隐式参数和 ClassTag [英] Implicit parameter and ClassTag

问题描述

谁能用下面的错误消息解释 Scala 编译器试图告诉我什么?

Can someone explain what the Scala compiler is trying to tell me with the error message below?

object Some {
  def apply[T: ClassTag](data: T)(implicit ordering: Ordering[T]): T = data
}
object Other {
  def apply[T: ClassTag](data: T)(implicit ordering: Ordering[T]): T =
    Some(data)(ordering.reverse)
}

编译器说:

not enough arguments for method apply: (implicit evidence$2: scala.reflect.ClassTag[T], implicit ordering: Ordering[T])T in object Some. Unspecified value parameter ordering.

当我将 ClassTag 添加到对象 Some 中的方法时出现错误，以便在那里使用一些内部数组.最初，代码是(并且编译时没有错误):

The error arose when I added the ClassTag to the method in object Some, in order to use some internal arrays there. Initially, the code was (and compiled without errors):

object Some {
  def apply[T](data: T)(implicit ordering: Ordering[T]): T = data
}
object Other {
  def apply[T: ClassTag](data: T)(implicit ordering: Ordering[T]): T =
    Some(data)(ordering.reverse)
}

我知道 ClassTag 添加了一个隐式的类型信息来克服擦除，但我不明白这与我的排序隐式参数有什么关系，或者为什么编译器突然认为排序没有价值...

I know ClassTag adds an implicit with type information to overcome erasure, but I don't understand what that has to do with my ordering implicit parameters, or why the compiler suddenly thinks that ordering has no value...

推荐答案

这个:

def apply[T: ClassTag](data: T)(implicit ordering: Ordering[T]): T = data

是用于此的语法糖:

def apply[T](data: T)(implicit evidence: ClassTag[T], ordering: Ordering[T]): T = data

当您明确指定隐式参数时，您必须同时提供它们.您可以使用 implicitly 来继承隐式 ClassTag:

When you explicitly specify the implicit parameters you must provide both of them. You can use implicitly to carry over the implicit ClassTag:

object Other { 
  def apply[T: ClassTag](data: T)(implicit ordering: Ordering[T]): T =
    Some(data)(implicitly, ordering.reverse)
}

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