一个更好的方法来映射一个需要IO列表的函数 [英] A better way to map a function that requires IO over a list

问题描述

所以最近我有一个字符串列表，并且需要独立地遍历每个字符串并执行一些 IO 函数。

所以基本上我得到的是：

  goOverList :: [字符串]  - > IO（）
 goOverList（x：[]）= do 
 putStrLn x 
 goOverList（x：xs）= do 
 goOverList [x] 
 goOverList xs 
 
 main = do 
 let myList = [first，second，third] 
 goOverList myList 
  

$ b $ IO 有点复杂，但它的要点是（需要有一个函数超过列表并根据列表成员执行 IO ）我希望有人会告诉我如何更好地做到这一点。 goOverList 函数几乎相当于 > mapM_ putStrLn 。 （几乎是因为 mapM _ 也适用于空列表，而你的函数没有）

mapM 是一个函数，它应用 a - >类型的函数。 IO b 1给 a s列表中的每个项目，并返回给您 IO ²列出 b s。 mapM _ 与 mapM 相同，不同之处在于它不会将结果存储在列表中对于返回（）像 putStrLn 那样的动作的感觉。

实际上它比这更一般：函数的类型是 a - > mb 其中 Monad m ，但在这种情况下 m 是 IO 。

²同样是m。

So lately I have a list of strings, and need to independently go over each one and perform some IO function.

So basically what I have is this:

goOverList :: [String] -> IO ()
goOverList (x:[]) = do
    putStrLn x
goOverList (x:xs) = do
    goOverList [x]
    goOverList xs

main = do
    let myList = ["first", "second", "third"]
    goOverList myList

My IO is a bit more complicated but that is the gist of it (needing to have a function go over a list and do IO based on the list member) I was hoping someone might show me how to do this better.

解决方案

Your goOverList function is almost equivalent to mapM_ putStrLn. (Just almost because mapM_ also works with the empty list while your function does not).

mapM is a function that applies a function of type a -> IO b¹ to each item in a list of as and gives you back an IO² with a list of bs. mapM_ is the same as mapM except that it doesn't store the results in a list (which doesn't make sense for actions that return () like putStrLn does).

¹ Actually it's more general than that: The function has type a -> m b where Monad m, but in this case m is IO.

² Again it's actually m.

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