Re: Read characters from without newline

[Mike Gran]

On Sat, Aug 28, 2021 at 02:09:52PM +0000, Zelphir Kaltstahl wrote:
> Hello Guile users,
> 
> I am trying to find a way to read a character from command line or REPL, 
> without
> having to enter a newline, confirming the input.
> 
> For example I found this for Python: https://stackoverflow.com/a/21659588
> <https://stackoverflow.com/a/21659588>
> 
> I read on https://www.gnu.org/software/guile/manual/html_node/Buffering.html
> <https://www.gnu.org/software/guile/manual/html_node/Buffering.html> about
> buffering and thought, that I could simply set the port to be unbuffered using
> (setvbuf port 'none) and then call (get-char port) as follows:
> 
> ~~~~
> (import
>  (except (rnrs base) let-values map)
>  (only (guile)
>        ;; lambda forms
>        lambda* ??
>        ;; input output
>        current-input-port)
>  (ice-9 textual-ports)
>  (ice-9 optargs))
> 
> (let ([port (current-input-port)])
>   (setvbuf port 'none)
>   (get-char port))
> ~~~~
> 
> However, this does not work. The terminal emulator might not send input
> immediately to the Guile program. Even in the Guile REPL and in Emacs Geiser
> this does not work. I guess it would work, if they did send each character
> immediately to the Guile program, because the following works:
> 
> ~~~~
> (display 
>   (call-with-input-string "ab"
>     (?? (in-port)
>       (setvbuf in-port 'none)
>       (get-char in-port))))
> ~~~~
> 
> There is no newline in the string "ab" and only the "a" is displayed.
> 
> So the question might be, how one can get into a mode, where Guile receives
> input immediately, taking over control of the terminal or REPL.

Hello. For this to work, your terminal needs to be in 'raw' mode
where it sends characters immediately.  Terminals are normally
in 'cooked' mode where it waits for a return before it sends everything.

The simple way to get into raw mode would be to run your script like

    stty raw && guile script.scm
    
But if you want to get the terminal into raw mode from within guile,
you need to get guile to call a function like 'tcsetattr' to set the
mode of your terminal. Or maybe just call the stty program from within
guile?

There is a binding of tcsetattr in the (ncurses extra) library
in guile-ncurses, but since you just need a couple of functions, you
could wrap them in FFI, I suppose.

-Mike