Feature Request: multiple names for fields

[ErinCall]

I'd like to be able to specify multiple/alternate names for a config field. I'm envisioning a rough equivalent to the EnvVars setting in urfave/cli:

type Config struct {
  SomeSetting string `envconfig:"setting_name,deprecated_name"`
}

Right now, we're working around it with multiple structs:

type LegacyConfig struct {
  SomeSetting string `envconfig:"deprecated_name"`
}

type Config struct {
  SomeSetting string `envconfig:"setting_name"`
}

func Main() {
  var lCfg LegacyConfig
  envconfig.Process(&lCfg)
  cfg := Config{
    SomeSetting: lCfg.SomeSetting,
  }
  envconfig.Process(&cfg)
}

[kelseyhightower]

How should ordering work? I think the big challenge will be dealing with the ambiguous nature of an env var being unset vs set to empty.

[afarbos]

Couldn't we use the order provided ?
In this case:

1. setting_name
2. deprecated_name

But I would argue that the order should not matter here actually. Users should only set one of these environment variables and envconfig pick the first one that is not nil.

[tomercagan]

Not exactly what you asked for but you can work around this by creating two entries and a function to return any non-empty one:

type Config struct {
   SomethingNew string `envconfig:setting_name`
   SomethingOld string `envconfig:deprecated_name`
}

func (cfg *Config) Something() string {
    if cfg.SomethingNew != "" {
        return cfg.SomethingNew
    }
    return cfg.SomethingOld
}

[jan-xyz]

Maybe the ordering is not that important. You won't likely see both being set explicitly, since you either specify the deprecated key name because you haven't migrated yet or migrated to the new key and won't be setting the old anymore. In that case it could be a either or setting that guards this but you should not specify both?

EDIT: maybe something like this:

type Config struct {
  SomeSetting string `envconfig:"setting_name" alternative:"deprecated_name"`
}

or

type Config struct {
  SomeSetting string `envconfig:"setting_name" deprecated:"deprecated_name"`
}

In the second example it could even be possible to add a way to handle the usage of deprecated keys (e.g. logging?)