code: func (v *Validator) TypeBool(field string) *ValidationResult {
  	if _, err := strconv.ParseBool(v.data.Get(field)); err != nil {
  		// note: "true or false" is a more natural colloquial term than "bool"
  		return v.addTypeError(field, "true or false")
  	} else {
  		return validationOk
  	}
  }
docs: // TypeBool will add an error to the Validator if the first
  // element of data.Values[field] cannot be converted to a bool.