decode.purs

type MyRecord = { foo :: String, bar :: Instant }

decodeinsomeway :: Int -> Either String Instant
decodeinsomeway i = eitherturnintintoinstantornot i

decodeMyRecord :: Json -> Either String MyRecord
decodeMyRecord json = do
  tempRecord <- decodeJson json -- this is decoded as { foo :: String, bar :: Int }
  newField <- decodeinsomeway tempRecord.bar
  pure $ tempRecord { bar = newField }