如何通过使用符号或类型对象将内容传递给泛型类型的函数？

huangapple
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英文:

How to pass into generic type using a Symbol or Type object into a generic typed function?

问题

在Scala中，是否可以将从Symbol或Type对象派生的类型传递给一个泛型函数？例如：

  1. case class Address(street: String, city: String, state: String, zipCode: String)
  2. case class Person(name: String, age: Int, address: Address)
  4. def a[T: TypeTag](): Unit = {
  5.     val fields: Seq[Symbol] = typeOf[T].members.filter(_.isMethod == false).toSeq
  6.     fields.foreach(x => {
  7.        b[x.getMyType]() // 如何将字段的"Type"传递给泛型类型的函数？
  8.     })
  9. }
  11. def b[T](): Unit = ???
  13. a[Person]()

从上面的示例中，我希望在调用a[Person]()时，在a()内部使用反射获取Person中的字段，然后使用每个字段的类型调用b[?]()。

英文:

In Scala, is it possible to pass a type derived from a Symbol or Type object into a generic typed function? For example:

  1. case class Address(street: String, city: String, state: String, zipCode: String)
  2. case class Person(name: String, age: Int, address: Address)
  4. def a[T: TypeTag](): Unit = {
  5.     val fields: Seq[Symbol] = typeOf[T].members.filter(_.isMethod == false).toSeq
  6.     fields.foreach(x => {
  7.        b[x.getMyType]() // How to pass field's "Type" into generic typed function?
  8.     })
  9. }
  11. def b[T](): Unit = ???
  13. a[Person]()

From the above example, I am interested in calling a[Person]() and within a(), use reflection to obtain the fields from Person to make calls into b[?]() using each field's type.

答案1

得分: 3

以下是您要翻译的内容：

是否可以将从Symbol或Type对象派生的类型传递给泛型类型的函数？

方法b的类型参数T必须在编译时知道，但x.typeSignature只在运行时知道。

您可以尝试使用编译时反射而不是运行时反射。然后，x.typeSignature将在宏的运行时（主要代码的编译时）变为已知。

  1. // 宏子项目
  3. import scala.language.experimental.macros
  4. import scala.reflect.macros.blackbox
  6. def a[T](): Unit = macro aImpl[T]
  8. def aImpl[T: c.WeakTypeTag](c: blackbox.Context)(): c.Tree = {
  9.   import c.universe._
  10.   val fields: Seq[Symbol] = weakTypeOf[T].members.filter(_.isMethod == false).toSeq
  11.   val bCalls = fields.map(x =>
  12.     q"b[${x.typeSignature}]()"
  13.   )
  14.   q"..$bCalls"
  15. }
  17. // 主要子项目
  19. case class Address(street: String, city: String, state: String, zipCode: String)
  20. case class Person(name: String, age: Int, address: Address)
  22. def b[T](): Unit = ???
  24. a[Person]()
  26. // scalac: {
  27. //  b[App.Address]();
  28. //  b[Int]();
  29. //  b[String]()
  30. //}

类似的事情也可以使用Shapeless完成。

  1. import shapeless.ops.hlist.{FillWith, Mapper}
  2. import shapeless.{Generic, HList, Poly0, Poly1}
  4. def b[T](): Unit = println("b")
  6. object bPoly extends Poly1 {
  7.   implicit def cse[X]: Case.Aux[X, Unit] = at(_ => b[X]())
  8. }
  10. object nullPoly extends Poly0 {
  11.   implicit def cse[X]: Case0[X] = at(null.asInstanceOf[X])
  12. }
  14. def a[T] = new PartiallyAppliedA[T]
  16. class PartiallyAppliedA[T] {
  17.   def apply[L <: HList]()(implicit
  18.     generic: Generic.Aux[T, L],
  19.     mapper: Mapper[bPoly.type, L],
  20.     fillWith: FillWith[nullPoly.type, L]
  21.   ): Unit = mapper(fillWith())
  22. }
  24. case class Address(street: String, city: String, state: String, zipCode: String)
  25. case class Person(name: String, age: Int, address: Address)
  27. a[Person]()
  29. //b
  30. //b
  31. //b

