在Haskell中，如何从XML文档中提取字符串？ [英] In Haskell how do you extract strings from an XML document?

问题描述

如果我有这样的XML文档：

 < root> 
< elem name =问候语> 
您好
< / elem> 
< elem name =Name> 
名称
< / elem> 
< / root> 
  

以及一些像这样的Haskell类型/数据定义：

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

我想用以下签名编写一个Haskell函数：

  getLocalizedStrings :: String  - > [LocalizedString] 
  

其中第一个参数是XML文本，返回的值是：

  [LSGreetingHello，LSNameName] 
  

我该怎么做？

如果HaXml是最好的工具，我会如何使用HaXml以达到上述目标？

谢谢！

解决方案

我从来没有真正想过弄清楚如何使用 HaXML ; HXT 满足了我所有的需求。

  { - ＃LANGUAGE箭头＃ - } 
导入Data.Maybe 
导入Text.XML.HXT.Arrow 
 
 type Name = String 
 type Value = String 
 data LocalizedString = LS Name Value 
 
 getLocalizedStrings :: String  - >也许[LocalizedString] 
 getLocalizedStrings =（。）listToMaybe。 runLA $ xread>>> getRoot 
 
 atTag :: ArrowXml a =>字符串 - > XmlTree XmlTree 
 atTag tag = deep $ isElem>>> hasName标记
 
 getRoot :: ArrowXml a => XmlTree [LocalizedString] 
 getRoot = atTagroot>>> listA getElem 
 
 getElem :: ArrowXml a => XmlTree LocalizedString 
 getElem = atTagelem>>> proc x  - > do 
 name<  -  getAttrValuename - < x 
值<  -  getChildren>>> getText  - < x 
 returnA  - < LS名称值
  

您可能想要多一点错误检查（即不要只是懒惰地使用 atTag 像我一样;实际验证< root> 是root，< elem> ; 是直接派生的，等等），但是在你的例子中这很好。

---

现在，如果您需要 Arrow 的介绍，不幸的是我不知道有什么好的。我自己学会了投入大海学习如何游泳的方式。

有些事情可能有助于记住， proc / - < 语法对于基本箭头操作（ arr ，>>> 等），就像 do / - 仅仅是基本monad操作的糖（ return ，>> = 等）。以下是相同的：

  getAttrValuename&&& （getChildren>>> getText）>>< uncurry LS 
 
 proc x  - > do 
 name<  -  getAttrValuename - < x 
值<  -  getChildren>>> getText  - < x 
 returnA  - < LS名称值
  

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!

解决方案

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

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