code: func EnvDefaultFunc(k string, dv interface{}) SchemaDefaultFunc {
  	return func() (interface{}, error) {
  		if v := os.Getenv(k); v != "" {
  			return v, nil
  		}
  
  		return dv, nil
  	}
  }
docs: // EnvDefaultFunc is a helper function that returns the value of the
  // given environment variable, if one exists, or the default value
  // otherwise.