wkoszek / book-real-world-haskell Goto Github PK

UglyStack.hs:59:13: error:
    • No instance for (Applicative MyApp)
        arising from the 'deriving' clause of a data type declaration
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Monad MyApp)
   |
59 |   deriving (Monad, MonadIO, MonadReader AppConfig, MonadState AppState)
   |             ^^^^^

instance Monad m => Applicative (MaybeT m) where
  pure = returnMT
  (<*>) a b = MaybeT $ do
    a' <- runMaybeT a
    case a' of Nothing -> return Nothing
               Just a'' -> do b' <- runMaybeT b
                              case b' of Nothing -> return Nothing
                                         Just b'' -> return $ Just $ a'' b''