有没有办法从IO monad打开一个类型？ [英] Is there a way to unwrap a type from an IO monad?

问题描述

我有这个非常简单的功能

I have this very simple function

import qualified Data.ByteString.Lazy as B

getJson :: IO B.ByteString
getJson = B.readFile jsonFile

readJFile :: IO (Maybe Response)
readJFile =  parsing >>= (\d ->
             case d of
                 Left err -> return Nothing
                 Right ps -> return (Just ps))
    where parsing = fmap eitherDecode getJson :: IO (Either String Response)

其中 jsonFile 是路径到我的硬盘上的一个文件（原谅我没有写法，但我发现这个更清楚了）

where jsonFile is a path to a file on my harddrive (pardon the lack of do-notation, but I found this more clear to work with)

我的问题是;有没有办法让我放弃 IO 部分，这样我就可以单独使用bytestring了？

my question is; is there a way for me to ditch the IO part so I can work with the bytestring alone?

我知道您可以在某些monad上进行模式匹配，例如 和可能以获取其值，但是您可以执行类似的操作与 IO ？

I know that you can pattern match on certain monads like Either and Maybe to get their values out, but can you do something similar with IO?

或者说不同：有没有办法让我做 readJFile 返回可能没有IO的响应？

Or voiced differently: is there a way for me to make readJFile return Maybe Response without the IO?

推荐答案

为了扩展我的评论，以下是你如何做到这一点：

To expand on my comments, here's how you can do it:

getJson :: IO B.ByteString
getJson = B.readFile jsonFile -- as before

readJFile :: B.ByteString -> Maybe Response -- look, no IO
readJFile b = case eitherDecode b of
                Left err -> Nothing
                Right ps -> Just ps

最后，您再次将所有内容组合在一个IO操作中：

In the end, you combine everything in one IO action again:

getAndProcess :: IO (Maybe Response)
getAndProcess = do
  b <- getJson
  return (readJFile b)

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