Control.Concurrent.Output

Concurrent output handling.

 import Control.Concurrent.Async
 import Control.Concurrent.Output

 main = withConcurrentOutput $
 	outputConcurrent "washed the car\n"
 		`concurrently`
	outputConcurrent "walked the dog\n"
		`concurrently`
 	createProcessConcurrent (proc "ls" [])

Documentation

withConcurrentOutput :: IO a -> IO a

Use this around any IO actions that use outputConcurrent or createProcessConcurrent

This is necessary to ensure that buffered concurrent output actually gets displayed before the program exits.

flushConcurrentOutput :: IO ()

Blocks until any processes started by createProcessConcurrent have finished, and any buffered output is displayed.

outputConcurrent :: String -> IO ()

Displays a string to stdout, and flush output so it's displayed.

Uses locking to ensure that the whole string is output atomically even when other threads are concurrently generating output.

When something else is writing to the console at the same time, this does not block. It buffers the string, so it will be displayed once the other writer is done.

createProcessConcurrent :: CreateProcess -> IO (Maybe Handle, Maybe Handle, Maybe Handle, ProcessHandle)

Wrapper around createProcess that prevents multiple processes that are running concurrently from writing to stdout/stderr at the same time.

If the process does not output to stdout or stderr, it's run by createProcess entirely as usual. Only processes that can generate output are handled specially:

A process is allowed to write to stdout and stderr in the usual way, assuming it can successfully take the output lock.

When the output lock is held (by another concurrent process, or because outputConcurrent is being called at the same time), the process is instead run with its stdout and stderr redirected to a buffer. The buffered output will be displayed as soon as the output lock becomes free.

waitForProcessConcurrent :: ProcessHandle -> IO ExitCode

This must be used to wait for processes started with createProcessConcurrent.

This is necessary because waitForProcess has a race condition when two threads check the same process. If the race is triggered, one thread will successfully wait, but the other throws a DoesNotExist exception.

lockOutput :: (MonadIO m, MonadMask m) => m a -> m a

Holds a lock while performing an action that will display output. While this is running, other threads that try to lockOutput will block, and calls to outputConcurrent and createProcessConcurrent will result in that concurrent output being buffered and not displayed until the action is done.