a.hs

menuField :: Context String
menuField = field "menu" $ \_ -> do
    path <- toFilePath <$> getUnderlying
    let menuPath = takeDirectory path </> "@menu.md"
    unsafeCompiler (readFile menuPath) >>= renderPandoc

error

    Couldn't match type ‘Item String’ with ‘[Char]’
    Expected type: String -> Compiler String
      Actual type: Item String -> Compiler (Item String)
    In the second argument of ‘(>>=)’, namely ‘renderPandoc’
    In a stmt of a 'do' block:
      unsafeCompiler (readFile menuPath) >>= renderPandoc