如何从Scala中的通用类型中获取字段名称和字段类型？

Question

在Scala中，给定一个泛型类型T，如何检索字段名称和字段类型的列表？例如，如果我有以下case类：

case class Person(name: String, age: Int, gender: Boolean)

并且这是通用函数：

def getFieldNamesAndTypes[T](): Seq[(String, String)]

我希望能够按照字段出现的顺序检索字段（名称、类型）的序列：

val fieldNamesAndTypes = getFieldNamesAndTypes[Person]()

- code

---

看起来像X/Y。您认为获取这样的信息的目标是实施解决方案的一种方式（以及哪种解决方案）？顺便说一句，运行时反射只应在边缘情况下考虑，因为它会破坏所有类型安全性。 - cchantep

我看到你最近发布了一些关于类和类型信息的问题。我建议你学习shapeless/magnolia库。所有你的问题都会得到解答。 - Artem Sokolov

Answer 1

似乎您需要使用反射API：

Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 1.8.0_222).
Type in expressions for evaluation. Or try :help.

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> def getFields[T: TypeTag] = typeOf[T].members.collect {
     |   case m: MethodSymbol if m.isCaseAccessor => (m.name.toString, m.returnType.toString)
     | }
getFields: [T](implicit evidence$1: reflect.runtime.universe.TypeTag[T])Iterable[(String, String)]

scala> case class Person(name: String, age: Int, gender: Boolean)
defined class Person

scala> getFields[Person]
res0: Iterable[(String, String)] = List((gender,Boolean), (age,Int), (name,String))

- esse

Answer 2

使用Shapeless可以在编译时实现此功能（即在幕后使用编译时反射）

import shapeless.labelled.FieldType
import shapeless.ops.hlist.{FillWith, Mapper, ToList}
import shapeless.{HList, LabelledGeneric, Poly0, Poly1, Typeable, Witness}

object fieldTypePoly extends Poly1 {
  implicit def cse[K <: Symbol, V](implicit
    witness: Witness.Aux[K],
    typeable: Typeable[V]
  ): Case.Aux[FieldType[K, V], (String, String)] =
    at(_ => witness.value.name -> typeable.describe)
}

object nullPoly extends Poly0 {
  implicit def cse[A]: Case0[A] = at(null.asInstanceOf[A])
}

def getFieldNamesAndTypes[T] = new PartiallyApplied[T]

class PartiallyApplied[T] {
  def apply[L <: HList, L1 <: HList]()(implicit
    generic: LabelledGeneric.Aux[T, L],
    mapper: Mapper.Aux[fieldTypePoly.type, L, L1],
    fillWith: FillWith[nullPoly.type, L],
    toList: ToList[L1, (String, String)]
  ): Seq[(String, String)] = toList(mapper(fillWith()))
}

case class Person(name: String, age: Int, gender: Boolean)

getFieldNamesAndTypes[Person]() // List((name,String), (age,Int), (gender,Boolean))

- Dmytro Mitin

---

https://stackoverflow.com/questions/55504430/get-case-class-fields-name-and-type-with-shapeless/ - Dmytro Mitin