Change type-definition of ValueSourceChain

[dennis-tra]

Just a fly-by comment. I'm currently looking into the changes of v3 vs v2 and saw the definition of:

type ValueSourceChain struct {
	Chain []ValueSource
}

Given the current usage of ValueSourceChain, I think this could be simplified to:

type ValueSourceChain []ValueSource

Complete Changeset:

type ValueChain []ValueSource

func NewValueChain(src ...ValueSource) ValueChain {
  return src
}

func (vsc ValueChain) Append(other ValueChain) {
  vsc = append(vsc, other...)
}

func (vsc ValueChain) EnvKeys() []string {
  vals := []string{}

  for _, src := range vsc {
    if v, ok := src.(*envVarValueSource); ok {
      vals = append(vals, v.Key)
    }
  }

  return vals
}

func (vsc ValueChain) String() string {
  s := []string{}

  for _, vs := range vsc {
    s = append(s, vs.String())
  }

  return strings.Join(s, ",")
}

func (vsc ValueChain) GoString() string {
  s := []string{}

  for _, vs := range vsc {
    s = append(s, vs.GoString())
  }

  return fmt.Sprintf("&ValueChain{Chain:{%[1]s}}", strings.Join(s, ","))
}

func (vsc ValueChain) Lookup() (string, bool) {
  s, _, ok := vsc.LookupWithSource()
  return s, ok
}

func (vsc ValueChain) LookupWithSource() (string, ValueSource, bool) {
  for _, src := range vsc {
    if value, found := src.Lookup(); found {
      return value, src, true
    }
  }

  return "", nil, false
}

This could make flag definitions a bit more ergonomic:

// before
&cli.StringFlag{
	Name: "log.level",
	Sources: cli.ValueSourceChain{
		Chain: []cli.ValueSource{cli.EnvVar("LOG_LEVEL")},
	},
	Usage:       "description",
	Destination: &cfg.LogLevel,
	Value:       cfg.LogLevel,
	Category:    "Logging",
}

// after
&cli.StringFlag{
	Name: "log.level",
	Sources: cli.ValueSourceChain{
		cli.EnvVar("LOG_LEVEL"),
	},
	Usage:       "description",
	Destination: &cfg.LogLevel,
	Value:       cfg.LogLevel,
	Category:    "Logging",
}

I wanted to create a quick issue because v3 isn't released yet and there's still an opportunity to do breaking changes.

[dearchap]

@dennis-tra We've gone through a couple of iterations of this. Can you look at the v3 changes for this particular piece of code and submit a PR if you'd like us to make the change ?