Haskell 分页：将参数值转换为整数时出错

我知道YesSOD有一个分页程序包，但我更喜欢一个简单的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，我无法找到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")

更新：

我厌倦了@Ankur的建议

pageNumber>=返回。（读：：String->Int）。从“1”）

中，我得到以下错误：

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1（

Data.Text.singleton“1”

），我仍然会收到完全相同的错误消息

---

谢谢

lookupGetParam

返回

ParamValue

，即

类型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") 

---

谢谢@Ankur。请查看上面我的更新--我仍然得到一个错误。事实上，我之前在fromMaybe中意外使用“1”时收到了此错误消息。那时我开始阅读文本，但由于缺乏Haskell知识，我很难进行调试。你有没有想过你的建议会出什么问题？

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...

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

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