crosspost :: Text -> Text -> IO ()
crosspost apiKey path = do
--        ^ Medium API key.
--               ^ Filepath to the blog post to crosspost.
  f <- Data.ByteString.readFile . Data.Text.unpack $ path
  case parseYamlFrontmatter f of
--     ^ Parse the frontmatter (and content) of the blog post to crosspost.
    Done postBS frontmatter -> do
--  ^ If the frontmatter (and content) was parsed successfully..
      let opts =
            defaults
              & Network.Wreq.auth ?~ Network.Wreq.oauth2Bearer (encodeUtf8 apiKey)
      r <- Network.Wreq.getWith opts "https://api.medium.com/v1/me"
      let id_ = r ^. responseBody . key "data" . key "id" . _String
--        ^ ..then get the user id associated with the API key and..

      let post = decodeUtf8 postBS
      let json = toJSON $ mkMediumPost path frontmatter post
--        ^ ..create the JSON body for the Medium endpoint and..
      let opts2 =
            defaults
              & Network.Wreq.auth ?~ Network.Wreq.oauth2Bearer (encodeUtf8 apiKey)
              & Network.Wreq.header "Content-Type" .~ [encodeUtf8 "application/json; charset=utf-8"]
      r2 <-
        Network.Wreq.postWith
--      ^ ..post the request to Medium to create the blog post.
          opts
          ("https://api.medium.com/v1/users/" <> Data.Text.unpack id_ <> "/posts")
          json
      print $ r2 ^? responseBody
    e ->
      error $ show e
--    ^ ..else stop execution and display the error `e`.

-- `MediumPost` is what we send to Medium.
data MediumPost = MediumPost
  { title :: Text,
    tags :: [Text],
    canonicalUrl :: Text,
    publishStatus :: Text,
    content :: Text,
    contentFormat :: Text,
    notifyFollowers :: Bool
  }
  deriving (Show, Generic)

instance ToJSON MediumPost where
  toJSON MediumPost {..} =
    object
      [ "title" .= title,
        "tags" .= tags,
        "canonicalUrl" .= canonicalUrl,
        "publishStatus" .= publishStatus,
        "content" .= content,
        "contentFormat" .= contentFormat,
        "notifyFollowers" .= notifyFollowers
      ]

-- `Front` is what we parse from the local blog post.
data Front = Front
  { title :: Text,
    description :: Text,
    tags :: [Text]
  }
  deriving (Show, Generic, FromJSON)


mkMediumPost :: Text -> Front -> Text -> MediumPost
mkMediumPost path Front {..} post = MediumPost {..}
--                      ^ Same as `{ title = title, description = description, tags = tags }`.
--                        Enabled by {-# LANGUAGE RecordWildCards #-}.
  where
    publishStatus = "draft"
    content = fold ["Originally posted on", " ", "[odone.io](", canonicalUrl, ").\n\n---\n\n", post]
    contentFormat = "markdown"
    canonicalUrl = urlFor path
    notifyFollowers = True

-- URL of the blog post on odone.io.
urlFor :: Text -> Text
urlFor path = fold [base, "/", name, ".html"]
  where
    base = "https://odone.io/posts"
    name = Data.Text.pack . System.FilePath.Posix.takeBaseName . Data.Text.unpack $ path

main :: IO ()
main = uncurry crosspost =<< execParser opts
--                           ^ Parses the command line input and returns a tuple (String, String).
--     ^ `uncurry` converts a function on two arguments to a function expecting a tuple.
  where
    opts =
      info
        (parser <**> helper)
        ( fullDesc
            <> progDesc "Crossposts POST to Medium"
        )

parser :: Options.Applicative.Parser (Text, Text)
parser =
  (,)
    <$> Options.Applicative.argument
      str
--    ^ The first mandatory argument is the API key to Medium.
      ( metavar "API_KEY"
          <> help "API_KEY to post on Medium"
      )
    <*> Options.Applicative.argument
      str
--    ^ The second mandatory argument is the path to the blog post to crosspost.
      ( metavar "POST"
          <> help "Path to blog POST to crosspost"
      )

./tomedium.hs
# Missing: API_KEY POST
#
# Usage: tomedium.hs API_KEY POST
#   Crossposts POST to Medium

./tomedium.hs --help
# Usage: tomedium.hs API_KEY POST
#   Crossposts POST to Medium
#
# Available options:
#   API_KEY                  API_KEY to post on Medium
#   POST                     Path to blog POST to crosspost
#   -h,--help                Show this help text