update DOM in Haskell

Question

I need to deep select a XML node from a document and transform it then move it to another position.

This example is taken from Haskell cafe and slightly changed:

Select the //d/e/f and then change the content at f and move the new f node under a.

<a>
...
  <d>
    <e>
      <f>some data to change
      </f>
    </e>
  </d>
...
</a>

I need to do this kind of operation for several times, so it's better if the update operation is composeable.

I am looking at xml-conduit package, but seems it lacks of DOM manipulation functions, what's the best way to do this kind of task?

What do you mean by DOM manipulation? The data structures used by xml-conduit- like most data structures in Haskell- are immutable. The way you transform an XML tree is by creating a new tree. If you describe in more detail what you're trying to achieve, you may get more informative answers. — Michael Snoyman, Mar 17 '14 at 06:57
@MichaelSnoyman Thanks Michael, I've updated the question, and I've read your solution to the problem, but hard code the selection logic into recursion is not what I want, it would be nice if xml-conduit has `update` or `insertAt` functions. — Sawyer, Mar 17 '14 at 07:29
@MichaelSnoyman also, seems xml-conduit is implemented using Zipper, it's very easy to select f `cur $// element "d" $/ element "e" $/ element "f"`, but if I made some changes and want to go back to the original tree with the changes, what should I do? The zipper hole and zipper context form the whole tree, it should have a way to go back right? I used to use this in Anti-XML, see here: http://anti-xml.org/zippers.html starts from the `Now comes the tricky part` paragraph. — Sawyer, Mar 17 '14 at 07:52
That usage isn't really encouraged by the xml-conduit API. However, it looks like exactly the kind of usage that a lens wrapper would support. I haven't used it myself, but I'd recommend having a look at [xml-lens](https://hackage.haskell.org/package/xml-lens). — Michael Snoyman, Mar 17 '14 at 14:59
@MichaelSnoyman `xml-lens` seems like a solely lens based XML selector, doesn't do the transformation, and the `xml-conduit` axes can also do the selection work. I feel like the `xml-conduit` only has the Zipper navigation functions, plus some filters, but doesn't have merging context and focus back the original tree features, which are also useful. — Sawyer, Mar 17 '14 at 15:18
possible duplicate of [How do i replace Nodes in HXT?](http://stackoverflow.com/questions/9483248/how-do-i-replace-nodes-in-hxt) — Paul Sweatte, Sep 05 '14 at 01:21

score 1 · Answer 1 · answered Feb 25 '15 at 14:19

I'm quite new to HXT, so there may be better ways, but I solved a similar problem like this:

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE Arrows, NoMonomorphismRestriction #-}

module Abc where

import Text.XML.HXT.Core

moveElts :: ArrowXml a => a XmlTree XmlTree
moveElts =
  processTopDown (
    -- Insert <f>'s under <a>:
    slurpF `when` hasName "a"
    >>>
    -- and remove the old <f>'s:
    removeChildrenExcept "f" `when` neg (hasName "a")
    )
  where
    removeChildrenExcept tag =
      replaceChildren (getChildren >>> neg (hasName tag))
    slurpF = proc a -> do
      elts <- deep (hasName "f") -< a
      returnA (insertChildrenAt 0 (constA elts)) -<< a


-- Test:
test :: IO ()
test = runX (readDocument [withValidate no] "abc.xml"
             >>> moveElts
             >>> writeDocumentToString [withIndent yes])
       >>= putStrLn . head

update DOM in Haskell

1 Answers1