Question

If I have an XML document like this:

<root>   <elem name='Greeting'>     Hello   </elem>   <elem name='Name'>     Name   </elem> </root> 

and some Haskell type/data definitions like this:

 type Name = String  type Value = String  data LocalizedString = LS Name Value 

and I wanted to write a Haskell function with the following signature:

 getLocalizedStrings :: String -> [LocalizedString] 

where the first parameter was the XML text, and the returned value was:

 [LS 'Greeting' 'Hello', LS 'Name' 'Name'] 

how would I do this?

If HaXml is the best tool, how would I use HaXml to achieve the above goal?

Thank!

Answer 1

I’ve never actually bothered to figure out how to extract bits out of XML documents using HaXML; HXT has met all my needs.

{-# LANGUAGE Arrows #-} import Data.Maybe import Text.XML.HXT.Arrow  type Name = String type Value = String data LocalizedString = LS Name Value  getLocalizedStrings :: String -> Maybe [LocalizedString] getLocalizedStrings = (.) listToMaybe . runLA $ xread >>> getRoot  atTag :: ArrowXml a => String -> a XmlTree XmlTree atTag tag = deep $ isElem >>> hasName tag  getRoot :: ArrowXml a => a XmlTree [LocalizedString] getRoot = atTag 'root' >>> listA getElem  getElem :: ArrowXml a => a XmlTree LocalizedString getElem = atTag 'elem' >>> proc x -> do     name <- getAttrValue 'name' -< x     value <- getChildren >>> getText -< x     returnA -< LS name value 

You’d probably like a little more error-checking (i.e. don’t just lazily use atTag like me; actually verify that <root> is root, <elem> is direct descendent, etc.) but this works just fine on your example.

---

Now, if you need an introduction to Arrows, unfortunately I don’t know of any good one. I myself learned it the ‘thrown into the ocean to learn how to swim’ way.

Something that may be helpful to keep in mind is that the proc/-< syntax is simply sugar for the basic arrow operations (arr, >>>, etc.), just like do/<- is simply sugar for the basic monad operations (return, >>=, etc.). The following are equivalent:

getAttrValue 'name' &&& (getChildren >>> getText) >>^ uncurry LS  proc x -> do     name <- getAttrValue 'name' -< x     value <- getChildren >>> getText -< x     returnA -< LS name value