provide an error message if completion errs in zsh

[gusega]

If the completion errs, then nothing is displayed to the user.

This change makes the returned completion (containing an error message) displayed on screen if the completion status is error in zsh.

Of course it would be better to do this also in all other shells (bash, powershell, etc), but for now this is for zsh only. Let me know if it's also desirable to look at other shells, maybe I could do that.

[marckhouzam]

Thanks @gusega. Could you give an example of how you imagine this being used?
It is also a bit tricky because we would start showing a message based on the completions being returned, but some projects may be blindly returning something but expecting it to be ignored when the ShellCompDirectiveError is used.

An alternative could be to use ActiveHelp in this case, which is already supported for both zsh and bash.
See this doc for ActiveHelp details: https://github.com/spf13/cobra/blob/main/active_help.md

[gusega]

Hi @marckhouzam

So, in your program, in completion function you'd return some error description as a single value of the completion slice, and the completion directive must be cobra.ShellCompDirectiveError, like this: return []string{"error description"}, cobra.ShellCompDirectiveError. The idea is that completion error means that something went wrong and user should know about it. Examples that I encountered: client application trying to get nouns completions when server is down, or the user has not authorised yet (and therefore cannot get nouns from the server).

I think I could do the same using active ActiveHelp, although it feels like misusing it, because it seems that ActiveHelp is intended for situations when completion cannot be tried, as user didn't provide enough info. And completion error means that all the stuff was provided, but the result is error.

Maybe we can introduce a new completion directive that would make completion function display the completion error, e.g. cobra.ShellCompDirectiveErrorShow, so that old directive still works as it is used to.

Meanwhile I will use ActiveHelp (didn't know about it) with something like this:

comps, err := getCompletions()
if err != nil {
    return cobra.AppendActiveHelp(make([]string, 0, 1), err.Error()), cobra.ShellCompDirectiveNoFileComp
}
return comps, cobra.ShellCompDirectiveNoFileComp

Thanks.