# Difference between revisions of "Solution4.html"

From HaskellWiki

(New page: <haskell> parent :: MonadPlus m => Sheep -> m Sheep parent s = (toMonad (father s)) `mplus` (toMonad (mother s)) grandparent :: MonadPlus m => Sheep -> m Sheep grandparent s = (toMonad (p...) |
(Another definition of grandparent.) |
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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. | ||

+ | |||

+ | Alternative grandparent: | ||

+ | <haskell> | ||

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

+ | grandparent s = parent s >>= parent | ||

+ | </haskell> |

## Revision as of 18:01, 21 September 2012

```
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.

Alternative grandparent:

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