0

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！

4

1 回答 1

1

lookupGetParam返回ParamValue是type ParamValue = String. 所以基本上它是字符串而不是文本。

试试这个：

pageNumber <- (lookupGetParam "p" >>= return . (read :: String -> Int) . fromMaybe "1")

更新：

实际上最新版本lookupGetParam是Text基于的，所以添加 OverloadedStrings 语言扩展应该可以完成工作：

将其放在{-# LANGUAGE OverloadedStrings #-}代码文件的开头并使用：

pageNumber <- (lookupGetParam "p" >>= return . (read :: String -> Int) . unpack . fromMaybe "1")
于 2013-07-18T09:08:35.193 回答