Validated Hamlet Textarea Form Field

Here is my custom textarea field that validates correct hamlet content

in the Form:
...
  (hamletHtmlResult, hamletHtmlView) <- mreq validatedHamletTextareaField
...

the field:

validatedHamletTextareaField :: Field (HandlerFor App) Textarea
validatedHamletTextareaField = checkM isValidHamlet textareaField
  where
    isValidHamlet :: Textarea -> Handler (Either AppMessage Textarea)
    isValidHamlet textarea@(Textarea text) = do
      eitherParsedTemplate <- tryParse text
      return $
        case eitherParsedTemplate of
          Left _ -> Left MsgGlobalInvalidHamlet
          Right _ -> Right $ Textarea text
      where
        tryParse :: Text -> Handler (Either SomeException HamletTemplate)
        tryParse text = try $ parseHamletTemplate defaultHamletSettings $ unpack text