# Solution4.html

### From HaskellWiki

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(Another definition of grandparent.) |
(Disclaimer for the alternative solution) |
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If the compiler cannot guess which MonadPlus to use you will need to specify it when the function is called. So, <hask>parent someSheep :: Maybe Sheep</hask> will use the Maybe monad and either <hask>parent someSheep :: [] Sheep</hask> or <hask>parent someSheep :: [Sheep]</hask> will use the list monad. |
If the compiler cannot guess which MonadPlus to use you will need to specify it when the function is called. So, <hask>parent someSheep :: Maybe Sheep</hask> will use the Maybe monad and either <hask>parent someSheep :: [] Sheep</hask> or <hask>parent someSheep :: [Sheep]</hask> will use the list monad. |
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− | Alternative grandparent: |
+ | |

+ | 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): |
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<haskell> |
<haskell> |
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grandparent :: (MonadPlus m) => Sheep -> m Sheep |
grandparent :: (MonadPlus m) => Sheep -> m Sheep |

## Latest revision as of 17:56, 10 June 2013

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

parent someSheep :: Maybe Sheep

parent someSheep :: [] Sheep

parent someSheep :: [Sheep]

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