将Text.Pandoc.writers打印到ghci :没有显示实例

Question

，我想做什么?

在ghci下打印格式和作者的关联列表。

请参见文档：

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

什么已经试过了

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

我期望自动导入相应的show实例，但情况似乎并非如此。我必须承认，我不知道如何定义一个实例声明(Show (WriterOptions -> Pandoc -> Char) )。作为解决办法，我尝试导入Pandoc库的其他模块，但仍然没有Show实例可用。

那么我应该自己来定义这个实例吗？

如果是的话，你有什么建议与我分享以完成这项任务。

如果我不应该，有什么问题吗？

提前谢谢你的帮助。

编辑

好吧，我想我看到我太太了：

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"]

我认为在我的元组里展示第二个东西是没有意义的。它是一种功能，所以我们不能展示它。

我想这应该是问题所在。

Answer 1

由于元组包含两种不同的类型，所以我试图做的事情没有任何意义。

第一个是特定编写器的标识符(类型为string)，第二个是写入器本身(然后是一个函数)。当然，如果我尝试全部打印，它将失败，因为函数没有显示实例。

然后，要检索Pandoc中可用的作者列表(目的是动态调用相应的函数)，只需检索标识符列表，如下所示：

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"]