Scala 2.12中的隐式ExecutionContext优先级

Question

在Scala 2.12中导入global执行上下文然后在作用域中定义了另一个隐式执行上下文导致模糊隐式,而在2.11中它可以正常工作.

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？

Answer 1

该规范将重载解析视为对类成员选择的歧义消除。但隐式解析使用静态重载解析来在非成员引用之间进行选择。

可以说，以下是对规范的误解，因为是在派生自以下zzz内容的类中定义的：Xyyy

$ 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中的隐式内容：

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.


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