[英]Scala polymorphic callback type mismatch
I'm sorry i couldn't find a better title.很抱歉我找不到更好的标题。
I'm trying to achieve something like the following我正在尝试实现以下目标
abstract class Person
case class User(uid: String, firstname: String, active: String) extends Person
case class Admin(id: String, pseudo: String, securityClearance: String) extends Person
def innerFunctionForUser(user: User): List[String] = {
List() :+ user.uid :+ user.firstname :+ user.active
}
def innerFunctionForAdmin(admin: Admin): List[String] = {
List() :+ admin.id :+ admin.psuedo :+ admin.securityClearance
}
def outerFunction(person: Person, innerFunction: (Person) => List[String]): List[String] = {
innerFunction(person)
}
so i could use it like that所以我可以这样使用它
val myUser = User("0c60c5b4-306d-4372-b60d-fd699c80e408", "joe", "false")
val myAdmin = Admin("178789", "jack", "high")
outerFunction(myUser, innerFunctionForUser)
outerFunction(myAdmin, innerFunctionForAdmin)
which does not type check不输入检查
type mismatch;
found : User => List[String]
required: Person => List[String]
and i can't have the innerFunction accept a type person like this而且我不能让 innerFunction 接受这样的类型的人
def innerFunctionForUser(user: Person): List[String] = {
List() :+ user.uid :+ user.firstname :+ user.active
}
I kept it simple here but i need case class with parameters of different type and different number of parameters.我在这里保持简单,但我需要具有不同类型和不同参数数量的参数的案例 class。 So i can't have them declared in the abstract class Person.
所以我不能在抽象的 class Person 中声明它们。 Which would give
哪个会给
value uid is not a member of Person
value firstname is not a member of Person
value active is not a member of Playground.Person
How can one make different case class with different parameters in types and numbers evaluate to the same type?如何使具有不同类型和数字参数的不同案例 class 评估为相同类型?
AND/OR和/或
How can one make aa callback polymorphic, kind a like this如何使 aa 回调多态,像这样
def outerFunction(person: Person, innerFunction: (SomeCaseClass) => List[String]): List[String] = {
innerFunction(person)
}
Hope this is clear enough.希望这足够清楚。
Thanks for reading, Have a good One.感谢阅读,祝您生活愉快。
User
and Admin
are subtypes of Person
but User => List[String]
and Admin => List[String]
are not subtypes of Person => List[String]
. User
和Admin
是Person
的子类型,但User => List[String]
和Admin => List[String]
不是Person => List[String]
的子类型。 User => List[String]
and Admin => List[String]
are actually supertypes of Person => List[String]
. User => List[String]
和Admin => List[String]
实际上是Person => List[String]
的超类型。 Function type A => B
is covariant with respect to B
but contravariant with respect to A
. Function 类型
A => B
相对于B
是协变的,但相对于A
是逆变的。
Try to make outerFunction
generic尝试使
outerFunction
泛型
def outerFunction[P <: Person](person: P, innerFunction: P => List[String]): List[String] =
innerFunction(person)
outerFunction(myUser, innerFunctionForUser) //List(0c60c5b4-306d-4372-b60d-fd699c80e408, joe, false)
outerFunction(myAdmin, innerFunctionForAdmin) //List(178789, jack, high)
You can also try to replace functions innerFunctionForUser
, innerFunctionForAdmin
with type class您也可以尝试用类型 class替换函数
innerFunctionForUser
, innerFunctionForAdmin
trait InnerFunction[P <: Person] {
def apply(person: P): List[String]
}
object InnerFunction {
implicit val forUser: InnerFunction[User] =
user => List(user.uid, user.firstname, user.active)
implicit val forAdmin: InnerFunction[Admin] =
admin => List(admin.id, admin.pseudo, admin.securityClearance)
}
def outerFunction[P <: Person](person: P)(implicit innerFunction: InnerFunction[P]): List[String] =
innerFunction(person)
outerFunction(myUser) //List(0c60c5b4-306d-4372-b60d-fd699c80e408, joe, false)
outerFunction(myAdmin) //List(178789, jack, high)
Since type class InnerFunction
acts on different data types now similarly (it produces list of values for all fields of a case class) you can even derive it:由于类型 class
InnerFunction
现在类似地作用于不同的数据类型(它为案例类的所有字段生成值列表),您甚至可以派生它:
trait InnerFunction[T] {
def apply(t: T): List[String]
}
object InnerFunction {
implicit def mkInnerFunction[T <: Product]: InnerFunction[T] =
_.productIterator.map(_.asInstanceOf[String]).toList
}
def outerFunction[T](t: T)(implicit innerFunction: InnerFunction[T]): List[String] =
innerFunction(t)
//or simply
// def outerFunction[T <: Product](t: T): List[String] =
// t.productIterator.map(_.asInstanceOf[String]).toList
//or
// def outerFunction(t: Product): List[String] =
// t.productIterator.map(_.asInstanceOf[String]).toList
outerFunction(myUser) //List(0c60c5b4-306d-4372-b60d-fd699c80e408, joe, false)
outerFunction(myAdmin) //List(178789, jack, high)
(this will fail at runtime if not all fields of T
are String
s) or (如果不是
T
的所有字段都是String
,这将在运行时失败)或
import shapeless.ops.hlist.ToList
import shapeless.{Generic, HList}
trait InnerFunction[T] {
def apply(t: T): List[String]
}
object InnerFunction {
implicit def mkInnerFunction[T <: Product, L <: HList](implicit
generic: Generic.Aux[T, L],
toList: ToList[L, String]
): InnerFunction[T] = generic.to(_).toList
}
def outerFunction[T](t: T)(implicit innerFunction: InnerFunction[T]): List[String] =
innerFunction(t)
//or simply
// def outerFunction[T, L <: HList](t: T)(implicit
// generic: Generic.Aux[T, L],
// toList: ToList[L, String]
// ): List[String] = generic.to(t).toList
outerFunction(myUser) //List(0c60c5b4-306d-4372-b60d-fd699c80e408, joe, false)
outerFunction(myAdmin) //List(178789, jack, high)
(this will guarantee at compile time that all fields of T
are String
s). (这将保证在编译时
T
的所有字段都是String
)。
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