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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Moduleconcurrent-output-1.10.21Haskell2010

System.Console.Regions

Console regions are displayed near the bottom of the console, and can be updated concurrently by threads.

Any other output lines displayed using outputConcurrent and createProcessConcurrent will scroll up above the open console regions.

For example, this program:

 import Control.Concurrent.Async
 import Control.Concurrent
 import System.Console.Concurrent
 import System.Console.Regions
 import System.Process
 
 main = displayConsoleRegions $ do
 	mapConcurrently download [1..5]
		`concurrently` mapM_ message [1..10]
		`concurrently` createProcessConcurrent (proc "echo" ["hello world"])
 
 message :: Int -> IO ()
 message n = do
 	threadDelay 500000
 	outputConcurrent ("Message " ++ show n ++ "\n")
 
 download :: Int -> IO ()
 download n = withConsoleRegion Linear $ \r -> do
 	setConsoleRegion r basemsg
 	go n r
   where
 	basemsg = "Download " ++ show n
	go c r
		| c < 1 = finishConsoleRegion r (basemsg ++ " done!")
 		| otherwise = do
 			threadDelay 1000000
 			appendConsoleRegion r " ... "
 			go (c-1) r

Will display like this:

Message 1
hello world
Message 2
Download 1 ...
Download 2 ...
Download 3 ...

Once the 1st download has finished, and another message has displayed, the console will update like this:

Message 1
hello world
Message 2
Download 1 done!
Message 3
Download 2 ... ...
Download 3 ... ...
  • 3 types
  • 2 classes
  • 14 values

Types

5 declarations
newtypenewtype ConsoleRegion
#

A handle allowing access to a region of the console.

Instances1Eq
  • Eq ConsoleRegionDefined in concurrent-output-1.10.21 · System.Console.Regions
datadata RegionLayout
#

Controls how a region is laid out in the console.

Here's an annotated example of how the console layout works. Each sequence of the same letter represents a distinct region.

scrolling......
scrolling......
scrolling......
aaaaaa......... -- Linear
bbbbbbbbbbbbbbb -- Linear
bbb............       (expanded to multiple lines)
ccccccccc...... -- Linear
ddddeeeefffffff -- [InLine]
fffffggggg.....       (expanded to multiple lines)

Constructors

Instances1Eq
  • Eq RegionLayoutDefined in concurrent-output-1.10.21 · System.Console.Regions
classclass ToRegionContent v where
#

Values that can be displayed in a region.

Methods

Instances4ToRegionContent
  • ToRegionContent StringDefined in concurrent-output-1.10.21 · System.Console.Regions
  • ToRegionContent TextDefined in concurrent-output-1.10.21 · System.Console.Regions
  • ToRegionContent TextDefined in concurrent-output-1.10.21 · System.Console.Regions

    Note that using a lazy Text in a region will buffer it all in memory.

  • ToRegionContent (STM Text)Defined in concurrent-output-1.10.21 · System.Console.Regions

    Makes a STM action be run to get the content of a region.

    Any change to the values that action reads will result in an immediate refresh of the display.

classclass LiftRegion (m :: Type -> Type) where
#

Many actions in this module can be run in either the IO monad or the STM monad. Using STM allows making several changes to the displayed regions atomically, with the display updated a single time.

Methods

Instances2LiftRegion
  • LiftRegion STMDefined in concurrent-output-1.10.21 · System.Console.Regions
  • LiftRegion IODefined in concurrent-output-1.10.21 · System.Console.Regions

Initialization

5 declarations
valuedisplayConsoleRegions :: (MonadIO m, MonadMask m) => m a -> m a
#

Handles all display for the other functions in this module.

Note that this uses lockOutput, so it takes over all output to the console while the passed IO action is running. As well as displaying the console regions, this handles display of anything buffered by outputConcurrent and createProcessConcurrent. So, withConcurrentOutput and flushConcurrentOutput should not be run while this is in use, and will block.

When standard output is not an ANSI capable terminal, console regions are not displayed.

Region content and display

5 declarations
valuesetConsoleRegion
  1. :: (ToRegionContent v, LiftRegion m)
  2. => ConsoleRegion
  3. -> v
  4. -> m ()
#

Sets the value of a console region. This will cause the console to be updated to display the new value.

It's fine for the value to be longer than the terminal is wide, or to include newlines ('n'). Regions expand to multiple lines as necessary.

The value can include ANSI SGR escape sequences for changing the colors of all or part of a region. For this to display properly, a reset escape sequence must be included to get the color back to default. System.Console.ANSI makes it easy to construct such values. For example:

 import System.Console.ANSI
 
 setConsoleRegion region 
 	( "hello "
 	<> setSGRCode [SetColor Foreground Vivid Red] 
	<> "Mars" 
	<> setSGRCode [Reset]
 	<> "!"
	)

Other ANSI escape sequences, especially those doing cursor movement, will mess up the layouts of regions. Caveat emptor.

ANSI SGR escape sequences that span multiple lines do not currently display as you might hope. (Patches would be accepted.)

valuefinishConsoleRegion
  1. :: (Outputable v, LiftRegion m)
  2. => ConsoleRegion
  3. -> v
  4. -> m ()
#

Closes the console region, and displays the passed value in the scrolling area above the active console regions. When Nothing is passed, displays the current value of the console region.

valuetuneDisplay :: LiftRegion m => ConsoleRegion -> (Text -> STM Text) -> m ()
#

Changes how a console region displays.

Each time the region's value changes, the STM action is provided with the current value of the region, and returns the value to display.

For example, this will prevent a region from ever displaying more than 10 characters wide, and will make it display text reversed:

tuneDisplay myregion $ pure . T.take 10
tuneDisplay myregion $ pure . T.reverse

Note that repeated calls to tuneDisplay are cumulative.

Normally, the STM action should avoid retrying, as that would block all display updates.

STM region contents

4 declarations

The ToRegionContent instance for STM Text can be used to make regions that automatically update whenever there's a change to any of the STM values that they use.

For example, a region that displays the screen size, and automatically refreshes it:

import qualified Data.Text as T
 r <- openConsoleRegion Linear s
 setConsoleRegion r $ do
 	w <- readTVar consoleWidth
 	h <- readTVar consoleHeight
 	return $ T.pack $ unwords
 		[ "size:"
		, show w
 		, "x"
		, show h
 		]
valueconsoleWidth :: STM Int
#

Gets the width of the console.

On Unix, this is automatically updated when the terminal is resized. On Windows, it is determined at start. On WASM, the console width is hard coded to 80 since WASI does not provide a way to determine it.

valueconsoleHeight :: STM Int
#

Get the height of the console.

On Unix, this is automatically updated when the terminal is resized. On Windows, it is determined at start. On WASM, the console heigth is hard coded to 25 since WASI does not provide a way to determine it.

valueregionList :: TMVar [ConsoleRegion]
#

All the regions that are currently displayed on the screen.

The list is ordered from the bottom of the screen up. Reordering it will change the order in which regions are displayed. It's also fine to remove, duplicate, or add new regions to the list.

valuewaitDisplayChange :: STM a -> IO a
#

Runs a STM action, and waits for the display to be fully updated with any changes that action makes to the displayed regions.