How do I code a tree of objects in Haskell with pointers to parent and children?

Question

I\'ve got the following problem: I have a tree of objects of different classes where an action in the child class invalidates the parent. In imperative languages, it is trivial

Answer 1

Couldn't laziness take care of making sure validation doesn't happen too often? That way, you don't need to store the m_valid field.

For example, if you only validate on save, then you can edit the objects to your hearts content, without revalidating all the time; only when the user presses the 'Save' button is the value of validateDoc computed. Since I don't know for sure what your notion of valid means and what you need it for, I might be totally of the mark.

Untried & incomplete code:

data Document = Document { subDocs :: [SubDoc] }
data SubDoc = SubDoc { content :: String }

addSubDoc :: SubDoc -> (Document -> Document)
addSubDoc = error "not yet implemented: addSubDoc"

modifySubDoc :: Int -> (SubDoc -> SubDoc) -> (Document -> Document)
modifySubDoc = error "not yet implemented: modifySubDoc"


validateDoc :: Document -> Bool
validateDoc = all validateSubDoc . subDocs

validateSubDoc :: SubDoc -> Bool
validateSubDoc = not . null . contents

I'm assuming the overall validity of the document depends only on the subdocuments (simulated here by ensuring that they contain a non-empty string).

By the way, I think you forgot a a.addChild(b); in main.