Add an ocamldebug command for back stepping

[aatxe]

I noticed when attempting to customize bindings for ocamldebug that Tuareg is actually missing a definition for backstep. This small change resolves that by creating an ocamldebug-back command (like the ocamldebug-next) command.

[leungbk]

It looks like C-b is already taken by break?

[aatxe]

Ah, that is true. It seems to take the first one defined, so I didn’t notice. I can change it, but I don’t know what would be the most suitable/mnemonic.

[bbatsov]

I'd just C-j, C-k or C-m as they are close to the keybinding C-n (for next line). Perhaps we can also add C-j and C-k as aliases for C-m and C-n, even if they are inspired by vim. :-)

[monnier]

This is old, but I think it's still relevant.
The "natural" binding for the opposite of C-n is C-p (C-b is the opposite of C-f), so the conflict with C-b == break is a non-issue (Yay!). Instead we have a conflict with C-p == print (bummer!).

But do we need a new binding or could we use a numeric prefix (like C-u - 2 C-n to backstep 2 lines)?

[gasche]

The OCaml manual (debugger chapter) mentions C-c C-k as the keybinding to step back, and this seems to have been implemented in caml-mode/camldebug.el in 1997. I'm not sure if Tuareg removed this keybinding (maybe due to a conflict with the kill keybinding?) or if it was forked from camldebug.el before it was added.

I do find it convenient to have support for both step and back(step), and I think it would make sense to reuse C-c C-k for this to follow the manual's documentation. This may require making C-k for the kill command not work anymore, and I think it would be a fair deal. (You only kill the program once in your debug section, but you may step back and forth quite a bit.)

[bbatsov]

@gasche Do you know who are the maintainers of the project these days? I've noticed there has been no activity in the past 18 months.

[gasche]

This isn't necessarily such a bad sign, this is old code that is in maintenance mode -- and maybe Emacs is good about not asking for too much code churn. If you want to ask for specific changes, I would try asking @monnier.

[monnier]

OMG, I can't believe I hadn't really registered the duplication between camldebug.el and ocamldebug.el. I remember having the feeling of deja-vu when I fixed the completion code, but somehow ....
Anyway, I have a PR #319 that partially syncs up with camldebug.el and that brings in C-c C-k for backstep as well as C-c C-d for display.

The new code passes the tests, but AFAIK none of our tests cover the ocamldebug.el code, so could some kind souls out there try that code of mine and confirms it seems to work at least for them?

[gasche]

Wow, very nice! (I'm not qualified to review the change unfortunately, but I'm in favor.)

[monnier]

Wow, very nice! (I'm not qualified to review the change unfortunately, but I'm in favor.)

Well, I merged it to `master`. So I think this can be closed.

[monnier]

Actually, I was confused. The camldebug.el code I brought in is not really a solution because it has both

(def-camldebug "backstep" "\C-k" "Step one event backward.")

and

(def-camldebug "kill"   "\C-k")

As already pointed out by @gasche (sorry, I was too dense to understand that comment).
[ Similarly, that patch supposedly added a C-c C-d binding for display, but it collides with the existing binding for delete. ]