[Haskell-cafe] Capturing the parent element as I parse XML using parsec

C K Kashyap ckkashyap at gmail.com
Mon Jul 30 11:53:17 CEST 2012

Thank you Richard and Antoine.

I think I see the pointlessness of my ask.


On Mon, Jul 30, 2012 at 4:14 AM, Richard O'Keefe <ok at cs.otago.ac.nz> wrote:

> On 29/07/2012, at 6:21 PM, C K Kashyap wrote:
> > I am struggling with an idea though - How can I capture the parent
> element of each element as I parse? Is it possible or would I have to do a
> second pass to do the fixup?
> Why do you *want* the parent element of each element?
> One of the insanely horrible aspects of the Document Object Model is that
> every
> element is nailed in place by pointers everywhere, with the result that you
> cannot share elements, and even moving an element was painful.
> I still do a fair bit of SGML/XML process in C using a "Document Value
> Model"
> library that uses hash consing, and it's so much easier it isn't funny.
> While you are traversing a document tree it is useful to keep track of the
> path from the root.  Given
>     data XML
>        = Element String [(String,String)] [XML]
>        | Text String
> you do something like
>     traverse :: ([XML] -> [a] -> a) -> ([XML] -> String -> a) -> XML -> a
>     traverse f g xml = loop [] xml
>       where loop ancs (Text s)           = g ancs  s
>             loop ancs e@(Element _ _ ks) = f ancs' (map (loop ancs') ks)
>                                            where ancs' = e:ancs
> (This is yet another area where Haskell's non-strictness pays off.)
> If you do that, then you have the parent information available without
> it being stored in the tree.
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