[Haskell-beginners] All about monads, exercise 4

[Nathan Hüsken]

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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