I have a solution, so for anyone interested I now do: father :: MonadPlus m => Sheep -> m Sheep father (Sheep Nothing _) = mzero father (Sheep (Just f) _) = return f mother :: MonadPlus m => Sheep -> m Sheep mother (Sheep _ Nothing) = mzero mother (Sheep _ (Just m)) = return m On 05/01/2011 01:14 AM, Nathan Hüsken wrote:

Hey,

I am currently reading "All about monads". Unfortanlty I have only a pdf without the solutions to the exercises.

in the previous exercises I defined:

data Sheep = Sheep (Maybe Sheep) (Maybe Sheep)

father :: Sheep -> Maybe Sheep father (Sheep f m) = f

mother :: Sheep -> Maybe Sheep mother (Sheep f m) = m

parent :: Sheep -> Maybe Sheep parent s = (mother s) `mplus` (father s)

parentList :: Sheep -> [Sheep] parentList s = (maybeToList (mother s)) `mplus` (maybeToList (father s))

Now I am supposed to combine "parent" and "parentList" to a function parent :: MonadPlus m => Sheep -> m Sheep

Problem, my parent function relies on "mother" and "father", and they give a Maybe monad. So I need: mother :: MonadPlus m => Sheep -> m Sheep

do I not? But I have no Idea of how the body of this function would look.

Anyone can give me a hint? Thanks! Nathan

but what I really need, is a function

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