Loading config file into input struct which contains configuration which is not exposed through CLI

[mitar]

So I have some configuration which I think it is too tricky to expose through CLI easily, e.g.:

type Config struct {
	Version kong.VersionFlag `short:"V" help:"Show program's version and exit."`
	Name string
	Config kong.ConfigFlag
	Sites []struct {
		Name string
		Domain string
		Dir string
	} `kong:"-"`
}

So I am thinking that it would be cool if I could load config first to set values in this struct, and then use CLI to add set CLI values. So I was thinking of having my own ConfigFlag which would use BeforeReset or BeforeResolve to load data from YAML file into config, with regular YAML loader, and then the rest is handled by kong. Does this sound reasonable and is there something already existing doing this I am missing? From what I was looking into config examples it looks like config files just set defaults for kong fields, but ignores other fields there might be in the struct/file?

[mitar]

So I came up with:

type ConfigFlag string

func (c ConfigFlag) BeforeResolve(app *kong.Kong, ctx *kong.Context, trace *kong.Path) error {
	path := string(ctx.FlagValue(trace.Flag).(ConfigFlag))
	file, err := os.Open(kong.ExpandPath(path))
	if err != nil {
		return errors.WithStack(err)
	}
	defer file.Close()
	decoder := yaml.NewDecoder(file)
	decoder.KnownFields(true)
	err = decoder.Decode(app.Model.Target.Addr().Interface())
	if err != nil {
		var yamlErr *yaml.TypeError
		if errors.As(err, &yamlErr) {
			e := "error"
			if len(yamlErr.Errors) > 1 {
				e = "errors"
			}
			return errors.Errorf("yaml: unmarshal %s: %s", e, strings.Join(yamlErr.Errors, "; "))
		}
		return errors.WithStack(err)
	}
	return nil
}

By setting yaml on struct fields, it looks like it works well. By setting yaml:"-" or kong:"-" I can control which config is available in CLI, which in config file, and which in both. The config struct is populated from the config file and then CLI parameters override some of those values. Am I missing something?

[mazzy89]

In my case I can pass flags and have a part of the config loaded via YAML file. let's say an hybrid combination which is what you are looking for to achieve but in order to do so you need to implement a custom mapper.