Solution4.html

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Revision as of 02:01, 23 August 2012 by Pronate (talk | contribs) (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...)

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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.