Support Description() on subcommands

[xvello]

A follow-up on #156 to address #139.

writeHelpForSubcommand checks whether the given subcommand implements the Described interface. If it does not, we fallback to the main command's description.

Testing it

package main
import "github.com/alexflint/go-arg"

func main() {
	arg.MustParse(new(rootCommand))
}

type rootCommand struct {
	One *firstCmd `arg:"subcommand:one"`
	Two *secondCmd `arg:"subcommand:two"`
}
func (*rootCommand) Description() string {
	return "main description"
}

type firstCmd struct {}
func (c *firstCmd) Description() string {
	return "subcommand description"
}

type secondCmd struct {}

± go run test/main.go --help | grep desc
main description
± go run test/main.go one --help | grep desc
subcommand description
± go run test/main.go two --help | grep desc
main description

Limitation

Due to how the commands and args are tokenized, the invocation below describes the main command instead of the one subcommand. I am not sure whether that's desirable or not.

± go run test/main.go --help one | grep desc
main description

[xvello]

Hello @alexflint,

I'll understand if your time is short, but I think this will be a quick review, to close a long-lasting feature request. Don't hesitate to directly push adjustments on the branch if your time is limited. Closing this issue is more important to me than being right.

[alexflint]

Thank you for putting this together @xvello. I should be able to find time to think this through soon, though I will be on silent retreat for a week starting tomorrow.

[xvello]

Hello @alexflint, did you get a chance to take a look at this PR?

[alexflint]

Thanks for this PR @xvello but I think we need a somewhat different approach. See my comment below.

[alexflint]

I don't think it makes sense to include !isZero(v) in here because v.IsValid already checks that (see https://cs.opensource.google/go/go/+/refs/tags/go1.17.5:src/reflect/value.go;l=1435)

The basic issue with this approach is that it won't work if the struct for this subcommand has a parent that is nil, since in that case we will end up here and then v.IsValid() will return true.

It would be better to store the type for each subcommand in the command struct and then instantiate a new struct of that type right here and check whether it satisfies the Described interface

[xvello]

Closing as I moved to https://github.com/alecthomas/kong in the meantime.