Clojure RuntimeException-标签db / id没有读取器功能

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问题描述

在Clojure中遇到此错误时会发生什么情况？

What is happening when I get this error in Clojure?

java.lang.RuntimeException: No reader function for tag db/id

标记文字

此错误消息与Clojure 1.7中引入的功能有关，
。标记文字是扩展
的一种简单方法，它可以在Clojure代码或
数据。

Tagged Literals

This error message is related to a feature introduced in Clojure 1.7,tagged literals. Tagged literals are a simple means for extendingwhat data types can be represented as literals in Clojure code or EDNdata.

Clojure附带了带有两个标记文字的阅读器，它们分别是 #inst 和
#uuid 允许 java.util.Date 和
<$ c $的文字表示c> java.util.UUID 。可以通过在类路径中将
data_readers.clj 文件包括为。

Clojure ships with readers for two tagged literals, #inst and#uuid allowing literal representations of java.util.Date andjava.util.UUID. Support for more tags can be added by includingdata_readers.clj files on the classpath as described in thedocumentation.

此错误的一些常见原因：

Some common causes of this error:

尝试读取包含＃db / id的Datomic事务数据文字

尝试使用，而不会在：readers

http://boot-clj.com/ rel = noreferrer>启动任务，这些任务通过传递
* data-readers * 来尝试使用EDN阅读器code>。

Trying to read Datomic transaction data containing #db/id literals
While attempting to use the EDN reader without passing :readers
In Boot tasks that try to use the EDN reader by passing*data-readers*.

一个常见问题导致此错误的原因是尝试使用，而未通过：readers 选项。从文档字符串中获取 clojure.edn / read ：

One common cause of this error is trying to use the EDN readerwithout passing the :readers option. From the doc string forclojure.edn/read:

:readers  - a map of tag symbols to data-reader functions to be
            considered before default-data-readers. When not
            supplied, only the default-data-readers will be used.

顾名思义， default-data-readers 仅是与Clojure一起提供的数据个读者的地图。也就是说，它不包括您正在使用的任何库的 data_readers.clj 文件中定义的任何读取器（例如Datomic）。在这种情况下，解决方案是通过当前加载的数据读取器集，该集合位于var * data-readers * 中。

As its name implies, default-data-readers is only the map of datareaders that ship with Clojure itself. That is, it does not includeany readers defined in the data_readers.clj files of any librariesyou are using (like, say, Datomic). The solution in this case is topass the current set of loaded data readers, found in the var*data-readers*.

(clojure.edn/read {:readers *data-readers*} <your-read-source>)

甚至更好的是，显式指定要使用的数据读取器的映射。例如，如果您只想使用原子数据读取器：

Or, even better, explicitly specify the mapping of the data readersyou want to use. For instance, if you want to be able to use only thedatomic data readers:

(require 'datomic.db
         'datomic.function
         'datomic.codec)

(edn/read {:readers {'db/id  datomic.db/id-literal
                     'db/fn  datomic.function/construct
                     'base64 datomic.codec/base-64-literal}}
          <your-read-source-here>)

当然，由于这些只是我们正在处理的Clojure映射，因此还可以包含默认数据读取器通过合并地图：

And of course, since these are just clojure maps we're dealing with,we could also include the default-data-readers by merging the maps:

(require 'datomic.db
         'datomic.function
         'datomic.codec)

(edn/read {:readers (merge
                     default-data-readers
                     {'db/id  datomic.db/id-literal
                      'db/fn  datomic.function/construct
                      'base64 datomic.codec/base-64-literal})}
          <your-read-source-here>)

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readers