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

规范将重载决策视为选择类成员的消歧。但隐式解析使用静态重载决策来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是在X派生的类中定义的，而yyy是：

$ 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隐藏的封闭范围：

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>

Scala 2.12中的隐式ExecutionContext优先级

1 个答案: