找不到类型类的隐式方法 [英] Implicit method for typeclass not found

问题描述

这是为类型类发散隐式扩展的延续.我想出了另一个仍然无法编译的版本，但现在出于另一个原因，因此是一个不同的问题.这是我所做的:

This is continuation of Diverging implicit expansion for type class. I've came up with another version that still doesn't compile, but now for another reason, hence a different question. Here's what I did:

我基本上添加了一个新的类型类 NestedParser，用于某些 case 类包含其他 case 类的情况.

I've basically added a new typeclass NestedParser for the cases where some case class consists of other case classes.

trait Parser[A] {
  def apply(s: String): Option[A]
}
trait NestedParser[A] {
  def apply(s: String): Option[A]
}

object Parser {
  def apply[A](s: String)(implicit parser: Parser[A]): Option[A] = parser(s)
}
object NestedParser {
  def apply[A](s: String)(implicit parser: NestedParser[A]): Option[A] = parser(s)
}

我已将之前的隐式函数更改为返回 NestedParser 以避免发散隐式扩展.否则和以前一样:

I've changed the previous implicit function to return the NestedParser to avoid diverging implicit expansions. Otherwise it's same as before:

implicit def nestedCaseClassParser[A, B, C]
  (
    implicit pb: Parser[B],
    pc: Parser[C],
    gen: Generic.Aux[A, B :: C :: HNil]
  ): NestedParser[A] = new NestedParser[A] {
    override def apply(s: String): Option[A] = {
      val tmp = s.span(_ != '|') match {
        case (h, t) =>
          for {
            a <- pb(h)
            b <- pc(t.substring(1))
          } yield a :: b :: HNil
      }
      tmp.map(gen.from)
    }
  }

案例类与以前相同:

case class Person(name: String, age: Int)
case class Family(husband: Person, wife: Person)

现在，当我尝试解析 Family 时，出现以下编译错误:

Now when I try to parse the Family, I get the following compile error:

scala> NestedParser[Family]("")
<console>:32: error: could not find implicit value for parameter parser: NestedParser[Family]
       NestedParser[Family]("")

然而，这对我来说没有意义.上面的函数清楚地提供了 NestedParser 的隐式实例.为什么它不满足编译器?

However, it doesn't make sense to me. The function above clearly provides the implicit instance of NestedParser. Why doesn't it satisfy the compiler?

推荐答案

嗯，据我所知，你没有提供任何 nestedCaseClassParser 需要的隐式 Parser 实例.

Well, as far as I can see you are not providing any implicit Parser instances which nestedCaseClassParser requires.

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