HappStack中的

Question

从…

http://happstack.com/docs/crashcourse/HappstackState.html

当我运行服务器时，peek计数器增加了

• 1当我看
• 2时，不看

有关守则如下：

handlers :: ServerPart Response
handlers = 
    msum [ dir "peek" $ do c <- query PeekCounter
                       ok $ toResponse $ "peeked at the count and saw: " ++ show (unCounter c)
         , do c <- update (AddCounter 1)
           ok $ toResponse $ "New count is: " ++ show (unCounter c)
         ]

但是，当我将其修改为

handlers :: ServerPart Response
handlers = 
    msum [ dir "peek" $ do c <- query PeekCounter
                           ok $ toResponse $ "peeked at the count and saw: " ++ show (unCounter c)
         , do ok $ toResponse $ "Stop here."
         , do c <- update (AddCounter 1)
              ok $ toResponse $ "New count is: " ++ show (unCounter c)
         ]

计数器增加了

• 0当我看
• 1时，不看

这是有意的行为吗？即使我看了一眼，也会感觉到第二次“漏水”。

Answer 1

由于浏览器每次加载页面时都请求/favicon.ico，所以计数会增加一次。由于最后一条路由是catch，所以对/favicon.ico的请求会导致增量。

最简单的解决方法是添加nullDir，以便它只对/进行增量，

handlers :: ServerPart Response
handlers = 
msum [ dir "peek" $ do c <- query PeekCounter
                   ok $ toResponse $ "peeked at the count and saw: " ++ show (unCounter c)
     , do nullDir
          c <- update (AddCounter 1)
          ok $ toResponse $ "New count is: " ++ show (unCounter c)
     ]

我已经更新了教程，以避免进一步的混乱。为了确认确实是/favicon.ico请求使事情变得一团糟，我们可以显式地处理对一个图标的请求：

handlers :: ServerPart Response
handlers = 
    msum [ dir "peek" $ do c <- query PeekCounter
                           ok $ toResponse $ "peeked at the count and saw: " ++ show (unCounter c)
         , dir "favicon.ico" $ notFound (toResponse "sorry, no favicon.ico")
         , do c <- update (AddCounter 1)
              ok $ toResponse $ "New count is: " ++ show (unCounter c)
         ]

现在我们看到了预期的行为。

总之，幸福栈没有什么问题。浏览器向非/peek的urls发出1或2个请求，因此计数增加了一两次。这是应用程序的预期行为。但是，由于人们并不期待/favicon.ico请求，这也会导致令人惊讶的行为。因此，现在应用程序已经更改为只有两个有效的urls，/peek和/。任何其他的结果都会导致404。

谢谢你的报告！