In ant design one can provide a custom validator like the following:

<Form.Item label=First Name>

  {getFieldDecorator(firstName, {

    rules: [

      {

        validator: (rule: any, value: string, cb: (msg?: string) => void) => {

          value.length < 3 ? cb(too short) : cb();

        }

      }

    ]

  })(<Input />)}

</Form.Item>

As you see I'm using typescript and cause its transpiler is really cool it wants me to use rule parameter of validator as well. I can't find any documentation on it and don't know what is good for. So if you can please explain briefly what is it and how it should be used?