Question

我想做什么？

在ghci下打印格式和作者的关联列表见doc：

writers :: [(String, Writer)]
Association list of formats and writers.

已尝试的内容

zurgl>>>import Text.Pandoc as P
zurgl>>>P.writers 

<interactive>:20:1:
    No instance for (Show (WriterOptions -> Pandoc -> [Char]))
      arising from a use of `print'
    Possible fix:
      add an instance declaration for
      (Show (WriterOptions -> Pandoc -> [Char]))
    In a stmt of an interactive GHCi command: print it

我希望自动导入相应的show实例，但似乎并非如此。我必须承认我没有任何线索如何为（Show（WriterOptions - ＆gt; Pandoc - ＆gt; [Char]）定义实例声明。作为解决方法，我试图导入Pandoc库的附加模块，但仍然没有可用的Show实例。

然后我应该自己定义这个实例吗？如果是，请告诉您与我分享完成此任务的任何提示如果我不应该是什么问题？

提前感谢您的帮助。

编辑

好吧，我想我看到了我的误解：

做：

zurgl>>>map (\x-> fst x) P.writers 
["native","json","html","html5","html+lhs","html5+lhs","s5","slidy","slideous","dzslides","docbook","opendocument","latex","latex+lhs","beamer","beamer+lhs","context","texinfo","man","markdown","markdown+lhs","plain","rst","rst+lhs","mediawiki","textile","rtf","org","asciidoc"]

我认为尝试在我的元组中显示第二个东西是没有意义的。它似乎是一个功能，然后我们无法显示它。

我想这应该是问题所在。

Answer 1

我尝试做的事情毫无意义，因为元组包含两种不同的类型第一个是特定编写器的标识符（类型为字符串），第二个是编写器本身（然后是函数）。当然，如果我尝试打印所有这些，它将失败，因为没有显示功能。

然后要检索Pandoc中可用编写器的列表（目的是动态调用相应的函数），我们只需检索标识符列表，如下：

zurgl>>>map fst P.writers 
["native","json","html","html5","html+lhs","html5+lhs","s5","slidy","slideous","dzslides","docbook","opendocument","latex","latex+lhs","beamer","beamer+lhs","context","texinfo","man","markdown","markdown+lhs","plain","rst","rst+lhs","mediawiki","textile","rtf","org","asciidoc"]

将Text.Pandoc.writers打印成ghci：No Show实例

1 个答案: