的setTitle与类型的错误IO字符串结果

Question

我想设定一个网页包含本年度标题为一个字符串，像这样：

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，我得到了在setTitle线以下错误：

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

我只是无法获得本年度出IO单子（或解除setTitle功能进去）。我尝试了各种各样的事情，但都没有成功，所以这可能归结为我还没有理解Haskell的类型系统;-)你能启发我吗？

Answer 1

耶，找到它了！解决方法是使用liftIO：

import Control.Monad.IO.Class (liftIO) 

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

Answer 2

您的title不是Html价值本身。它被包裹在IO中。你必须先提取它。

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