从子树中提取值 [英] Extracting Values from a Subtree

问题描述

我用 HXT 解析一个XML文件，我试图将一些节点提取分解为模块化的部分（我一直使用它作为我的指南）。不幸的是，一旦我进行了第一级解析，我无法弄清楚如何应用一些选择器。

  import Text.XML.HXT.Core 
 
 let node tag = multi（hasName tag）
 xml<  -  readFiletest.xml
 let doc = readString [withValidate yes，withParseHTML no，withWarnings no] xml 
 books<  -  runX $ doc>>>节点book
  

我发现书籍的类型为 [XmlTree]

 ：t books 
 books :: [XmlTree] 
  

现在我想获取 books 的第一个元素，然后提取一些子树内的值。

  let b = head（books）
 runX $ b>>>节点成本
 
无法与'IOSLA（XIOState（））XmlTree'类型的'Data.Tree.NTree.TypeDefs.NTree'
匹配
预期类型：IOSLA （XIOState（））XmlTree XNode 
实际类型：XmlTree 
在'（>>）'的第一个参数中，即'b'
在第二个参数' $）'，即'b>>>节点成本'
  

我有一个 XmlTree 并且我正在显示上述不正确的用法来说明我想要的内容。我知道我可以这样做：

  runX $ doc>>>节点书>>>节点成本/> getText 
 [55.9，95.0] 
  

但我不仅对成本，但还包含 book 中的更多元素。 XML文件非常深，所以我不想用< +> 来嵌套所有内容，而更多的评价者更喜欢提取我想要的块，然后提取子元素在一个单独的功能。

示例（编制）XML文件：

 < ？xml version =1.0encoding =UTF-8？>< start xmlns =http://www.example.com/namespacexmlns：xsi =http://www.w3.org/ 2001 / XMLSchema的实例> 
< books> 
< book> 
< author> 
<名称> 
< first> Joe< / first> 
< last> Smith< / last> 
< / name> 
< city>纽约市< / city> 
< / author> 
<发布> 1990-11-15< /发布> 
< isbn> 1234567890< / isbn> 
< publisher> X发布者< / publisher> 
<成本> 55.9< / cost> 
< / book> 
< book> 
< author> 
<名称> 
< first> Jane< / first> 
< last> Jones< / last> 
< / name> 
< city>旧金山< / city> 
< / author> 
<发布> 1999-01-19< /发布> 
< isbn> 0987654321< / isbn> 
< publisher> Y发布者< / publisher> 
<成本> 95.0< / cost> 
< / book> 
< / books> 
< / start> 
  

有人可以帮我理解，如何提取的子元素？理想情况下，使用>>> 和节点之类的东西，这样我就可以定义我自己的函数，例如 getCost ， getName 等等，每一个都大致具有签名 XmlTree - > [String]

解决方案

doc is不是你认为的那样。它的类型是 IOStateArrow s b XmlTree 。你真的应该再次阅读你的指南，所有你想知道的是在标题避免IO。

箭头基本上是函数。 SomeArrow a b 可以被看作是类型为 a - >的泛化/专用函数。 B'/ code>。 >>> 和范围中的其他操作符都是用于箭头组合的，类似于函数组合。你的 books 有类型 [XmlTree] ，所以它不是箭头，不能用箭头组成。满足你的需求的是 runLA ，它将节点tag这样的箭头转换成一个普通函数：

 模块Main其中
 
 import Text.XML.HXT.Core 
 
 main =做
 html<  -  readFiletest.xml
 let doc = readString [withValidate yes，withParseHTML no，withWarnings no] html 
 books<  -  runX $ doc>>> ;节点book
  -  runLA（节点成本/> GT; getText）:: XmlTree  - > [String] 
 let cost = books>> = runLA（节点成本/> getText）
打印成本
 
节点标签=多（hasName标签）
  




I am parsing an XML file with HXT and I am trying to break up some of the node extraction into modular pieces (I have been using this as my guide). Unfortunately, I cannot figure out how to apply some of the selectors once I do the first level parsing.

 import Text.XML.HXT.Core

 let node tag = multi (hasName tag)
 xml <- readFile "test.xml"
 let doc = readString [withValidate yes, withParseHTML no, withWarnings no] xml
 books <- runX $ doc >>> node "book"

I see that books has a type [XmlTree]

 :t books
 books :: [XmlTree]

Now I would like to get the first element of books and then extract some values inside the sub-tree.

 let b = head(books)
 runX $ b >>> node "cost"

Couldn't match type ‘Data.Tree.NTree.TypeDefs.NTree’
               with ‘IOSLA (XIOState ()) XmlTree’
Expected type: IOSLA (XIOState ()) XmlTree XNode
  Actual type: XmlTree
In the first argument of ‘(>>>)’, namely ‘b’
In the second argument of ‘($)’, namely ‘b >>> node "cost"’

I cannot find selectors once I have an XmlTree and I am showing the above incorrect usage to illustrate what I would like to. I know I can do this:

 runX $ doc >>> node "book" >>> node "cost" /> getText
 ["55.9","95.0"]

But I am not only interested in cost but also many more elements inside book. The XML file is pretty deep so I don't want to nest everything with <+> and much rater prefer extract the chunk I want and then extract the sub-elements in a separate function.

Example (made-up) XML File:

 <?xml version="1.0" encoding="UTF-8"?><start xmlns="http://www.example.com/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
     <books> 
         <book>
             <author>
                 <name>
                     <first>Joe</first>
                     <last>Smith</last>
                 </name>
                 <city>New York City</city>
             </author>
             <released>1990-11-15</released>
             <isbn>1234567890</isbn>
             <publisher>X Publisher</publisher>
             <cost>55.9</cost>
         </book>
         <book>
             <author>
                 <name>
                     <first>Jane</first>
                     <last>Jones</last>
                 </name>
                 <city>San Francisco</city>
             </author>
             <released>1999-01-19</released>
             <isbn>0987654321</isbn>
             <publisher>Y Publisher</publisher>
             <cost>95.0</cost>
         </book>
     </books>
  </start> 

Can someone help me understand, how to extract the sub-elements of book? Ideally with something as nice as >>> and node so I can define my own functions such as getCost, getName, etc. that each will roughly have the signature XmlTree -> [String]

解决方案

doc is not what you thought it is. It has type IOStateArrow s b XmlTree. You really should read your guide again, all you want to know was concluded under the title "Avoiding IO".

Arrows are basically functions. SomeArrow a b can be considered as a generalized/specialized function of type a -> b. >>> and other operators in the scope are for arrow composition, similar to function composition. Your books has type [XmlTree] so it's not an arrow and cannot be composed with arrows. What fulfills your needs is runLA, it transforms an arrow like node "tag" to a normal function:

module Main where

import           Text.XML.HXT.Core

main = do
  html <- readFile "test.xml"
  let doc = readString [withValidate yes, withParseHTML no, withWarnings no] html
  books <- runX $ doc >>> node "book"
  -- runLA (node "cost" /> getText) :: XmlTree -> [String]
  let costs = books >>= runLA (node "cost" /> getText)
  print costs

node tag = multi (hasName tag)

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