how to default string flag to "auto"

[AlejandroSuero]

I am working on a feature for freeze introducing a flag --window.title which if passed like --window.title should equal to --window.title=auto.

// config.go
type Config struct {
  // ...
  Title      string    `json:"title,omitempty" help:"Display input file as title. {{--window.title=auto}} as default." prefix:"window." group:"Window" default:"auto"`
  // ...
}

When parsing the args from the command line like kong.Parse(os.Args[1:]) if I use the flag like ./freeze cut.go --window.title, I get the following error:

--window.title: expected string value but got "EOL" (<EOL>)

Is there a way to use a string with the use case of --window.title or if I just don't include it in the command line args it will just default to auto?

[alecthomas]

If you don't included it it will default to "auto".

[AlejandroSuero]

I know that, maybe my question should be the next one:

1. freeze cut.go --window will include --window.title=auto as is not explicitly written in the args.
2. freeze cut.go --window --window.title won't default to "auto" or be "" since it errors expecting a string.

Is there a way to use case 2 to behave like case 1? freeze cut.go --window --window.title defaulting to "auto" or after parsing being "", to act like a bool so if it is included defaults to "auto" or is "" and if --window.title="foo" after parsing would be "foo".

[alecthomas]

No, that is not possible.

[AlejandroSuero]

Okay, thank you for answering 😁

[VonC]

@AlejandroSuero I usually have a pkg/infrastructure/cli/args_initialization.go with a InitializeArgs() function where I do read and modify os.Args. I called it from my main() function.

You can modify a --window.title to --window.title=auto, or add a --window.title=auto arg if it is missing, and if --window is present.

Then you can call kong.Parse(os.Args[1:])

[ccoVeille]

It's an hack, but I like it anyway
Hack because, the app should allow to parse flag the way we want.

But thanks, it gives ideas

[alecthomas]

If anyone is wondering why this is the case, it's because it can easily be ambiguous. If you had a flag with a default that could be omitted, and you also had positional arguments, eg. [--window.title=<string>] [<arg>...], then in the following case it's not clear whether the value is the positional argument or the flag argument:

--window.title something

Kong supports flag values via either --foo=bar or --foo bar, so the example above is confusing. Does it mean:

1. --window.title using the default value, and something is the positional argument or
2. --window.title with something as the value, and no positional argument

And even if Kong picks one of these, what will the users expectations be?

[ccoVeille]

Thanks for this interesting and valuable feedback