我想从本地存储读取和写入整数。
我的代码看起来像这样(到目前为止只是试图编译):
loadFromStorage = do
mr <- getItem "budget"
case mr of
Left _ -> return (0 :: Integer)
Right _ -> return (1 :: Integer)
但我得到:
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:
我必须做什么才能使这段代码编译?
我必须怎么做才能读写一个整数?