Hello, I end up with this solution in order to parse this data type. If you have some advice in order to simplify or beautify the Parser part do not hesitate. Cheers Fred data MaskLocation = MaskLocation Text | MaskLocation'Tmpl Text | MaskLocation'Or MaskLocation MaskLocation deriving (Eq, Generic, Show) deriving anyclass (FromJSON, ToJSON) instance FieldEmitter MaskLocation where fieldEmitter (MaskLocation t) = t fieldEmitter (MaskLocation'Tmpl t) = t fieldEmitter (MaskLocation'Or l r) = fieldEmitter l <> " | " <> fieldEmitter r instance FieldParsable MaskLocation where fieldParser = do let loc :: Text -> MaskLocation loc t = if "{scannumber:" `Data.Text.isInfixOf` t then MaskLocation'Tmpl (strip t) else MaskLocation (strip t) t <- takeTill (== '|') if t == "" then fail "MaskLocation is Empty" else do mc <- peekChar case mc of Nothing -> pure $ loc t Just '|' -> do _ <- char '|' -- extract the '|' char MaskLocation'Or (loc t) <$> fieldParser Just c -> fail ("MaskLocation " <> [c])