如何解决有界泛型的隐式查找? [英] How to resolve implicit lookup by bounded generic?

问题描述

我有一系列的Foo课:

I have a series of class Foo:

trait Foo
class Foo1 extends Foo
class Foo2 extends Foo
//...

并且我有一个类型类和所有Foos的实例:

and I have a type class and instances for all of the Foos:

trait CanBar[T] { def bar: Unit }
implicit val foo1: CanBar[Foo1] = null
implicit val foo2: CanBar[Foo2] = null

然后我尝试从方法中获取类型类实例:

and I try to get the type class instance from a method:

def bar[T <: Foo](foo: T) = {
  val canBar = implicitly[CanBar[T]]
  //...
}

即使我导入了所有 CanBar [Foo] 实例，编译器仍会抱怨未找到参数e:CanBar [T] 的隐式变量.

The compiler complains No implicits found for parameter e: CanBar[T], even though I imported all the CanBar[Foo] instances.

我的假设是编译器正在寻找T(它是Any或Foo)而没有找到任何T.我是正确的，在这种情况下(没有宏)如何使它工作

My assumption is that the compiler is looking for T (which is Any or Foo) and did not find any. Am I correct and how can I make it work in this case (without macros)

推荐答案

即使我导入了所有 CanBar [Foo] 实例，编译器仍会抱怨未找到参数e:CanBar [T] 的隐式变量.

The compiler complains No implicits found for parameter e: CanBar[T], even though I imported all the CanBar[Foo] instances.

CanBar [Foo] 不是 CanBar [T] .

添加上下文绑定

def bar[T <: Foo : CanBar](foo: T) = {
  val canBar = implicitly[CanBar[T]]
  //...
}

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