问题描述
在Scala中,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?例如:
case class Address(street: String,city: String,state: String,zipCode: String)
case class Person(name: String,age: Int,address: Address)
def a[T: TypeTag](): Unit = {
val fields: Seq[Symbol] = typeOf[T].members.filter(_.isMethod == false).toSeq
fields.foreach(x => {
b[x.getMyType]() // How to pass field's "Type" into generic typed function?
})
}
def b[T](): Unit = ???
a[Person]()
在上面的示例中,我感兴趣于调用a[Person]()
,并且在a()中,使用反射从Person中获取字段,以使用每个字段的类型调用b[?]()
。
解决方法
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?
在编译时必须知道方法T
的类型参数b
,但是x.typeSignature
仅在运行时才知道。
您可以尝试使用compile-time reflection而不是运行时之一。然后x.typeSignature
在宏的运行时即主要代码的编译时就知道了。
// macros subproject
import scala.language.experimental.macros
import scala.reflect.macros.blackbox
def a[T](): Unit = macro aImpl[T]
def aImpl[T: c.WeakTypeTag](c: blackbox.Context)(): c.Tree = {
import c.universe._
val fields: Seq[Symbol] = weakTypeOf[T].members.filter(_.isMethod == false).toSeq
val bCalls = fields.map(x =>
q"b[${x.typeSignature}]()"
)
q"..$bCalls"
}
// main subproject
case class Address(street: String,city: String,state: String,zipCode: String)
case class Person(name: String,age: Int,address: Address)
def b[T](): Unit = ???
a[Person]()
// scalac: {
// b[App.Address]();
// b[Int]();
// b[String]()
//}
类似的事情可以用Shapeless完成。
import shapeless.ops.hlist.{FillWith,Mapper}
import shapeless.{Generic,HList,Poly0,Poly1}
def b[T](): Unit = println("b")
object bPoly extends Poly1 {
implicit def cse[X]: Case.Aux[X,Unit] = at(_ => b[X]())
}
object nullPoly extends Poly0 {
implicit def cse[X]: Case0[X] = at(null.asInstanceOf[X])
}
def a[T] = new PartiallyAppliedA[T]
class PartiallyAppliedA[T] {
def apply[L <: HList]()(implicit
generic: Generic.Aux[T,L],mapper: Mapper[bPoly.type,fillWith: FillWith[nullPoly.type,L]
): Unit = mapper(fillWith())
}
case class Address(street: String,address: Address)
a[Person]()
//b
//b
//b
或者,如果您确实要使用运行时反射,则必须将b[...]()
的编译推迟到运行时。您可以使用toolbox来做到这一点。
import scala.reflect.runtime.currentMirror
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
val toolbox = currentMirror.mkToolBox()
def a[T: TypeTag](): Unit = {
val fields: Seq[Symbol] = typeOf[T].members.filter(_.isMethod == false).toSeq
val bCalls = fields.map(x =>
q"b[${x.typeSignature}]()"
)
toolbox.eval(q"""
import Obj._
..$bCalls
""")
}
object Obj {
def b[T](): Unit = println("b")
}
case class Address(street: String,address: Address)
a[Person]()
//b
//b
//b