[Haskell-cafe] HXT: desperatedly trying to "concat"

Steffen Schuldenzucker Steve.Schuldenzucker at web.de
Thu Apr 2 09:26:21 EDT 2009


I've got a problem with the Haskell XML Toolkit (hxt). I want to write a little app that performs REST requests from a certain (rather simple) XML format. 
A example procedure Call looks like testData defined below.
What I'd like to do is to transform this xml tree into a GET variable string using an XmlArrow. The task sounds easy, and it has to be easy, but I've been sitting here for about a day now, staring at my code.

It looks like this (the transformation should be done by the arrow "mkGetStr"):

-- Rest.hs
-- This is also on HPaste: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3210#a3210

{-# LANGUAGE NoMonomorphismRestriction #-}

module Rest where

import Text.XML.HXT.Arrow
import Data.List

getParamsA = hasName "param" >>> getChildren >>> isElem
         >>> (getName &&& (getChildren >>> getText)) >>> arr2 mkGetPair

mkMethodStr = ("method=" ++) 
mkGetPair k v = k ++ "=" ++ v

getMethodA = hasName "method" >>> getChildren >>> getText >>> arr mkMethodStr

mkGetStr = isElem
       >>> (getMethodA <+> getParamsA) 
       >>. intercalate "&" 

-- Try it with: runX (testData >>> mkGetStr) >>= print
testData = xread <<< constA (
 ++ "<param>"
 ++     "<foo_arg>Foo</foo_arg>"
 ++     "<bar_arg>Bar</bar_arg>"
 ++ "</param>" )

-- End of Rest.hs

What I get out of it is this (in ghci):

*Rest> runX (testData >>> mkGetStr) >>= print

There is an "&" missing right after "method=my.Method"!
Why? I've tried many variants of this and they give me either this or a similar result or multiple results (what I don't want either).

I'd be really happy if someone could save my day and help me with this issue.

Thanks in advance,

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