Portable getch?

[andyl]

I’ve been unable to find a simple & portable getch implementation for Elixir. The use case is to detect single keypress events, without needing the return key.

Comparable tools like inquirer/npm and tty/ruby use getch to build features like type-ahead, pull-down menus, etc.

To implement getch I've looked at ex_ncurses, rustler, System.cmd, Ports, etc. This is the best I’ve come up with so far:

#!/usr/bin/env elixir

ruby_cmd = ~S"""
  ruby -e '
    require "io/console"
    @output = IO.new(4)
    @output.sync = true
    char = STDIN.getch
    @output.write(char)
  '
"""

IO.write("Press a key > ")
port = Port.open({:spawn, ruby_cmd}, [:binary, :nouse_stdio]) 
receive do
  {^port, {:data, result}} -> IO.puts("\nChar: #{result}")
end

This works on my Linux dev machine, but depends on Ruby.

Does anyone know of a portable getch implementation? Maybe with Rustler or precompiled binaries or ???

[fuelen]

I found this crate https://crates.io/crates/getch
it is quite small, we can play with it and Rustler and if it works okay, then implement as a separate package

[antedeguemon]

@fuelen @andyl I tried using Rustler + getch but couldn't get it to work. I think it is due to BEAM blocking the stdin port (here).

So I came with a dirty solution that closes the stdin port opened by the user driver and re-opens it on a process I can control:
https://gist.github.com/antedeguemon/565d65718a4625dcec2d83d51614a9c5

It works well but it has the big downside of overriding the IO port, meaning regular IO operations won't work. Here is an asciicast of the script:

[fuelen]

Hey @antedeguemon

Cool, that's interesting. I went to Google to find what is tty_sl and quickly found this repo https://github.com/swalkerhppr/elixir-io_tty :)
Seems like manual stop of the port can be replaced with -noinput flag. @andyl have you seen the repo?

[andyl]

@antedeguemon thanks for posting your code I was able to run it locally. Studying.

@fuelen - have not seen elixir-io_tty before - it looks interesting!

[andyl]

I had another look - seems like tty_sl is an Erlang module that makes it possible to read character-by-character, and is used in IEx.CLI.start. I am unable to find any docs on tty_sl. Anyone have a link for this?

[fuelen]

Anyone have a link for this?

Unfortunately no

[andyl]

Here's a thread with some good resources:

https://groups.google.com/g/elixir-lang-talk/c/WtzbZcAk6XQ

[zachallaun]

Hi friends! I’ve been reading through the various linked resources from this thread this morning and wanted to follow up to see if any conclusions had been drawn.

It looks to me like the key is:

Port.open({:spawn, "tty_sl -c -e"}, [:binary, :eof])

which will send input characters to the opening process. The question then is what are the additional responsibilities that process has to keep everything working? Some misc things that I believe would need to be handled:

• Obvious stuff like writing characters back to the terminal (if desired)
• Handle cursor position
• Intercept output from other processes (register as group leader)

Other questions:

• How does the interact with IEx? E.g. running with iex -S mix … and hitting a dbg
• I’m sure there are other implications here…

[andyl]

Here's a gist for a 'getch' demo script. It works for me on Ubuntu 22.04 with OTP25 and Elixir 1.15. Give it a try!

[zachallaun]

Worked well on Ubuntu 20.04 via WSL, but only had a moment to test it.

Things to try:

• Ctrl-c / Ctrl-g
• Starting with IEx, is there a way to hijack control from IEx/yield it back later?

[fuelen]

Also, here are some upcoming changes which may change the current state erlang/otp#6144
I've just tried to run the script using Erlang from master (OTP 26) and the script prints the following error:

PRESS ANY KEY> sh: line 1: exec: tty_sl: not found

[zachallaun]

There may be some hints for what can be used on OTP 26 here: https://github.com/erlang/otp/blob/master/lib/kernel/src/prim_tty.erl

[andyl]

I get the same error tty_sl: not found using erlang 26.0.2 and elixir 1.15.4.otp-26 on Ubuntu 22.04. Had a look at prim_tty.erl & not able to figure out a solution. Anyone have a working example to share?

[zachallaun]

I'd love a "native" solution for OTP 24-26, but to be honest, I've lately been looking more towards Rustler/Zigler to wrap a more "native" getch solution based on termbox2 (C) or crossterm (rust). What I really, really want is something that puts the term into raw mode only for handling getch. I think it's possible by implementing the IO protocol and making sure the appropriate stuff gets cached while a getch is pending, but I've not looked into the edge cases.

Basically: I want to have my cake and eat it too 😂 I want to be able to just puts my output and keep things simple, but getch a single char to improve UX.

[andyl]

The source for the prim_tty nifs is more than 1000 lines. Probably somewhere in there is code that works like tty_sl, but there is no documentation. A standalone getch implementation would be great.

[andyl]

This issue has a demo of getch for Erlang/OTP26 that uses prim_tty...