设置Ext_yaml_metadata_block后，Pandoc Haskell代码为何不输出YAML元数据块？

我尝试用Markdown写入器还原读取的Markdown文件，测试发现命令行指令pandoc -s -t markdown input.md -o output.md可以生成带有YAML头的文件。查看源码后得知需要启用Ext_yaml_metadata_block扩展，但在我的Haskell代码中，尽管读写环节都设置了该扩展，写入器却只输出内容，不生成YAML头。请问还需要额外配置什么，或者代码中存在哪些问题？

{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE ScopedTypeVariables   #-}

module MarkdownYAMLquestion where

import qualified Text.Pandoc as Pandoc
import           Text.Pandoc.Definition as Pandoc(Pandoc(..))
import           Uniform.PandocImports(unPandocM) 
import           UniformBase(s2t,putIOwords, runErr, ErrIO, Text, showT)
 
 
smallMD :: Text 
smallMD = s2t $ unlines 
    ["---"
    , "keywords: somewords"
    , "showframe: False"
    , "---"
    , ""
    , "some content text"
    ]
testyaml :: IO ()
-- a minimal example 
testyaml = do 
    res <- runErr $ do 
        pan1 <- readMarkdown3 smallMD
        putIOwords ["the pandoc", showT pan1] -- the YAML header is parsed!
        mdRes <- writeToMarkdown3 pan1 
        putIOwords ["\n\nthe file reproduced\n", mdRes]
    putIOwords ["testyaml end res", showT res]


writeToMarkdown3 :: Pandoc -> ErrIO Text
-- write with yaml header
writeToMarkdown3  pan1= unPandocM $ Pandoc.writeMarkdown 
                            markdownWriterOptionsx pan1

markdownWriterOptionsx :: Pandoc.WriterOptions
markdownWriterOptionsx = Pandoc.def {Pandoc.writerExtensions = exts}
    where
        exts = mconcat [ Pandoc.extensionsFromList
                            [Pandoc.Ext_yaml_metadata_block]
                            ]

readMarkdown3 ::  Text ->   ErrIO Pandoc
-- | reads the markdown text and produces a pandoc structure
-- the filename is only used for the error message
readMarkdown3 text1 = unPandocM  $ Pandoc.readMarkdown markdownOptions text1

markdownOptions :: Pandoc.ReaderOptions
markdownOptions = Pandoc.def { Pandoc.readerExtensions = exts }
  where
    exts = mconcat
        [ Pandoc.extensionsFromList
            [ Pandoc.Ext_yaml_metadata_block
            -- , Pandoc.Ext_fenced_code-block -- code blocks with ~
            , Pandoc.Ext_backtick_code_blocks
            , Pandoc.Ext_fenced_code_attributes  -- eg for haskell code snippets
            , Pandoc.Ext_auto_identifiers
            -- , Pandoc.Ext_raw_html   -- three extension give markdown_strict
            , Pandoc.Ext_raw_tex   --Allow raw TeX (other than math)
            , Pandoc.Ext_latex_macros -- allow tex for math
            , Pandoc.Ext_tex_math_dollars 
            , Pandoc.Ext_shortcut_reference_links
            , Pandoc.Ext_spaced_reference_links
            , Pandoc.Ext_footnotes  -- all footnotes
            , Pandoc.Ext_inline_notes
            , Pandoc.Ext_citations           -- <-- this is the important extension for bibTex
            , Pandoc.Ext_implicit_figures  -- a figure alone in a para will have a caption
            , Pandoc.Ext_header_attributes -- for {.unnumbered}
            , Pandoc.Ext_lists_without_preceding_blankline
            , Pandoc.Ext_superscript  -- start and closing ^
            , Pandoc.Ext_subscript -- start and closing ~
            -- , Pandoc.Ext_short_subsuperscripts  -- only start ^ and ~
            , Pandoc.Ext_strikeout  -- require ~~ two! before and after
            , Pandoc.Ext_smart -- Smart quotes, apostrophes, ellipses, dashes
            ]
        , Pandoc.githubMarkdownExtensions
        ]

你的代码有两个关键问题导致YAML头无法输出：

1. Writer扩展集合被错误替换：你直接用Ext_yaml_metadata_block覆盖了默认的Markdown扩展集合，丢失了很多必要的基础扩展。正确的做法是在默认扩展基础上启用目标扩展，而不是完全替换。
2. 未开启独立文档模式：命令行里的-s参数对应Haskell API中的writerStandalone选项，只有开启这个选项，pandoc才会将元数据以YAML块的形式输出。

修改后的markdownWriterOptionsx应该如下：

markdownWriterOptionsx :: Pandoc.WriterOptions
markdownWriterOptionsx = Pandoc.def 
  { Pandoc.writerExtensions = Pandoc.enableExtension Pandoc.Ext_yaml_metadata_block Pandoc.def
  , Pandoc.writerStandalone = True
  }

另外，读取端的扩展集合同时混合了手动添加的扩展和githubMarkdownExtensions，可能存在潜在冲突，建议保持扩展集合的一致性，但这不是YAML头不输出的直接原因。

内容的提问来源于stack exchange，提问作者user855443