Case.hs

case mc of
  Nothing -> return $ FailFatal $ serverErr' Forbidden
  Just jwt -> do
    mcs <- verifyClaims $ decodeUtf8 jwt
    case mcs of
      Nothing -> return $ FailFatal $ serverErr' Forbidden
      Just clms -> do
        let user = lookup "user" $ toList $ unregisteredClaims clms :: Maybe Value
        case user of
          Nothing -> return $ FailFatal $ serverErr' Forbidden
          Just user' -> do
            let usr = handleCTypeH (Proxy :: Proxy list) (cs contentTypeH) (encode user')
            case usr of
              Nothing -> return $ FailFatal $ serverErr' Forbidden
              Just (Left e) -> return $ FailFatal err403
              Just (Right v) -> return $ Route v

fromMaybeT (FailFatal $ serverErr' Forbidden) $ do
  jwt <- MaybeT $ return mc
  clms <- MaybeT $ verifyClaims $ decodeUtf8 jwt
  user <- MaybeT $ return $ lookup "user" $ toList $ unregisteredClaims clms :: Maybe Value
  usr <- MaybeT $ return $ handleCTypeH (Proxy :: Proxy list) (cs contentTypeH) (encode user')
  case usr of
    Left e -> return $ FailFatal err403
    Right v -> return $ Route v

fromMaybeT :: Monad m => m a -> MaybeT m a -> m a
fromMaybeT x y = y <|> lift x

@Gurkenglas wrote this as well. Putting here for reference:

fromMaybe (FailFatal $ serverErr' Forbidden) <$> (ala EndoT . alaf MaybeT) foldMap
  [ return . Just . either (const $ FailFatal err403) Route
  , return . handleCTypeH (Proxy :: Proxy list) (cs contentTypeH) . encode
  , return . lookup "user" . toList . unregisteredClaims
  , verifyClaims . decodeUtf8 jwt
  , return
  ] mc

newtype EndoT m a = EndoT {runEndoT :: a -> m a} deriving Wrapped

instance Monad m => Monoid (EndoT m) where
  mempty = coerce return
  mappend = coerce (<=<)