mother 2 = Just 5
mother 3 = Just 10
mother _ = Nothing

parent2 :: Sheep -> Maybe Sheep
parent2 s = (mother s) `mplus` (father s)

grandparent2 :: Sheep -> Maybe Sheep
grandparent2 s = (parent2 s) >>= parent2

parent3 :: Sheep -> [Sheep]
parent3 s = (maybeToList $ mother s) `mplus` (maybeToList $ father s)

grandparent3 :: Sheep -> [Sheep]
grandparent3 s = (parent3 s) >>= parent3

parent4 :: (MonadPlus m) => Sheep -> m Sheep
parent4 s =  (maybeToMonad $ mother s) `mplus` (maybeToMonad $ father s)

grandparent4 :: (MonadPlus m) => Sheep -> m Sheep
grandparent4 s = (parent4 s) >>= parent4