code: func BoolStringPtrState(v interface{}) string {
  	b, err := strconv.ParseBool(v.(string))
  	if err != nil {
  		return ""
  	}
  	return strconv.FormatBool(b)
  }
docs: // BoolStringPtrState is a state normalization function for stringified 3-state
  // bool pointers.
  //
  // The function silently drops any result that can't be parsed with ParseBool,
  // and will return an empty string for these cases.
  //
  // This is designed to address the current lack of HCL and Terraform to be able
  // to distinguish between nil states and zero values properly. This is a
  // systemic issue that affects reading, writing, and diffing of these values.
  // These issues will eventually be addressed in HCL2.