1

您可以这样做以获取到链的隐式转换：

package language

object chainedImplicits {
  implicit def chainImplicits[A, B, C](a: A)(implicit conv1: A => B, conv2: B => C): C = conv2(conv1(a))
}

但这显然不安全。

不过，我看不出这个版本有什么问题：

package language

package chainedImplicits {

  final class Chained[A, B] private[chainedImplicits] (val f: A => B)

  trait Low { this: `package`.type =>
    implicit def startChaining(implicit conv: A => B): Chained[A, B] = new Chained[A, B](conv)
    implicit def chainImplicits[A, B, C](implicit conv1: Chained[A, B], conv2: B => C): Chained[B, C] = new Chained(conv1.f andThen conv2)
  }

}

package object chainedImplicits extends Low {
  implicit def endChain[A, B](a: A)(implicit conv: Chained[A, B]): B = conv.f(a)
}

这里有问题吗？

4

1 回答 1

0

首先，我无法让您的第一个“显然不安全”的示例做任何事情：

import language.implicitConversions

object Test {
  implicit def chain[A, B, C](a: A)(implicit ab: A => B, bc: B => C): C = bc(ab(a))

  case class X()
  case class Y()
  case class Z()

  implicit def xy(x: X) = Y()
  implicit def yz(y: Y) = Z()

  val z: Z = X()
}

...

type mismatch;
  found   : Test.X
  required: Test.Z
   val z: Z = X()
               ^

接下来，第二个示例中明显的“捕获”是它无法编译。你真的试过了吗？无论如何，在“修复”它之后，它仍然不能满足你的要求：

import language.implicitConversions

class Convert[A, B](val convert: A => B) extends AnyVal
trait Convert0 {
  implicit def convert[A, B](implicit ab: A => B): Convert[A, B] = new Convert(ab)
  implicit def convert[A, B, C](implicit ab: Convert[A, B], bc: B => C): Convert[A, C] = new Convert(ab.convert andThen bc)
}
object Convert extends Convert0 {
  implicit def convert[A, B](a: A)(implicit convert: Convert[A, B]): B = convert.convert(a)
}

object Test {
  case class X()
  case class Y()
  case class Z()

  implicit def xy(x: X) = Y()
  implicit def yz(y: Y) = Z()

  val z: Z = X()
}

这给出了相同的类型不匹配错误。据我所知，如果你想隐式转换为链，你必须明确它：

import language.implicitConversions

object test {
  case class X()
  case class Y()
  case class Z()

  // anything convertible to X is convertible to Y
  implicit def xy[A <% X](x: A) = Y()

  // anything convertible to Y is convertible to Z
  implicit def yz[B <% Y](y: B) = Z()

  val z: Z = X()
}
于 2013-04-19T21:54:43.667 回答