Scala:从通用类型到第二通用类型的隐式转换

Question

假设我有两组类,第一组继承自Foo,第二组继承自Bar.

class Foo
class Baz extends Foo
class Bar
class Qux extends Bar

我想创建一个通用的隐式转换函数,将任何Foo转换为Bar,因为范围内有隐式转换器类型.

trait Converter[A <: Foo, B <: Bar] {
  def convert(a : A) : B
}

implicit object BazToQuxConverter extends Converter[Baz, Qux] {
  def convert(a : Baz) : Qux = new Qux
}

implicit def FooToBar[A <: Foo, B <: Bar](a : A)(implicit converter : Converter[A, B]) : B = converter.convert(a)

不幸的是,这似乎不像我期望的那样有效.当我将以下行插入REPL时:

val a : Baz = new Baz
val b : Qux = a

...我收到以下错误:

<console>:17: error: type mismatch;
 found   : Baz
 required: Qux
       val b : Qux = a
                     ^

有没有办法让这个工作？我能够得到的最接近的是:

implicit def BadFooToBar[A <: Foo, B <: Bar](a : A)(implicit converter : Converter[A, _]) : B = converter.convert(a).asInstanceOf[B]

这适用于我之前的示例,但它不是非常类型安全的.

class Qax extends Bar
val a : Baz = new Baz
val b : Qax = a

这将编译得很好,但它会在运行时爆炸,因为Qux(结果converter.convert(a))不能转换为Qax(asInstanceOf[Qax]).理想情况下,我想要它,以便在编译时捕获上面的行,因为没有任何Converter[Bax,Qax]范围.

Answer 1

这是一个在2.11中修复的错误.看起来自PR 2822以来它已被修复,相关票据是SI-3346.

Welcome to Scala version 2.11.0-20131030-090728-c38235fd44 (OpenJDK 64-Bit Server VM, Java 1.7.0_45).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :paste
// Entering paste mode (ctrl-D to finish)

  class Foo
  class Baz extends Foo
  class Bar
  class Qux extends Bar

  trait Converter[A <: Foo, B <: Bar] {
    def convert(a : A) : B
  }

  implicit object BazToQuxConverter extends Converter[Baz, Qux] {
    def convert(a : Baz) : Qux = new Qux
  }

  import scala.language.implicitConversions
  implicit def FooToBar[A <: Foo, B <: Bar](a : A)(implicit converter : Converter[A, B]) : B = converter.convert(a)

  val a : Baz = new Baz
  val b : Qux = a

// Exiting paste mode, now interpreting.

defined class Foo
defined class Baz
defined class Bar
defined class Qux
defined trait Converter
defined object BazToQuxConverter
import scala.language.implicitConversions
FooToBar: [A <: Foo, B <: Bar](a: A)(implicit converter: Converter[A,B])B
a: Baz = Baz@4f4db2ac
b: Qux = Qux@760d62e0