1

我想做什么？

在 ghci 下打印格式和作者的关联列表。
见文档： 

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

什么已经尝试过

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节目实例，但似乎并非如此。而且我必须承认我不知道如何为 (Show (WriterOptions -> Pandoc -> [Char]) 定义实例声明。作为一种解决方法，我尝试导入 Pandoc 库的附加模块，但是仍然没有可用的 Show 实例。

那我应该自己定义这个实例吗？
如果是，您有什么提示可以与我分享以完成此任务。
如果我不应该有什么问题？

在此先感谢您的帮助。

编辑

好的，我想我看到了我的误解：

正在做 ：

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。它应该是一个函数，然后我们无法显示它。

我想这应该是问题所在。

4

1 回答 1

2

我试图做的事情没有意义，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（字符串类型），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 Show 函数的实例。

然后要检索 Pandoc 中可用的 writer 列表（目的是动态调用相应的函数），我们只需要检索标识符列表，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"]
于 2013-02-15T15:21:07.353 回答