在Haste中使用本地存储

Question

我想从本地存储器读取一个整数并将其写入本地存储器。

我的代码看起来像这样(到目前为止我只是想让它编译一下)：

loadFromStorage = do
    mr <- getItem "budget"
    case mr of
        Left _ -> return (0 :: Integer)
        Right _ -> return (1 :: Integer)

但我得到了：

No instance for (Serialize a0) arising from a use of ‘getItem’
    The type variable ‘a0’ is ambiguous
    Note: there are several potential instances:
      instance Serialize JSON -- Defined in ‘Haste.Serialize’
      instance Serialize JSString -- Defined in ‘Haste.Serialize’
      instance (Serialize a, Serialize b) => Serialize (Either a b)
        -- Defined in ‘Haste.Serialize’
      ...plus 13 others
    In a stmt of a 'do' block: mr <- getItem "budget"
    In the expression:
      do { mr <- getItem "budget";
           case mr of {
             Left _ -> return (0 :: Integer)
             Right _ -> return (1 :: Integer) } }
    In an equation for ‘loadFromStorage’:
        loadFromStorage
          = do { mr <- getItem "budget";
                 case mr of {
                   Left _ -> return (0 :: Integer)
                   Right _ -> return (1 :: Integer) } }

问题是：

要编译这段代码，我必须做些什么？

我必须做什么才能读写一个整数？

Answer 1

即使您没有使用mr，也需要指定它的类型。

指定mr类型的一种方法是添加let _ = mr :: ...语句：

loadFromStorage = do
    mr <- getItem "budget"
    let _ = mr :: MyType
    case mr of
        Left _ -> return (0 :: Integer)
        Right _ -> return (1 :: Integer)

Answer 2

这对我来说很有效：

toStorage :: Int -> IO ()
toStorage n = do
    setItem "page" n
    return ()

fromStorage :: IO Int
fromStorage = do
                 value <- getItem "page"
                 either (\_ -> return 0)
                        (\n -> return n)
                        value