Re: [Haskell-cafe] Loop with the StateT monad

19 Mar 2017

      Am 19.03.2017 um 01:27 schrieb Brandon Allbery:
...
On Sat, Mar 18, 2017 at 7:57 PM, Ben Franksen 
wrote:
...
As long as you leave control over these things to the terminal, yes. But
what if your program wants to control that itself? E.g. react to
keystrokes, rather than edited lines, as the OP seemed to want?
https://downloads.haskell.org/~ghc/8.0.2/docs/html/libraries/unix-2.7.2.1/Sy...
Can System.Posix.Terminal be used on Windows?
...
Admittedly some wrappers would be nice... but with care, lest you reproduce
the same issues stack is having.
Right. With hindsight it seems obvious how to do this right: save the
initial terminal settings and restore them when the program exits.

Let's see if haskeline does the right thing. The following seems to work:

  import System.Console.Haskeline
  getKey = runInputT defaultSettings (getInputChar "")
  main = getKey >>= print

Now we want to see if this leaves the terminal in the same state as it
was before. Unfortunately the obvious thing doesn't work:

  import System.Console.Haskeline
  import System.Posix.Terminal

  getKey = runInputT defaultSettings (getInputChar "")

  main = do
    before <- getTerminalAttributes 0
    getKey >>= print
    after <- getTerminalAttributes 0
    print (before == after)

because there is no Eq instance for TerminalAttributes. So I asked stty:

  before=$(stty -a)
  ./test
  after=$(stty -a)
  if test "$before" = "$after"; then echo good; else echo bad; fi

and that reports "good".

So, perhaps a better answer to the OP would have been:

  import System.Console.Haskeline

then define

  getKey = runInputT defaultSettings (getInputChar "")

and use that instead of getChar.

Cheers
Ben