import Text.Pandoc.Definition
import qualified Data.Map.Strict as Map
import Text.Pandoc.JSON
import Misc
import Control.Monad.Writer
import Data.Monoid

-- | replaces the @date@ field, which starts in YYYY-MM-DD format, with
-- something prettier
main :: IO ()
main = toJSONFilter \(Meta m) -> do
  (res', mdate) <- run $ Map.alterF reformat' "date" m
  let res = maybe m (\d -> Map.insert "date-iso" d res') mdate
  pure $ Meta res

type T = WriterT (Alt Maybe MetaValue) IO

run :: T a -> IO (a, Maybe MetaValue)
run = fmap (fmap getAlt) . runWriterT

reformat' :: Maybe MetaValue -> T (Maybe MetaValue)
reformat' Nothing = pure Nothing
reformat' (Just d) = do tell $ pure d; Just <$> reformat d

reformat :: MetaValue -> T MetaValue
reformat (toText -> Just txt) = MetaString . showDate <$> parseIsoDate txt
reformat d = fail $ "date is\n" <> show d <> "\nwanted a string"