如何修复这个类型类示例? [英] How to fix this typeclass example?

问题描述

这是我之前的的后续问题:

假设我创建了以下测试 converter.scala:

Suppose I create the following test converter.scala:

trait ConverterTo[T] {
  def convert(s: String): Option[T]
}

object Converters {
  implicit val toInt: ConverterTo[Int] =
    new ConverterTo[Int] { 
      def convert(s: String) = scala.util.Try(s.toInt).toOption
    }
}

class A {
  import Converters._
  def foo[T](s: String)(implicit ct: ConverterTo[T]) = ct.convert(s)
}

现在，当我尝试在 REPL 中调用 foo 时，它无法编译:

Now when I tried to call foo in REPL it fails to compile:

scala> :load converter.scala
Loading converter.scala...
defined trait ConverterTo
defined module Converters
defined class A

scala> val a = new A()

scala> a.foo[Int]("0")
<console>:12: error: could not find implicit value for parameter ct: ConverterTo[Int]
          a.foo[Int]("0")
                    ^

推荐答案

import Converters._ in class A 没有删减.您可以删除它，代码仍将编译.编译器需要在实际隐式中找到的时刻不在 class A 中，其中 foo 刚刚声明.

import Converters._ in class A does not cut it. You can remove it and the code will still compile. The moment the compiler needs to find in actual implicit is not in class A, where foo is just declared.

在您调用 REPL 中的 a.foo[Int](..) 时，编译器需要在隐式作用域中找到一个 ConverterTo[Int].所以这就是需要导入的地方.

The compiler needs to find a ConverterTo[Int] in implicit scope at the moment you call a.foo[Int](..) that is in the REPL. So this is where the import needs to be.

如果 object Converters 和 trait ConverterTo 被命名为相同的(所以会有一个伴随对象)，将不需要导入.

Had object Converters and trait ConverterTo been named the same (so there would be a companion object) the import would not be needed.

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