当typeclass不在专用的源文件中时，为什么Scala不能在同伴对象中隐式定义我的typeclass实例? [英] Why can't Scala find my typeclass instance defined implicitly in the companion object, when the typeclass is not in a dedicated source file?

问题描述

请参考下面的源代码.所有源代码都在同一包中定义.当我在单个源文件ShowMain.scala中定义所有代码时，会出现编译错误，但是，当object ShowMain在ShowMain.scala中定义并且trait Show和object Show在Show.scala中定义时，没有编译错误.

Please refer to the source code below. All source code is defined in the same package. When I define all of the code within a single source file ShowMain.scala, I get a compile error, however when object ShowMain is defined in ShowMain.scala and trait Show and object Show are defined in Show.scala, there is no compile error.

我的问题:原因是什么?我违反了什么语言规则?

My question: What is the cause for this? What language rule have I run afoul of?

示例代码:

object ShowMain {

  def main(args: Array[String]): Unit = {
    output("hello")
  }

  def output[A](a: A)(implicit show: Show[A]) =
    println(show.show(a))

}

trait Show[-A] {
  def show(a: A): String
}

object Show {

  implicit object StringShow extends Show[String] {
    def show(s: String) = s"[String: $s]"
  }

}

编译错误:

(包含output("hello")的行上的ScalaIDE/Scala 2.11.2)

(ScalaIDE/Scala 2.11.2 on line containing output("hello"))

Multiple markers at this line
    - not enough arguments for method output: (implicit show: test.typeclasses.show1.Show[String])Unit. Unspecified value 
     parameter show.
    - not enough arguments for method output: (implicit show: test.typeclasses.show1.Show[String])Unit. Unspecified value 
     parameter show.
    - could not find implicit value for parameter show: test.typeclasses.show1.Show[String]
    - could not find implicit value for parameter show: test.typeclasses.show1.Show[String]

推荐答案

有一个规则，必须在编译单元中较早地定义隐式.

There's a rule that implicit has to be defined earlier in the compilation unit.

因此，将对象Show移到顶部并编译.

So, move object Show to the top and it compiles.

或者，

object Show {
  //implicit object StringShow extends Show[String] {
  implicit val x: Show[String] = new Show[String] {
    def show(s: String) = s"[String: $s]"
  }
}

请参阅有关类型的说明:

see the explanation about the type:

https://stackoverflow.com/a/2731285/1296806

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