可能会与yesod叠在一起 [英] Deep maybe stack with yesod

问题描述

我试图设置一个授权方案，用于检查1.用户是否已登录2.用户可以访问某个对象。为此，我首先调用 maybeAuthId ，然后尝试获取当前对象，然后将'join'添加到列出权限的另一个表中。有两种级别的可能情况和一级空白情况。我想过使用MaybeT，但是我太累了，无法使其工作，或者不是真正的monad变压器 - handler-transformers不能与MaybeT一起使用。有没有一种很好的方式来处理深层的maybes？

I'm trying to set an authorization scheme where I check that 1. user is logged in 2. user has access to a certain object. For this I first call maybeAuthId, then try to get the current object, and 'join' to another table which lists permissions. There are two levels of maybe-cases and one level of empty-list case. I thought of using MaybeT, but either I'm too tired to get it to work or the "not really monad transformer"-handler-transformers can't be used with MaybeT. Is there a nice way to handle deep maybes?

编辑：

我看起来有点不清楚。我的意思是说我有这样的事情：

I was a bit unclear it seems. I meant that I have something like this:

case foo of
   Nothing -> something
   Just foo' -> do
      bar <- somethingelse
      case bar of
         Nothing -> ...
         Just bar' -> ...

推荐答案

您完全可以使用<$ c $对Yesod来说，c> MaybeT 。只需要这样做：

You can totally use MaybeT for Yesod. Just do it like this:

runMaybeT $ do
  uid <- MaybeT maybeAuthID
  car <- MaybeT . runDB . getBy $ UniqueCarOwner uid
  location <- MaybeT . liftIO . ciaLocateLicensePlate . licensePlate $ car
  country <- MaybeT . findCountry $ location
  return (car, country)

正如你所说，大部分函数都不是'针对Yesod中的通用错误处理进行了优化。但是，如果您有某种形式的 Monad m =>你可以简单地使用 MaybeT 来将它转化为 Monad m => m（也许a）也许（m a）。

As you said, most functions aren't optimized for generic error handling in Yesod. However, if you have something of the form Monad m => m (Maybe a), you can simply use MaybeT to turn it inside out into Monad m => Maybe (m a).

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