# Difference between revisions of "Solution4.html"

From HaskellWiki

(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: |
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+ | |||

+ | 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
```

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
```