本文介绍了如何从Haskell中的monad中取出一个值?的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

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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10-29 16:16