Haskell read raw keyboard input

by

This might be the simplest solution, resembling typical code in other programming languages:

import System.IO (stdin, hReady)

getKey :: IO [Char]
getKey = reverse <$> getKey' ""
  where getKey' chars = do
          char <- getChar
          more <- hReady stdin
          (if more then getKey' else return) (char:chars)

It works by reading more than one character “at a time”. Allowing E.g. the ↑ key, which consists of the three characters ['\ESC','[','A'] to be distinguished from an actual \ESC character input.

Usage example:

import System.IO (stdin, hSetEcho, hSetBuffering, NoBuffering)
import Control.Monad (when)

-- Simple menu controller
main = do
  hSetBuffering stdin NoBuffering
  hSetEcho stdin False
  key <- getKey
  when (key /= "\ESC") $ do
    case key of
      "\ESC[A" -> putStr "↑"
      "\ESC[B" -> putStr "↓"
      "\ESC[C" -> putStr "→"
      "\ESC[D" -> putStr "←"
      "\n"     -> putStr "⎆"
      "\DEL"   -> putStr "⎋"
      _        -> return ()
    main

This is a bit hackish, since in theory, a user could input more keys before the program gets to the hReady. Which could happen if the terminal allows pasting. But in practice, for interactive input, this is not a realistic scenario.

Fun fact: The cursor strings can be putStrd to actually move the cursor programmatically.

More Related Contents:

  1. A Haskell function of type: IO String-> String
  2. What’s so bad about Lazy I/O?
  3. Wrong IO actions order using putStr and getLine
  4. Should do-notation be avoided in Haskell?
  5. Haskell: I/O and Returning From a Function
  6. Converting IO Int to Int
  7. How to catch a no parse exception from the read function in Haskell?
  8. In what sense is the IO Monad pure?
  9. How do I define Lisp’s apply in Haskell?
  10. Operating on a return from a Maybe that contains “Just”
  11. Is there a Haskell idiom for updating a nested data structure?
  12. Positive integer type
  13. Explicitly import instances
  14. How to compare two functions for equivalence, as in (λx.2*x) == (λx.x+x)?
  15. Why do 3 and x (which was assigned 3) have different inferred types in Haskell? [duplicate]
  16. List of different types?
  17. What is the purpose of the reader monad?
  18. Ordering of parameters to make use of currying
  19. No instance for (Fractional Int) arising from a use of `/’
  20. Prefix form of unary operator in Haskell
  21. How can I generate different random values in Haskell?
  22. Can you overload + in haskell?
  23. Is there a way to see the list of functions in a module, in GHCI?
  24. Haskell GHC: what is the time complexity of a pattern match with N constructors?
  25. Creating unique labels in Haskell
  26. What’s the status of multicore programming in Haskell?
  27. Tools for analyzing performance of a Haskell program
  28. Why does OCaml sometimes require eta expansion?
  29. Explanation of “tying the knot”
  30. Return specific type within Haskell

Leave a Comment