Question

我正在尝试将网页标题设置为包含当前年份的字符串，如下所示：

getCurrentYear :: IO String
getCurrentYear = do
    now <- getCurrentTime
    let today = utctDay now
    let (year, _, _) = toGregorian today
    return $ show year

title :: IO Html
title = do
    y <- getCurrentYear
    return $ toHtml $ "Registration " ++ y

getRootR :: Handler RepHtml
getRootR = do
    (widget, enctype) <- generateFormPost personForm -- not important for the problem at hand, comes from the example in the yesod book
    defaultLayout $ do
        setTitle title -- this is where I get the type error
[...]

当我尝试编译时，我在setTitle行收到以下错误：

Couldn't match expected type `Html' with actual type `IO Html'
In the first argument of `setTitle', namely `title'
[...]

我无法将当前年份从IO monad中取出（或将setTitle函数提升到其中）。我尝试了各种各样的事情没有成功，所以这可能归结为我仍然没有理解Haskell的类型系统;-)你能启发我吗？

Answer 1

您的title本身不是Html值。它包含在IO中。你必须先提取它。

getRootR = do
    [...]
    defaultLayout $ do
        unwrappedTitle <- title
        setTitle unwrappedTitle
    [...]

Answer 2

耶，发现它！解决方案是使用liftIO：

import Control.Monad.IO.Class (liftIO)

getRootR = do
    [...]
    defaultLayout $ do
        unwrappedTitle <- liftIO title
        setTitle unwrappedTitle
    [...]

带有IO String的setTitle会导致类型错误

2 个答案: