Scala:无法从对象中找到隐式内容

Question

我有以下scala代码。我不明白为什么编译器没有找到隐式。我还尝试将导入行放在 Main 中。但请注意，当在 Main 中创建隐式对象时，代码会正确运行

import LoggingAddon._
object Main {
def main(args: Array[String]): Unit = {
val dog = new Dog
Util.act(dog)
}
}
class Dog {
def bark(): Unit = {
println("woof")
}
}
trait Action[A] {
def action(x: A): Unit
}
trait WithoutLogging[A] extends Action[A] {
}
trait WithLogging[A] extends Action[A] {
}
object LoggingAddon {
implicit object DogWithLogging extends WithLogging[Dog] {
override def action(x: Dog): Unit = {
println("before")
x.bark()
print("after")
}
}
}
object NoLoggingAddion {
implicit object DogWithoutLogging extends WithoutLogging[Dog] {
override def action(x: Dog): Unit = {
x.bark()
}
}
}
object Util {
def act(x: Dog)(implicit nolog: Action[Dog]): Unit = {
nolog.action(x)
}
}

我已经从 LoggingAddon 导入了必要的隐式，但 scala 编译器仍然说could not find implicit Action[Dog]

我要做的就是做一个可插拔的类型类。与其更改任何代码段，不如仅更改导入语句以产生不同的副作用

Answer 1

只需移动导入隐式的使用顺序，我在以下示例中移至底部

class Dog {
def bark(): Unit = {
println("woof")
}
}
trait Action[A] {
def action(x: A): Unit
}
trait WithoutLogging[A] extends Action[A] {
}
trait WithLogging[A] extends Action[A] {
}
object LoggingAddon {
implicit object DogWithLogging extends WithLogging[Dog] {
override def action(x: Dog): Unit = {
println("before")
x.bark()
print("after")
}
}
}
object NoLoggingAddion {
implicit object DogWithoutLogging extends WithoutLogging[Dog] {
override def action(x: Dog): Unit = {
x.bark()
}
}
}
object Util {
def act(x: Dog)(implicit nolog: Action[Dog]): Unit = {
nolog.action(x)
}
}
import LoggingAddon._
object Main {
def main(args: Array[String]): Unit = {
val dog = new Dog
Util.act(dog)
}
}