up/down arrow history support

[pop]

Hey! I really like using shticker_book_unwritten (installed via Nix). My one ask is to store a history of commands with up/down arrow to cycle through, similar to how bash/sh/zsh allow you to cycle through history.

I'm going to take a stab at implementing this, so if you have thoughts on how to best implement this, or if this is a feature you think s.b.u. should include let me know! I'm very open to feedback and would love some guidance.

[JonathanHelianthicusDoe]

I think this to be a useful feature for sure. I never bothered to implement it, because it would require the way that inputs are handled to be considerably less “dumb”. Right now, inputs are simply read via std::io::Stdin::read_line. This implies two important things:

• The input is only received after pressing Enter — that is, when a newline is encountered. The only exception is EOF (typically Ctrl+d), which is already handled by exiting.
• The input is necessarily a string (of UTF-8 in particular, but the encoding is largely immaterial here). This means that, for example, if you wanted to read a keypress of ↑, then you would need to somehow parse it out of an ordinary string. For most Unices, this is going to be an ANSI escape code ^[[A. But this string is also displayed by the terminal (as part of the user’s input), which is presumably not what you want either.

If someone didn’t know any better, then they might think the former problem to be a consequence of using a function called read_line — it is in the name, after all. But the deeper reason is that stdin is typically line-buffered: unless you use an OS-specific API, your std::io::Stdin handle only ever observes full lines; specifically, of text (even if you use its Read implementation). This is, naturally, also the reason for the second problem.

So you need a way to access OS-specific APIs that allow for reading keystrokes directly. Realistically, the best way is to just depend on a crate like console. This solves the other problem that you have:

• You have to know how to clear lines, move the cursor, &c.