7

在 Scala 2.12 中导入global执行上下文，然后在作用域中定义另一个隐式执行上下文会导致模棱两可的隐式，而在 2.11 中它工作得很好。

import scala.concurrent._
import scala.concurrent.ExecutionContext.Implicits.global

class A(implicit ec: ExecutionContext) {
  x = implicitly[ExecutionContext]
}

编译器给出错误：

error: ambiguous implicit values:
 both lazy value global in object Implicits of type => scala.concurrent.ExecutionContext
 and value ec in class A of type scala.concurrent.ExecutionContext
 match expected type scala.concurrent.ExecutionContext
       val x = implicitly[ExecutionContext]
                         ^

这是什么原因以及如何在代码中解决它？

4

1 回答 1

2

该规范将重载解析视为对类成员选择的消歧。但是隐式解析使用静态重载解析在不是成员的引用之间进行选择。

可以说，以下是对规范的误解，因为zzz它是在派生自Xmuch as yyyis 的类中定义的：

$ scala
Welcome to Scala 2.12.0 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_101).
Type in expressions for evaluation. Or try :help.

scala> import concurrent._, ExecutionContext.global
import concurrent._
import ExecutionContext.global

scala> trait X { implicit val xxx: ExecutionContext = global }
defined trait X

scala> class Y extends X { implicit val yyy: ExecutionContext = global ; def f = implicitly[ExecutionContext] }
defined class Y

scala> class Z extends X { def f(implicit zzz: ExecutionContext) = implicitly[ExecutionContext] }
<console>:16: error: ambiguous implicit values:
 both value xxx in trait X of type => scala.concurrent.ExecutionContext
 and value zzz of type scala.concurrent.ExecutionContext
 match expected type scala.concurrent.ExecutionContext
       class Z extends X { def f(implicit zzz: ExecutionContext) = implicitly[ExecutionContext] }
                                                                             ^

目前，您必须依靠命名来隐藏封闭范围的隐含：

scala> class Z extends X { def f(implicit xxx: ExecutionContext) = implicitly[ExecutionContext] }
defined class Z

或者，

scala> :pa
// Entering paste mode (ctrl-D to finish)

package object p { import concurrent._ ; implicit val xxx: ExecutionContext = ExecutionContext.global }
package p { import concurrent._ ;
  class P { def f(implicit xxx: ExecutionContext) = implicitly[ExecutionContext]
            def g = implicitly[ExecutionContext] }
}

// Exiting paste mode, now interpreting.


scala>
于 2016-11-05T21:45:07.427 回答