Scala 2.13中如何实现Case Class的多类型联合？

问题背景 #

我定义了三个case class：

case class Teacher(name: String, subject: String, age: Int)

case class Student(name: String, subject: String, section: String, class: Int)

case class School(user: Teacher | Student)

希望School的user字段只能是Teacher或Student中的一种，请问在Scala 2.13中该如何实现？

补充1 #

有评论建议用Either，但如果需要支持三种类型呢？比如：

case class School(user: Teacher | Student | Principal)

补充2 #

我尝试用sealed trait实现，但写法存在问题，不清楚如何实例化。比如怎么创建包含Principal(name="ABC",year=2022)的School实例？我的代码如下：

sealed trait SchoolUser
case class Student(name: String, subject: String, section: String, `class`: Int)
case class Teacher(name: String, subject: String, age: Int)
case class Principal(name: String, year: Int)

case object Student extends SchoolUser
case object Teacher extends SchoolUser
case object Principal extends SchoolUser

case class School(user: SchoolUser)

val school = School(user = ???)

println("Complete")

解决方案 #

1. 修正Sealed Trait的写法 #

你之前的代码错误在于重复定义了同名的case class和case object，导致Student/Teacher/Principal的case class并未继承SchoolUser特质。正确的写法是让每个实体case class直接继承SchoolUser，不需要额外的case object：

// 密封特质，限定所有实现类必须在当前文件中定义
sealed trait SchoolUser

// 让每个实体类继承SchoolUser
case class Student(name: String, subject: String, section: String, `class`: Int) extends SchoolUser
case class Teacher(name: String, subject: String, age: Int) extends SchoolUser
case class Principal(name: String, year: Int) extends SchoolUser

// School类接收SchoolUser类型的参数，自然限定了user只能是上述三种类型之一
case class School(user: SchoolUser)

2. 实例化方式 #

现在可以直接创建具体的实体实例，传入School的构造函数：

// 创建包含Principal的School实例
val principal = Principal(name = "ABC", year = 2022)
val schoolWithPrincipal = School(user = principal)

// 创建包含Student的School实例
val student = Student(name = "小明", subject = "数学", section = "A", `class` = 3)
val schoolWithStudent = School(student)

// 创建包含Teacher的School实例
val teacher = Teacher(name = "李老师", subject = "英语", age = 35)
val schoolWithTeacher = School(teacher)

3. Sealed Trait的优势 #

• 编译时类型检查：因为SchoolUser是sealed的，编译器会确保所有实现类都在当前文件中，后续无法新增未声明的类型，严格限制School.user的可选范围。
• 完善的模式匹配支持：对School.user做模式匹配时，编译器会检查是否覆盖了所有可能的类型，避免遗漏分支：

schoolWithPrincipal.user match {
  case Student(n, s, sec, cls) => println(s"学生：$n，班级：$cls")
  case Teacher(n, s, a) => println(s"老师：$n，年龄：$a")
  case Principal(n, y) => println(s"校长：$n，任职年份：$y")
}

4. 为什么不推荐嵌套Either？ #

Either仅适用于二选一的场景，当需要支持三种及以上类型时，嵌套Either（比如Either[Teacher, Either[Student, Principal]]）会让代码结构复杂、可读性下降，而sealed trait可以优雅支持任意数量的类型。

内容的提问来源于stack exchange，提问作者Shivam Sahil