Scala：無法從對象中找到隱式

Question

我有以下的scala代碼。我不明白爲什麼這些隱含的東西沒有被編譯器弄明白。我也嘗試在Main中放置導入線。但請注意，隱式對象是內部主要創建時，那麼代碼正確運行

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) 
    } 
} 

我已經導入了必要的隱含從LoggingAddon但仍Scala編譯器說：could not find implicit Action[Dog]

所有我想do做一個可插入的類型類。而不是改變任何一段代碼，僅改變導入語句以具有不同的副作用

Answer 1

只需移動，其中隱式導入使用順序，我在下面的例子中

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) 
    } 
}