There is an Ext_alert extension in pandoc which handles alerts like this:

↪ echo "> [!WARNING]" | pandoc -t html -f gfm
<div class="warning">
<div class="title">
<p>Warning</p>
</div>
</div>

I’m trying to use it inside my hakyll project manually:

myPandocCompiler :: Compiler (Item String)
myPandocCompiler =
    pandocCompilerWithTransformM ropt wopt (pygmentsHighlight . shiftHeaders 1)
    where
        ropt = defaultHakyllReaderOptions { readerExtensions = enableExtension Ext_alerts pandocExtensions }
        wopt = defaultHakyllWriterOptions { writerExtensions = enableExtension Ext_alerts pandocExtensions }

But it doesn’t work.

I can bypass hakyll and use pandoc only with the same result:

λ> ropt = defaultHakyllReaderOptions { readerExtensions = getDefaultExtensions "gfm" }
λ> wopt = defaultHakyllWriterOptions { writerExtensions = getDefaultExtensions "gfm" }
λ> readerExtensions ropt
Extensions (fromList [Ext_alerts,Ext_auto_identifiers,Ext_autolink_bare_uris,Ext_emoji,Ext_footnotes,Ext_gfm_auto_identifiers,Ext_pipe_tables,Ext_raw_html,Ext_strikeout,Ext_task_lists,Ext_tex_math_dollars,Ext_tex_math_gfm,Ext_yaml_metadata_block])
λ>
λ>
λ> runPure $ writeHtml5String wopt =<< readMarkdown ropt  ("> [!WARNING]" :: Text)
Right "<blockquote>n<p>[!WARNING]</p>n</blockquote>"

What am I doing wrong?
How to readMarkdown and writeHtml5String using pandoc, converting alerts?