What is a simple way to wait for and then detect keypresses in Haskell?

Question

I'm pretty new to Haskell, so I'm looking for a simple-ish way to detect keypresses, rather than using getLine.

If anyone knows any libraries, or some trick to doing this, it would be great!

And if there is a better place to ask this, please direct me there, I'd appreciate it.

There is a similar thread here http://stackoverflow.com/questions/2983974/haskell-read-input-character-from-console-immediately-not-after-newline — Pedro Rodrigues, Oct 09 '10 at 01:17
Most GUI libraries (ex: gtk2hs) have a onKeyPress type operation - if you're doing anything large or for distribution then consider those. — Thomas M. DuBuisson, Oct 09 '10 at 04:59

score 16 · Accepted Answer · answered Oct 09 '10 at 15:42

If you don't want blocking you can use hReady to detect whether a key has been pressed yet. This is useful for games where you want the program to run and pick up a key press whenever it has happened without pausing the game.

Here's a convenience function I use for this:

ifReadyDo :: Handle -> IO a -> IO (Maybe a)
ifReadyDo hnd x = hReady hnd >>= f
   where f True = x >>= return . Just
         f _    = return Nothing

Which can be used like this:

stdin `ifReadyDo` getChar

Returning a Maybe that is Just if a key was pressed and Nothing otherwise.

I'd also mention about `hSetEcho` function, which allows suppress echoing characters into `stdout`, while using `getChar`. — Dmitry Bespalov, Oct 16 '12 at 17:40

score 9 · Answer 2 · answered Oct 08 '10 at 22:47

9

import System.IO

main :: IO ()
main = do
  hSetBuffering stdin NoBuffering
  x <- getChar
  putStrLn ("You pressed: " ++ [x])

I don't know when this is guaranteed to work. Putting the terminal into a "raw" mode is a system-dependent process. But it works for me with GHC 6.12.1 on Linux.

answered Oct 08 '10 at 22:47

keegan

1,617
10
13

That's the issue I've found, it works rather poorly on Windows, and requires a newline before it is submitted. – TaslemGuy Oct 12 '10 at 22:26

What is a simple way to wait for and then detect keypresses in Haskell?

2 Answers2

Linked