集合的 Scala 类型类派生

Question

我想编写一个类型类来静态编码我可以输出的所有内容到Printer：

  import java.io.PrintWriter

  trait Write[A] {
    def apply(out: PrintWriter)(x: A): Unit
  }
  object Write {
    def apply[A] = new Write[A] {
      def apply(out: PrintWriter)(x: A) = out.print(x)
    }
    implicit val string     : Write[String]     = Write[String]
    implicit val char       : Write[Char]       = Write[Char]
    implicit val boolean    : Write[Boolean]    = Write[Boolean]
    implicit val int        : Write[Int]        = Write[Int]
    implicit val long       : Write[Long]       = Write[Long]
    implicit val bigInt     : Write[BigInt]     = Write[BigInt]
    implicit val double     : Write[Double]     = Write[Double]
    implicit val bigDecimal : Write[BigDecimal] = Write[BigDecimal]    
  }

现在我可以这样写了：

def output[A(out: PrintWriter, x: A)(implicit write: Write[A]) = write(out)(x)

output(out, "hello") // compiles
output(out, 1) //compiles
output(out, Set.empty[String]) // does not compile

但是，现在我想为所有Iterables 写一个Write：

   implicit def iterable[A](implicit write: Write[A]) = new Write[Iterable[A]] {
      def apply(out: PrintWriter)(xs: Iterable[A]) = {
        xs.foreach(write(out))
        out.println()
      }
    }

但是，我还是编译失败：

output(out, Set.empty[String]) // should compile!!

理想情况下，我也应该能够派生嵌套的可迭代对象，例如：

output(out, Array.ofDim[Int](100, 100)) // should compile!!

我该怎么办？我查看了CanBuild，但这在构建集合时很有用；在这种情况下，我正在解构一个集合。

Answer 1

你可以像这样在 trait Write 中声明类型参数 A 是逆变的：

trait Write[-A] {
  def apply(out: PrintWriter)(x: A): Unit
}

然后应该编译