Support automatic subcommands for completion

[colemickens]

It seems to be something of a pattern for apps to support completion via the following:

$ source <(app completion zsh)
# or
$ source <(app completion bash)

It'd be nice if there were a helper on App so that we could call .add_completion_subcommands() and have this wired up automatically.

[kbknapp]

This would be an easy addition, although it'd probably get implemented as an AppSettings variant.

The one thing that gives me pause, is that I've seen this done in a few different ways by different applications, such as $ prog completion[s] [--shell] <shell>.

Is the prog completion <shell> (singular form of "completion") a standard? Because if it were only up to me, I'd implement it as prog completions <shell> (plural form of "completion" with a single positional argument accepting the shell name), but if there's a written standard I haven't seen I'd rather follow the standard.

[colemickens]

I don't know if one is more popular/"standard" than another. I thought I had other examples up my sleeve, but the only one I can think of now is kubectl which takes the form kubectl completion zsh. I have only a tiny preference for the subcommand style, rather than flag, but it seems to just be cosmetic.

[kbknapp]

In all honesty it'd be possible to give options for both, which is how I think I'll tackle this.

• AppSettings::GenerateCompletionsSubcommand
  • Will generate prog completions <shell>
• AppSettings::GenerateCompletionsArg
  • Will generate prog --shell-completions <shell>

[rharriso]

Is this possible anymore?

The clap_generate package depends on clap. Won't this be a circular package dependency?
Also. I'm having trouble using clap_generate

working in this branch:
https://github.com/rharriso/clap/tree/v3-master

[pksunkara]

@rharriso Try the clap_generate now, it should work. But yeah, this would become a circular dependency. I am not sure we want this.

Maybe we could do some kind of macro.

[pksunkara]

@CreepySkeleton Would love your input here.

[CreepySkeleton]

That's possible. We can make clap_generate just another module in clap (i.e it would be no longer a separate crate), possibly behind a feature flag.

Pros:

• Very convenient for end users.

• We will no longer need the #[doc(hidden)] pub things.

  Let me catch your attention here: currently, a lot of stuff in clap is doc(hidden) precisely and only because clap_generate needs it. It's much more that you might have thought; many things (structs and enums) are public only because they are parts of such a "public" field, or a part of some function's signature (also pub hidden one). The point is: this pseudo-publicity expands onto other things transitively, pulling them into the club even though they have no need to be publich.

  In my estimation, about 30% of code is pub hidden now, and those erase the line between "internal API used by clap_generate" and really "public API". Once/if we embrace the generate into the clap itself, we'll be able to change them to pub(crate), as it should be. A big improvement if you ask me.

Cons:
* clap's own codebase would grow a bit. Well, we're the ones who's gonna have to maintain it one way or another 🤷‍♂ .

[TeXitoi]

@CreepySkeleton You mean clap_generate instead of clap_derive ?

[pksunkara]

There must have been a reason Kevin wanted to make it a separate crate.

…

On Tue, Mar 3, 2020, 16:47 Guillaume P. ***@***.***> wrote: @CreepySkeleton <https://github.com/CreepySkeleton> You mean clap_generate instead of clap_derive ? — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#810?email_source=notifications&email_token=AABKU35NWB52OG4OZCVHKO3RFURBVA5CNFSM4C3WHVVKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOENUAHZY#issuecomment-594019303>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABKU3ZGNHAF4PG3EOK2DJ3RFURBVANCNFSM4C3WHVVA> .

[CreepySkeleton]

@TeXitoi right 😨

[CreepySkeleton]

I don't see any reason, honestly. Not any I would call a good one.

[TeXitoi]

The advantage is that you will not compile it if you don't use it. The other solution is features, and I personally prefer to keep features to the bare minimum as they complexify the code considerably.

But, having this implementation hole is quite dirty. I think the clean way would be:

• having the introspection API used by clap_generate public and documented
• having clap_generate in the clap crate

The second way seems easier now. But that's just my humble opinion, I didn't even seen the code.

[pksunkara]

I made as much of introspection API public. The remaning stuff are doc(hidden) because they are macro helpers for the introspection API. If the whole system isn't macros, they would have been private.

[CreepySkeleton]

The other solution is features, and I personally prefer to keep features to the bare minimum as they complexify the code considerably.

That's gonna be one module level cfg and one another cfg somewhere in AppSettings. Sounds simple enough.

I've seen the code, I've been trying to implement an introspection API of sorts (you just read my mind 😃, ha!), and I feel like I'm failing. calp_generate and clap are so closely tied to each other that we may as well make them all public. I'll show you the result of this attempt soon, but I vote for pulling it into clap.

I made as much of introspection API public.

You did? Mind elaborate?

[CreepySkeleton]

The remaning stuff are doc(hidden) because they are macro helpers for the introspection API. If the whole system isn't macros, they would have been private.

I hate to be rude, but that's actually not true at all. Just grep for #[doc(hidden)]: 140 matches across 17 files. Ridiculous.

[CreepySkeleton]

@TeXitoi They seem to be implementing explicit support for features in docs.rs, see dtolnay/syn#734 . We might as well do that too.

Looks neat unless you have lots of cfg'ed items (we don't): https://docs.rs/tokio/0.2.13/tokio/index.html#modules

[CreepySkeleton]

@pksunkara @kbknapp So can we finally decide whether we want this or not? More specifically, are we open to inclusion clap_generate as a clap module? If we aren't, than we can close this as impossible.

[pksunkara]

I will take care of this. I have a few ideas on how to fix this.

[epage]

btw another way of solving this is instead of having a setting for this, to provide types that implement Args and Subcommand (through derive or not). A user could then choose to flatten those into their arguments.