blog/blog-meta/nice-date.hs

import Text.Pandoc.Definition
import qualified Data.Map.Strict as Map
import Text.Pandoc.JSON
import Misc

-- | replaces the @date@ field, which starts in YYYY-MM-DD format, with
-- something prettier
main :: IO ()
main = toJSONFilter \(Pandoc (Meta m) body) -> do
  m' <- Map.alterF reformat "date" m
  pure $ Pandoc (Meta m') body

reformat :: Maybe MetaValue -> IO (Maybe MetaValue)
reformat Nothing = pure Nothing
reformat (Just (toText -> Just txt)) =
  Just . MetaString . showDate <$> parseIsoDate txt
reformat (Just d) = fail $ "date is\n" <> show d <> "\nwanted a string"