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.

Answer 1

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.

Answer 2

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.

Answer 3

You can use getChar instead of getLine. This probably isn't what you're looking for, but it's the simplest way.

pressKey :: IO ()
pressKey = do x <- getChar
              return x

But there's an even simpler way. Just write getChar:

pressKey :: IO ()
pressKey = getChar >> putStr "I am a String"

pressKey = putStrLn "Hi" >> getChar >> putStrLn "Bye"