我知道 Yesod 有一个 Paginator 包,但我更喜欢更简单的 UI,所以我为我的应用程序创建了一个简单的分页逻辑。但是,我想不出将参数值转换为整数的方法。
import Data.Text (unpack, singleton)
import Data.Maybe
one = singleton '1' -- convert char to Text, required by fromMaybe
getTestPanelR :: Handler Html
getTestPanelR = do
ptext <- lookupGetParam "p" -- guessing returns Maybe Text
p <- fromMaybe one ptext -- ??? does not work
-- pn <- ??? Once p is extracted successfully, how to convert to an integer?
s <- runDB $ selectList [] [Asc PersonName, LimitTo 10 , OffsetBy $ (pn - 1) * 10]
(widget, enctype) <- generateFormPost $ entryForm Nothing
defaultLayout $ do
$(widgetFile "person")
当我运行上面的代码时,我收到以下错误消息:
No instance for (MonadHandler Maybe)
arising from a use of `lookupGetParam'
Possible fix: add an instance declaration for (MonadHandler Maybe)
In the second argument of `($)', namely `lookupGetParam "p"'
In a stmt of a 'do' block:
p <- fromMaybe one $ lookupGetParam "p"
In the expression:
...
当我用它写出“ptext”时, #{show ptext}
它会显示Just "1"
. 获得 GET 参数后,如何将其转换为整数以便进行分页?(需要为“下一个”加 1,为“上一个”减 1)
FWIW,当我使用 GHCi 尝试这个时,它工作正常:
Prelude Data.Maybe Data.Text> let one = singleton '1'
Prelude Data.Maybe Data.Text> let x = Just $ singleton '5'
Prelude Data.Maybe Data.Text> let y = fromMaybe one x
Prelude Data.Maybe Data.Text> y
"5"
Prelude Data.Maybe Data.Text> read $ Data.Text.unpack y ::Int -- This is probably unsafe because I cannot trust 'y' in my web app
5
更新:
我厌倦了@Ankur 的建议pageNumber <- (lookupGetParam "p" >>= return . (read :: String -> Int) . fromMaybe "1")
,我收到以下错误:
Couldn't match expected type `String' with actual type `Text'
Expected type: Maybe Text -> String
Actual type: Maybe Text -> Text
In the return type of a call of `fromMaybe'
In the second argument of `(.)', namely `fromMaybe "1"'
Build failure, pausing...
如果将“1”更改为一个 ( Data.Text.singleton '1'
),我仍然会收到完全相同的错误消息。
谢谢!