或者，如果您确实想使用运行时反射，您必须推迟b[...]()的编译直到运行时。您可以使用toolbox实现这一点。

  1. import scala.reflect.runtime.currentMirror
  2. import scala.reflect.runtime.universe._
  3. import scala.tools.reflect.ToolBox
  5. val toolbox = currentMirror.mkToolBox()
  7. def a[T: TypeTag](): Unit = {
  8.   val fields: Seq[Symbol] = typeOf[T].members.filter(_.isMethod == false).toSeq
  9.   val bCalls = fields.map(x =>
  10.     q"b[${x.typeSignature}]()"
  11.   )
  12.   toolbox.eval(q"""
  13.     import Obj._
  14.     ..$bCalls
  15.   """)
  16. }
  18. object Obj {
  19.   def b[T](): Unit = println("b")
  20. }
  22. case class Address(street: String, city: String, state: String, zipCode: String)
  23. case class Person(name: String, age: Int, address: Address)
  25. a[Person]()
  27. //b
  28. //b
  29. //b
英文:

> is it possible to pass a type derived from a Symbol or Type object into a generic typed function?

Type parameter T of method b must be known at compile time but x.typeSignature becomes known only at runtime.

You can try to use compile-time reflection rather than runtime one. Then x.typeSignature becomes known at runtime of the macro, which is compile time of the main code.

  1. // macros subproject
  3. import scala.language.experimental.macros
  4. import scala.reflect.macros.blackbox
  6. def a[T](): Unit = macro aImpl[T]
  8. def aImpl[T: c.WeakTypeTag](c: blackbox.Context)(): c.Tree = {
  9.   import c.universe._
  10.   val fields: Seq[Symbol] = weakTypeOf[T].members.filter(_.isMethod == false).toSeq
  11.   val bCalls = fields.map(x =&gt; 
  12.     q&quot;b[${x.typeSignature}]()&quot;
  13.   )
  14.   q&quot;..$bCalls&quot;
  15. }
  17. // main subproject
  19. case class Address(street: String, city: String, state: String, zipCode: String)
  20. case class Person(name: String, age: Int, address: Address)
  22. def b[T](): Unit = ???
  24. a[Person]()
  26. // scalac: {
  27. //  b[App.Address]();
  28. //  b[Int]();
  29. //  b[String]()
  30. //}

Similar thing can be done with Shapeless.

  1. import shapeless.ops.hlist.{FillWith, Mapper}
  2. import shapeless.{Generic, HList, Poly0, Poly1}
  4. def b[T](): Unit = println(&quot;b&quot;)
  6. object bPoly extends Poly1 {
  7.   implicit def cse[X]: Case.Aux[X, Unit] = at(_ =&gt; b[X]())
  8. }
  10. object nullPoly extends Poly0 {
  11.   implicit def cse[X]: Case0[X] = at(null.asInstanceOf[X])
  12. }
  14. def a[T] = new PartiallyAppliedA[T]
  16. class PartiallyAppliedA[T] {
  17.   def apply[L &lt;: HList]()(implicit
  18.     generic: Generic.Aux[T, L],
  19.     mapper: Mapper[bPoly.type, L],
  20.     fillWith: FillWith[nullPoly.type, L]
  21.   ): Unit = mapper(fillWith())
  22. }
  24. case class Address(street: String, city: String, state: String, zipCode: String)
  25. case class Person(name: String, age: Int, address: Address)
  27. a[Person]()
  29. //b
  30. //b
  31. //b

Alternatively, if you really want to use runtime reflection, you have to postpone compilation of b[...]() till runtime. You can do this with toolbox.

  1. import scala.reflect.runtime.currentMirror
  2. import scala.reflect.runtime.universe._
  3. import scala.tools.reflect.ToolBox
  5. val toolbox = currentMirror.mkToolBox()
  7. def a[T: TypeTag](): Unit = {
  8.   val fields: Seq[Symbol] = typeOf[T].members.filter(_.isMethod == false).toSeq
  9.   val bCalls = fields.map(x =&gt; 
  10.     q&quot;b[${x.typeSignature}]()&quot;
  11.   )
  12.   toolbox.eval(q&quot;&quot;&quot;
  13.     import Obj._
  14.     ..$bCalls
  15.   &quot;&quot;&quot;)
  16. }
  18. object Obj {
  19.   def b[T](): Unit = println(&quot;b&quot;)
  20. }
  22. case class Address(street: String, city: String, state: String, zipCode: String)
  23. case class Person(name: String, age: Int, address: Address)
  25. a[Person]()
  27. //b
  28. //b
  29. //b

huangapple
  • 本文由 发表于 2020年8月14日 09:13:11
  • 转载请务必保留本文链接：https://go.coder-hub.com/63405194.html
  • generics
  • java
  • reflection
  • scala
  • scala-reflect
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