[Haskell-cafe] hxt and pickler combinations

[Andrea Rossato]

On Thu, Jun 26, 2008 at 04:11:58PM +0200, Andrea Rossato wrote:
> Hello,
> 
> I'm using HXT for writing a Citation Style Language
> (http://xbiblio.sourceforge.net) implementation in Haskell and I'm
> trying to use the hxt pickler library to parse XML data contained in
> elements that can be interleaved, that is to say, elements that can
> appear in any order within other elements.
> 
> For instance:
> <data>
>   <string>ciao</string>
>   <int>2</int>
> </data>
> or
> <data>
>   <int>2</int>
>   <string>ciao</string>
> </data>
> 
> are both permitted.
> 
> I'm not able to write picklers able to parse such kind of data. I
> indeed noticed that this is not possible with interleaved elements,
> but it is possible with attributes.

Thanks to a suggestion from Uwe (I contacted the HXT authors too,
since I thought it could be a non intended behaviour: instead it is,
in order to conform to the standard DTD validation), I came up with
this solution. I'm leaving it here too, for the archives.

This is a pickler that search the element in the contents and match it
without regard to the elements' order:

xpElem'  :: String -> PU a -> PU a
xpElem' name pa
    = PU { appPickle   = ( \ (a, st) ->
                           let
                           st' = appPickle pa (a, emptySt)
                           in
                           addCont (XN.mkElement (mkName name) (attributes st') (contents st')) st
                                                    )
         , appUnPickle = \ st -> fromMaybe (Nothing, st) (unpickleElement st)
         , theSchema   = scElem name (theSchema pa)
         }
      where
      unpickleElement st
          = do
            let t = contents st
            n <- mapM XN.getElemName t
            case elemIndex name (map qualifiedName n) of
              Nothing -> fail "element name does not match"
              Just i  -> do
                let cs = XN.getChildren (t !! i)
                al <- XN.getAttrl (t !! i)
                res <- fst . appUnPickle pa $ St {attributes = al, contents = cs}
                return (Just res, st {contents = take i t ++ drop (i + 1) t})


Andrea