[Haskell-cafe] hxt and pickler combinations

Andrea Rossato mailing_list at istitutocolli.org
Thu Jun 26 10:11:58 EDT 2008


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:

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.

To make myself hopefully clearer I included some code below. Suppose
we have a data structure like:

data Term = T Int String deriving ( Show )

If encoded in XML without respecting the ordering (first the Int and
then the String), picklers seem to fail. But if I use attributes to
store the values, this doesn't happen.

In other word, the xp1 pickler (taken from the example below) will
fail with such an xml doc:


since it requires the 'int' element to appear before the 'string'

To test this behaviour, run the code below and see that:

- test doc1 xp1 will fail
- test doc2 xp1 will succeed


- test doc3 xp2 will succeed
- test doc4 xp2 will succeed

What am I getting wrong? It's just a matter of wrong combinator I'm
choosing or I'm getting wrong something more fundamental?



The code:

import Text.XML.HXT.Arrow

test :: String -> PU Term -> IO ()
test t xp = do
      p <- runX ( constA t
                  xunpickleVal xp
      putStrLn (show p)

data Term = T Int String deriving ( Show )

xp1, xp2 :: PU Term
xp1 = xpElem "data" $
      xpWrap (uncurry T, \(T i s) -> (i, s)) $
      xpPair (xpElem "int"    xpickle)
             (xpElem "string" xpText )
xp2 = xpElem "data" $
      xpWrap (uncurry T, \(T i s) -> (i, s)) $
      xpPair (xpAttr "int"    xpickle)
             (xpAttr "string" xpText )

doc1, doc2, doc3, doc4 :: String
doc1 = "<data><string>ciao</string><int>2</int></data>"
doc2 = "<data><int>2</int><string>ciao</string></data>"
doc3 = "<data int=\"2\" string=\"ciao\" />"
doc4 = "<data string=\"ciao\" int=\"4\" />"

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