scala - scala 伴生对象可以有抽象成员吗?
问题描述
编辑:使问题更清楚
我有一个尝试将 a 解析Map[String, String]
为ReadRequests
and的函数WriteRequests
。我需要MapConvert
从抽象类隐式访问Request
。
// Conversion methods to make conversions from Map[String, String] to ReadRequest and WriteRequest implicit
trait MapConvert[A] {
def convert(values: Map[String, String]): A
}
object Map2ClassHelpers {
implicit class Map2Class(values: Map[String, String]) {
def extract[A](implicit mapper: MapConvert[A]): A = mapper convert values
}
}
// abstract Request class
abstract class Request[R <: Request[R]]() {
def companion: RequestCompanion[R]
}
trait RequestCompanion[R <: Request[R]] {
implicit val m: MapConvert[R]
}
// ReadRequest and companion object
case class ReadRequest(source: String, data: String)
extends Request[ReadRequest] {
override def companion: RequestCompanion[ReadRequest] = ReadRequest
}
object ReadRequest extends RequestCompanion[ReadRequest] {
implicit val m = new MapConvert[ReadRequest] {
def convert(values: Map[String, String]) = ReadRequest(
source = values("source"),
data = values("data")
)
}
}
// WriteRequest and companion object
case class WriteRequest(destination: String, data: String)
extends Request[WriteRequest] {
override def companion: RequestCompanion[WriteRequest] = WriteRequest
}
object WriteRequest extends RequestCompanion[WriteRequest] {
implicit val m = new MapConvert[WriteRequest] {
def convert(values: Map[String, String]) = WriteRequest(
destination = values("destination"),
data = values("data")
)
}
}
case class Spec(requestType: String, args: Map[String, String])
def fromMapToRequest[R <: Request[R]](spec: Spec)(implicit mR: Manifest[R]): R = {
/* this extract method comes from Map2ClassHelpers
* the result should be either a ReadRequest or a WriteRequest
* the error happens at this call saying "could not find implicit value for parameter mapper
* my guess is because it can't implicitly find the correct MapConvert from the abstract Request class
*/
// a bunch of checks ...
spec.args.extract[R]
}
def runExample() = {
val spec1: Spec = Spec("read", Map("source" -> "/readPath", "data" -> "ABC"))
val spec2: Spec = Spec("write", Map("destination" -> "/writePath", "data" -> "ABC"))
val specs = Seq(spec1, spec2)
specs.map(spec => {
spec.requestType match {
case "read" => fromMapToRequest[ReadRequest](spec)
case "write" => fromMapToRequest[WriteRequest](spec)
}
})
}
当我将其更改为:
def fromMapToRequest[R <: Request[R]](spec: Spec)(eval: spec => Request)(implicit mR: Manifest[R]): R = {
// a bunch of checks ...
import Map2ClassHelpers._
eval(spec)
}
def runExample() = {
val spec1: Spec = Spec("read", Map("source" -> "/readPath", "data" -> "ABC"))
val spec2: Spec = Spec("write", Map("destination" -> "/writePath", "data" -> "ABC"))
val specs = Seq(spec1, spec2)
import Map2ClassHelpers._
specs.map(spec => {
// this part is kind of ugly compared to the first way
spec.requestType match {
case "read" => fromMapToRequest[ReadRequest](spec)(_.args.extract[ReadRequest])
case "write" => fromMapToRequest[WriteRequest](spec)(_.args.extract[WriteRequest])
}
})
}
但我想要更通用的第一个选项,因此只需在 fromMapToRequest 中指定一次类型。
解决方案
(可能由于问题编辑而过时)
为伴生对象定义一个特征:
trait FoobarCompanion[X <: Foobar[X]] {
implicit val m: Converter[X]
}
让伴生对象扩展它:
object Foo extends FoobarCompanion[Foo] {
implicit val m = Converter[Foo] { ... }
}
object Bar extends FoobarCompanion[Bar] {
implicit val m = Converter[Bar] { ... }
}
companion
为自身添加一个方法Foobar
:
abstract class Foobar[F <: Foobar[F]] {
def companion: FoobarCompanion[F]
}
现在,只要您Foobar
通过companion
.
您可能想看看GenericCompanion周围的代码是如何组织的。
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