# Solution4.html

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Revision as of 17:56, 10 June 2013 by Prajit (talk | contribs) (Disclaimer for the alternative solution)

```
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
```