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Clojure世界：XML處理
XML處理也是個常見的編程工作，雖然說在Clojure裏你很少使用XML做配置文件，但是跟遺留係統集成或者處理和其他係統通訊，可能都需要處理XML。Clojure的標準庫clojure.xml就是用來幹這個事情的。一個簡單的例子如下，首先我們要解析的是下麵這個簡單的XML：
<?xml version="1.0" encoding="UTF-8"?>
<books>
<book>
<title>The joy of clojure</title>
<author>Michael Fogus / Chris House</author>
</book>
<book>
<title>Programming clojure</title>
<author>Stuart Halloway</author>
</book>
<book>
<title>Practical clojure</title>
<author>Luke Van der Hart</author>
</book>
</books>
<books>
<book>
<title>The joy of clojure</title>
<author>Michael Fogus / Chris House</author>
</book>
<book>
<title>Programming clojure</title>
<author>Stuart Halloway</author>
</book>
<book>
<title>Practical clojure</title>
<author>Luke Van der Hart</author>
</book>
</books>
解析xml用clojure.xml/parse方法即可，該方法返回一個clojure.xml/element這個struct-map組成的一棵樹：
user=> (use '[clojure.xml])
nil
user=> (parse "test.xml")
{:tag :books, :attrs nil, :content
[{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}]}
nil
user=> (parse "test.xml")
{:tag :books, :attrs nil, :content
[{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}]}
這是一個嵌套的數據結構，每個節點都是clojure.xml/element結構，element包括：
(defstruct element :tag :attrs :content)
tag、attrs和content屬性，tag就是該節點的標簽，attrs是一個屬性的map，而content是它的內容或者子節點。element是一個struct map，它也定義了三個方法來分別獲取這三個屬性：
user=> (def x (parse "test.xml"))
#'user/x
user=> (tag x)
:books
user=> (attrs x)
nil
user=> (content x)
[{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}]
books節點是root node，它的content就是三個book子節點，子節點組織成一個vector，我們可以隨意操作：#'user/x
user=> (tag x)
:books
user=> (attrs x)
nil
user=> (content x)
[{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}]
user=> (tag (first (content x)))
:book
user=> (content (first (content x)))
[{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]
user=> (content (first (content (first (content x)))))
["The joy of clojure"]
:book
user=> (content (first (content x)))
[{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]
user=> (content (first (content (first (content x)))))
["The joy of clojure"]
額外提下，clojure.xml是利用SAX API做解析的。同樣它還有個方法，可以將解析出來的結構還原成xml，通過emit：
user=> (emit x)
<?xml version='1.0' encoding='UTF-8'?>
<books>
<book>
<title>
The joy of clojure
</title>
<author>
Michael Fogus / Chris House
</author>
</book>
<book>
<?xml version='1.0' encoding='UTF-8'?>
<books>
<book>
<title>
The joy of clojure
</title>
<author>
Michael Fogus / Chris House
</author>
</book>
<book>
如果你要按照深度優先的順序遍曆xml，可以利用xml-seq將解析出來的樹構成一個按照深度優先順序排列節點的LazySeq，接下來就可以按照seq的方式處理，比如利用for來過濾節點：
user=> (for [node (xml-seq x)
:when (= :author (:tag node))]
(first (:content node)))
("Michael Fogus / Chris House" "Stuart Halloway" "Luke Van der Hart")
:when (= :author (:tag node))]
(first (:content node)))
("Michael Fogus / Chris House" "Stuart Halloway" "Luke Van der Hart")
通過:when指定了條件，要求節點的tag是author，這樣就可以查找出所有的author節點的content，是不是很方便？就像寫英語描述一樣。
更進一步，如果你想操作parse解析出來的這棵樹，你還可以利用clojure.zip這個標準庫，它有xml-zip函數將xml轉換成zipper結構，並提供一係列方法來操作這棵樹：
user=>(def xz (xml-zip x))
#'user/xz
user=> (node (down xz))
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
user=> (-> xz down right node)
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
user=> (-> xz down right right node)
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}
user=> (-> xz down right right lefts)
({:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]})
#'user/xz
user=> (node (down xz))
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
user=> (-> xz down right node)
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
user=> (-> xz down right right node)
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}
user=> (-> xz down right right lefts)
({:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["The joy of clojure"]} {:tag :author, :attrs nil, :content ["Michael Fogus / Chris House"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]})
是不是酷得一塌煳塗？可以通過up,down,left,right,lefts,rights,來查找節點的鄰近節點，可以通過node來得到節點本身。一切顯得那麼自然。更進一步，你還可以“編輯“這棵樹，比如刪除The joy of clojure這本書：
user=> (def loc-in-new-tree (remove (down xz)))
#'user/loc-in-new-tree
user=> (root loc-in-new-tree)
{:tag :books, :attrs nil, :content
[{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}]}
#'user/loc-in-new-tree
user=> (root loc-in-new-tree)
{:tag :books, :attrs nil, :content
[{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Programming clojure"]} {:tag :author, :attrs nil, :content ["Stuart Halloway"]}]}
{:tag :book, :attrs nil, :content [{:tag :title, :attrs nil, :content ["Practical clojure"]} {:tag :author, :attrs nil, :content ["Luke Van der Hart"]}]}]}
ok,隻剩下兩本書了，更多方法還包括replace做替換，edit更改節點等。因此編輯XML並重新生成，你一般可以利用clojure.zip來更改樹，最後利用clojure.xml/emit將更改還原為xml。
生成xml除了emit方法，還有一個contrib庫，也就是prxml，這個庫的clojure 1.3版本有人維護了一個分支，在這裏。主要方法就是prxml，它可以將clojure的數據結構轉換成xml：
user=>(prxml [:p [:raw! "<i>here & gone</i>"]])
<p><i>here & gone</i></p>
<p><i>here & gone</i></p>
顯然，它也可以用於生成HTML。
xpath的支持可以使用clj-xpath這個開源庫，遺憾的是它目前僅支持clojure 1.2。
文章轉自莊周夢蝶 ，原文發布時間2012-02-18
最後更新：2017-05-18 20:36:13