code: function(inputValue, ruleValue, attribute, callback) {
      var fn = this.isMissed() ? missedRuleValidator : this.fn;
  
      return fn.apply(this, [inputValue, ruleValue, attribute, callback]);
    }
docs: Apply validation function
  
  @param  {mixed} inputValue
  @param  {mixed} ruleValue
  @param  {string} attribute
  @param  {function} callback
  @return {boolean|undefined}