在Arrow proc上下文中提取多个元素

Question

我想解析以下示例XML文件，而没有pickler模块。

<?xml version="1.0" encoding="utf-8" ?>
<Groups> 
    <Name>ABC</Name>
    <GroupA>
        <Name>Foo</Name>
        <Sum>100</Sum>
    </GroupA>
    <GroupB>
        <Name>Bar</Name>
        <Sum>0</Sum>
    </GroupB>
</Groups>

我结束了这个：

{-# language Arrows #-}

import Text.XML.HXT.Core

data Groups = Groups GroupA GroupB deriving Show
data GroupA = GroupA String String deriving Show
data GroupB = GroupB String String deriving Show


readGroup :: LA XmlTree Groups
readGroup = deep (isElem >>> hasName "Groups") >>> getChildren >>>
  proc root -> do
    a <- readGroupA -< root
    b <- readGroupB -< root
    returnA -< Groups a b

readGroupA :: LA XmlTree GroupA
readGroupA = isElem >>> hasName "GroupA" >>> getChildren >>>
  proc root -> do
    n <- isElem >>> hasName "Name" /> getText -< root
    s <- isElem >>> hasName "Sum"  /> getText -< root
    returnA -< GroupA n s

readGroupB :: LA XmlTree GroupB
readGroupB = isElem >>> hasName "GroupB" >>> getChildren >>>
  proc root -> do
    n <- isElem >>> hasName "Name" /> getText -< root
    s <- isElem >>> hasName "Sum"  /> getText -< root
    returnA -< GroupB n s

不幸的是，这不起作用。如果我尝试在proc上下文中仅提取单个元素，那么它将起作用。但是尝试提取多个元素将始终失败\返回空列表。我可能对构图有误解>>>。

我用这个例子 runLa (xreadDoc >>> readGroups)