Clojure XML 流关闭异常

Question

我在解析带有clojure.data.xml 的 XML 文件时遇到异常，因为在解析完成之前流正在关闭。

我不明白为什么doall 不强制在with-open 关闭XML 数据之前对其进行评估（如this related answer 所建议）：

(:require [clojure.java.io :as io]
          [clojure.data.xml :as xml])

(defn file->xml [path] 
  (with-open [rdr (-> path io/resource io/reader)] 
    (doall (xml/parse rdr))))

抛出异常：

(file->xml "example.xml")
;-> XMLStreamException ParseError at [row,col]:[80,1926]
Message: Stream closed com.sun.org.apache.xerces.internal.impl.XMLStreamReaderImpl.next

如果我删除 with-open 包装器，它会按预期返回 XML 数据（因此文件是合法的，尽管不能保证阅读器已关闭）。

我看到 (source xml/parse) 产生惰性结果：

(defn parse
  "Parses the source, which can be an
   InputStream or Reader, and returns a lazy tree of Element records. 
   Accepts key pairs with XMLInputFactory options, see http://docs.oracle.com/javase/6/docs/api/javax/xml/stream/XMLInputFactory.html
   and xml-input-factory-props for more information. 
   Defaults coalescing true."
   [source & opts]
     (event-tree (event-seq source opts)))

所以也许这是相关的，但我拥有的功能与clojure.data.xml README 上的“往返”示例非常相似。

我在这里错过了什么？

Answer 1

我很惊讶看到这种行为。似乎clojure.data.xml.Element（返回类型）实现了一种不受doall影响的“惰性映射”类型。

这是一个将惰性值转换为法线贴图的解决方案：

(ns tst.clj.core
  (:use clj.core clojure.test tupelo.test)
  (:require
    [tupelo.core :as t]
    [clojure.string :as str]
    [clojure.pprint :refer [pprint]]
    [clojure.java.io :as io]
    [clojure.data.xml :as xml]
    [clojure.walk :refer [postwalk]]
  ))
(t/refer-tupelo)

(defn unlazy
  [coll]
  (let [unlazy-item (fn [item]
                      (cond
                        (sequential? item) (vec item)
                        (map? item) (into {} item)
                        :else item))
        result    (postwalk unlazy-item coll) ]
    result ))

(defn file->xml [path]
  (with-open [rdr (-> path io/resource io/reader) ]
    (let [lazy-vals    (xml/parse rdr)
          eager-vals   (unlazy lazy-vals) ]
      eager-vals)))
(pprint (file->xml "books.xml"))

{:tag :catalog,
 :attrs {},
 :content
 [{:tag :book,
   :attrs {:id "bk101"},
   :content
   [{:tag :author, :attrs {}, :content ["Gambardella, Matthew"]}
    {:tag :title, :attrs {}, :content ["XML Developer's Guide"]}
    {:tag :genre, :attrs {}, :content ["Computer"]}
    {:tag :price, :attrs {}, :content ["44.95"]}
    {:tag :publish_date, :attrs {}, :content ["2000-10-01"]}
    {:tag :description,
     :attrs {},
     :content
     ["An in-depth look at creating applications\n      with XML."]}]}
  {:tag :book,
   :attrs {:id "bk102"},
   :content
   [{:tag :author, :attrs {}, :content ["Ralls, Kim"]}
    {:tag :title, :attrs {}, :content ["Midnight Rain"]}
    {:tag :genre, :attrs {}, :content ["Fantasy"]}
    {:tag :price, :attrs {}, :content ["5.95"]}
    {:tag :publish_date, :attrs {}, :content ["2000-12-16"]}
    {:tag :description,
     :attrs {},
     :content
     ["A former architect battles corporate zombies,\n      an evil sorceress, and her own childhood to become queen\n      of the world."]}]}
  {:tag :book,
   :attrs {:id "bk103"},
   :content .....

由于clojure.data.xml.Element 实现了clojure.lang.IPersistentMap，因此使用(map? item) 返回true。

这里是sample data for books.xml

请注意：

clojure.data.xml 与 clojure.xml 不同。您可能需要探索这两个库，才能找到最适合您需求的库。

• https://clojuredocs.org/clojure.xml
• https://github.com/clojure/data.xml

您也可以在需要时使用crossclj.info 查找 api 文档：

• https://crossclj.info/doc/org.clojure/clojure/latest/clojure.xml.html
• https://crossclj.info/doc/org.clojure/data.xml/0.2.0-alpha2/index.html

更新：

在我看到这个问题大约一周后，我遇到了一个 XML 解析问题，就像这个需要 unlazy 函数的问题一样。你现在可以找到unlazyin the Tupelo library。