如何从Haskell中的monad中取出一个值？ [英] How to take out a value out of a monad in Haskell?

问题描述

有没有办法从monad中取出东西？

我正在开发一款游戏，现在我正在尝试了解数据库。我发现 happstack

例如，我有这个功能（希望您熟悉 happstack ）

  getAllThings :: MonadIO m => m [事物] 
 getAllThings =做
 elems<  - 查询GetThings 
返回元素
  

所以我得到 m [物品] ，但我不能在我的模型中使用它！例如

  doSomeThingWithThings :: [Thing]  - > 
  

我搜索了这个，我什么也没找到。

解决方案

您不应该以这种方式退出IO monad（除 unsafePerformIO 函数），但仍然可以在内部使用您的函数它：

  process :: MonadIO m => m（）
 process = do 
 elems<  -  getAllThings 
 let smth = doSomeThingWithThings elems 
  -  ... 
  

Is there any way to take "things" out of a monad?

I am developing a game, and I am now trying to understand about databases. I found happstack really nice, but I can't get the thing.

For example, I have this function (hope you are familiar with happstack)

getAllThings :: MonadIO m => m [Thing]
getAllThings = do
            elems <- query GetThings
            return elems

So I get m [Things], but I can't use this in my model! For instance

doSomeThingWithThings :: [Thing] -> Something

I googled this and I found nothing.

解决方案

You are not supposed to exit IO monad this way (except unsafePerformIO function), but you can still use your function inside it:

process :: MonadIO m => m ()
process = do
          elems <- getAllThings
          let smth = doSomeThingWithThings elems
          -- ...

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