Question

I want to get access to the data of an XML file like

<?xml version="1.0"?>
<MY>
  <Foo id="1" name="test">
    <Argument name="a" />
  </Foo>
  <Foo id="2" name="test2">
    <Argument name="a" />
    <Argument name="b" />
  </Foo>
  <Other id="2" name="someOther"/>
</MY>

I want e.g. to read out each Foo with its Arguments, how can I do this with Haskell? (I would like to use the HaXml module)

I don’t know where to start.

Answer 1

I cannot find up to date documentation and examples for haXml.

There is however some documentation for HXT available.
I know that’s probably an overkill for your example, but anyway.

If you want to use tagsoup, perhaps the following answers might help:
xml-tree parser (Haskell) for graph-library
In Haskell how do you extract strings from an XML document?

Here is the documentation an examples for HXT:
http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_XML
http://www.haskell.org/haskellwiki/HXT
http://www.haskell.org/haskellwiki/HXT/Practical
http://en.wikibooks.org/wiki/Haskell/XML

Now the code using HXT.
(warning I am not sure if this is the correct way)

I followed the tutorial:
http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_XML

you need your xml file as “data.xml”

import Data.Map (Map, fromList, toList)
import Text.XML.HXT.Core


type Foos = Map String [Foo]

data Foo = Foo
     { 
       fooId :: String 
     , fooName :: String
     , arguments :: [Argument]  
     } 
          deriving (Show, Eq)



data Argument = Argument
      { argName  :: String
      }
           deriving (Show, Eq)

instance XmlPickler Foo where
  xpickle = xpFoo


instance XmlPickler Argument where
  xpickle = xpArgument

-- WHY do we need this?? no clue            
instance XmlPickler Char where
    xpickle = xpPrim

-- this could be wrong
xpFoos :: PU Foos
xpFoos
  = xpWrap (fromList
          , toList
          ) $
  xpList $
      xpElem "MY" $ 
      xpickle

xpFoo :: PU Foo
xpFoo
  = xpElem "Foo" $
     xpWrap ( uncurry3 Foo
            , \ f -> (fooId f
                      , fooName f
                      , arguments f
                     )  
           ) $          
    xpTriple (xpAttr "id" xpText) 
              (xpAttr "name" xpText)
              (xpList xpickle)


xpArgument :: PU Argument
xpArgument
    = xpElem "Argument" $
       xpWrap ( \ ((a)) -> Argument a
               , \ t -> (argName t)
              ) $
       (xpAttr "name" xpText )


main    :: IO ()
main
     = do
       runX ( xunpickleDocument xpFoos
                                [ withValidate no
                                , withTrace 1
                                , withRemoveWS yes
                                , withPreserveComment no
                                ] "data.xml"
         >>>
             arrIO ( \ x -> do {print x ; return x}) 
            )
       return ()

RESULT (you need you xml example as “data.xml”):

-- (1) getXmlContents
-- (1) readDocument: "data.xml" (mime type: "text/xml" ) will be processed
-- (1) readDocument: "data.xml" processed
fromList [("",[Foo {fooId = "1", fooName = "test", arguments = [Argument {argName = "a"}]},
Foo {fooId = "2", fooName = "test2", arguments = [Argument {argName = "a"},
Argument     {argName = "b"}]}])]