Look into a pure Python readline replacement

[andychu]

This is kind of vague, but it's a good first issue for somebody who already has expertise in the area. It's not something I know a ton about (other than the fact that I can't live without set -o vi).

The author/maintainer of bpython has a lot of interesting things to say about terminals and REPLs.

http://ballingt.com/blog/

• python-prompt-toolkit
• curtsies
• blessings
• the stdlib curses module? That doesn't do what readline does.

[watk]

Excuse my ignorance, but would you mind writing some rationale for this?

[andychu]

Here's one motivation. Start zsh and then try to complete 2 or more flags, like

ls --a<TAB>

then pick a suggestion

ls --all --s<TAB>

zsh doesn't redraw the prompt. Now compare with bash.

Maybe readline can already do this? Again it's something I don't know much about :)

---

Another motivation is that GNU readline is GPL, which I think caused problems for Python at one point. It would be nice to have an alternative.

I think Python links with libedit or something on Mac? Oil only links with readline.

Anyway, this is not all that solid an issue, hence the disclaimer that it's vague :) Some people have higher standards for interactive shells (namely zsh users) so I was just throwing this out there.

As far as being pure Python, I think it would help iterate more quickly on the UI. There is a lot of precedent for that in the 3 libraries I mentioned.

[andychu]

Notes:

A big difference between bash and fish/zsh is that the latter two both display flag descriptions.

I think it's possible to write a custom readline display hook to do this, but I'm not sure.

In other words, it might be worthwhile to reuse readline for vi editing mode, but try to replace its completion display routines. That is not trivial because I think it has to respond to SIGWINCH and stuff like that.

[andychu]

In summary, things I want:

• Don't reprint the prompt line on successive tab completions, which preserves vertical screen space
  • Also erase the prompt on Ctrl-C
• Display "first words" and flags with additional info like description
  • Maybe smarter prioritization of candidates. Readline has a coarse prompt: "Display all 3000 possibilities?"
• Preserve vi bindings of readline

[andychu]

Update: This is less urgent because I figured out how to use readline in the way I want, with demoish.py. It has those 3 properties.

/~https://github.com/oilshell/blog-code/tree/master/interactive-shell

[andychu]

Update: a problem with using readline is that I don't know how to detect when the user typed past the last column in the terminal, and then clear that next line of completion suggestions.

[andychu]

Closing in favor of #460, and #653 (ble.sh)