Better error message about missing arguments for subcommands

[freehck]

% bf2 buildpkg       
bf2: required argument PLATFORM is missing
Usage: bf2 buildpkg [OPTION]... PKGNAME VERSION PLATFORM

Obviously it should be reported about the missing argument PKGNAME.

I suppose the problem is in intertal command matching. Cmdliner sees that the 3rd parameter of command is missed. And reported that there's not the 3rd parameter of subcommand in the error message.

The code to reproduce it:

let buildpkg_cmd =
  let doc = "..." in
  let man = help_secs in

  let pkgname =
    Arg.(required & pos 0 (some string) None & info [] ~docv:"PKGNAME") in
  let version =
    Arg.(required & pos 1 (some string) None & info [] ~docv:"VERSION") in
  let platform =
    Arg.(required & pos 2 (some string) None & info [] ~docv:"PLATFORM") in

  Term.(...),
  Term.info "buildpkg" ~doc ~man

and eventually:

let () = match Term.eval_choice default_cmd [buildpkg_cmd] with
  | `Error _ -> exit 1
  | _ -> exit 0

[dbuenzli]

Obviously it should be reported about the missing argument PKGNAME.

Well obviously the PLATFORM parameter is also missing so this is not a wrong error message... but yes maybe I could start to report from the left. Which error is reported if you respectively specify one, and two positional arguments ?

[dbuenzli]

Also a fully reproducible test case would be nice.

[dbuenzli]

I think from a user point of view we should simply report all missing required arguments, not stop at the first one.

[hcarty]

Agreed, I'd prefer a report of all missing required arguments if that's reasonable.

If only one is reported then the first missing would be nice to have.

[freehck]

I'll supply you with reproducible examples after holidays, okay? I'm a little bit busy now.
Well, I'd prefer an error message either with only first missed parameter because I can easily determine the set of missed parameters from the 2nd line with "Usage" string.

[dbuenzli]

Well, I'd prefer an error message either with only first missed parameter because I can easily determine the set of missed parameters from the 2nd line with "Usage" string.

I'd say that the usage line only allows you to see at which position the missing arguments are due.

If the error message only tells you "required argument PKGNAME is missing" your brain will likely focus to fill in PKGNAME, failing to realize other required arguments are missing. In other words it's a misleading error message that will likely bring you to a new erroring interaction.

If the error is "required arguments PKGNAME, VERSION and PLATFORM are missing", you'll be more likely to take care to try to fill in all the arguments and the usage line will help you figure out at which position the arguments are due.

[dbuenzli]

In fact the problem is even worse: the actual argument that is reported depends on the way you construct your term.

[dbuenzli]

This should be fixed, all missing required arguments should be reported, in order. Thanks for the report.

[dbuenzli]

This introduced the regression that we can no longer as for --help if the tool has required arguments and they are not provided. We should test if help is requested before raising the error.