[英]Haskell Put IO String
I am trying to read and print the output from the "readProcess" command mapped onto a list of filenames: 我试图读取并打印映射到文件名列表的“readProcess”命令的输出:
files <- readProcess "ls" [] []
let mdList = map ( \file -> do
md <- readProcess "mdls" [file] []
return md ) $ splitOn "\n" files in
map (\md -> putStrLn md) mdList
putStrLn "Complete"
Each time I try to map putStrLn onto the mdList, I get this error: 每次我尝试将putStrLn映射到mdList时,我都会收到此错误:
Couldn't match type ‘IO String’ with ‘[Char]’
I have read many StackOverflow answers that seem to just use putStrLn on an IO String but I am unable to do so. 我已经阅读了许多StackOverflow答案,似乎只是在IO字符串上使用putStrLn但我无法这样做。 Also, I am new to Haskell so any other tips are also appreciated.
此外,我是Haskell的新手,所以任何其他提示也值得赞赏。
You are using 您正在使用
map :: (a -> b) -> [a] -> [b]
which specializes to 哪个专攻
map :: (a -> IO b) -> [a] -> [IO b]
The final result, [IO b]
is not what we need. 最终结果,
[IO b]
不是我们需要的。 That is a list of IO
actions, ie the equivalent of a list of non-executed no-arguments imperative functions. 这是一个
IO
动作列表,即相当于一个未执行的无参数命令功能列表。 Instead, we want a single IO
action, whose result is a list of b
: that would be IO [b]
instead of [IO b]
. 相反,我们需要一个单独的
IO
操作,其结果是b
的列表: IO [b]
而不是[IO b]
。
The library provides that as well: 图书馆也提供了:
mapM :: (a -> IO b) -> [a] -> IO [b]
or, if we don't care about collecting the results 或者,如果我们不关心收集结果
mapM_ :: (a -> IO b) -> [a] -> IO ()
The library also provides variants with flipped arguments: 该库还提供带有翻转参数的变体:
for :: [a] -> (a -> IO b) -> IO [b]
for_ :: [a] -> (a -> IO b) -> IO ()
So, the original code can be fixed as follows: 因此,原始代码可以修复如下:
import Data.Foldable
files <- readProcess "ls" [] []
for_ files $ \file -> do
md <- readProcess "mdls" [file] []
putStrLn md
putStrLn "Complete"
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