Haskell 实战：惰性地读取子进程输出
source link: https://blog.lilydjwg.me/2012/4/18/haskell-in-action-read-subprocess-output-lazily.33103.html
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Haskell 实战：惰性地读取子进程输出
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突然想给 locate 命令写个 wrapper，把输出中的家目录和一些因加密而引入的软链接显示为~。自然，这需要读取 locate 命令的输出。在 process 这个库中看到了readProcess函数，似乎是自己想要的（完整代码）：
readLocate :: [String] -> IO StringreadLocate args = getArgs >>= \cmd ->let args' = args ++ cmdin readProcess "locate" args' ""结果却发现，原本 locate 命令是边查找边输出的，现在变成了先静默，然后一下子全部吐出来。没有按 Haskell 惯常的「懒惰」脾气来。这样一来，当我发现输出项目太多想按Ctrl-C中断时已经晚了。
Google 了一下，找到这个：
I guess people who want laziness can implement it themselves directly, taking care to get whatever laziness it is that they want.
好吧。我先下回 process 库的源码看看readProcess为什么不是惰性的：
readProcess :: FilePath -- ^ command to run-> [String] -- ^ any arguments-> String -- ^ standard input-> IO String -- ^ stdoutreadProcess cmd args input = do(Just inh, Just outh, _, pid) <-createProcess (proc cmd args){ std_in = CreatePipe,std_out = CreatePipe,std_err = Inherit }-- fork off a thread to start consuming the outputoutput <- hGetContents outhoutMVar <- newEmptyMVar_ <- forkIO $ C.evaluate (length output) >> putMVar outMVar ()-- now write and flush any inputwhen (not (null input)) $ do hPutStr inh input; hFlush inhhClose inh -- done with stdin-- wait on the outputtakeMVar outMVarhClose outh-- wait on the processex <- waitForProcess pidcase ex ofExitSuccess -> return outputExitFailure r -> ioError (mkIOError OtherError ("readProcess: " ++ cmd ++ ' ':unwords (map show args) ++ " (exit " ++ show r ++ ")")Nothing Nothing)原来是另开了一 IO 线程读输出，然后等待进程结束后关闭管道。这解释为什么它不是惰性的——它得进程善后处理。
那好吧，改用createProcess好了：
doLocate :: IO (String, ProcessHandle)doLocate = doargv0 <- getProgNamelet args = case argv0 of"lre" -> ["-b", "--regex"]_ -> []args' <- getArgslet args'' = args ++ args'(_, Just out, _, p) <- createProcess (proc "locate" args''){ std_in = Inherit,std_out = CreatePipe,std_err = Inherit }hSetBuffering out LineBuffering(,) <$> hGetContents out <*> return p改进后的程序，不会等待进程结束，而是返回输出和进程句柄。进程句柄用来等待子进程结束，同时获取退出状态。至于那个管道就不关闭了，留给操作系统解决好了。
main = do(out, p) <- doLocateputStr $ transform outwaitForProcess p >>= exitWith改进版的完整程序在此。
9 年前
期待你再改一下 indet/haskell.vim ，
单行 where 缩进两格，下一行缩进两格
do 也是缩进两格
两格 = shiftwidth / 2
像 emacs 的 haskell-mode 那样
f = f
where
f = f
g = do
return f
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