我试图理解Scala对Case Classes的处理方式,使它们免受类型擦除警告的影响。
假设我们有以下简单的类结构。这基本上是一个Either
:
abstract class BlackOrWhite[A, B]
case class Black[A,B]( val left: A ) extends BlackOrWhite[A,B]
case class White[A,B]( val right: B ) extends BlackOrWhite[A,B]
而你正在尝试像这样使用它:
object Main extends App {
def echo[A,B] ( input: BlackOrWhite[A,B] ) = input match {
case Black(left) => println( "Black: " + left )
case White(right) => println( "White: " + right )
}
echo( Black[String, Int]( "String!" ) )
echo( White[String, Int]( 1234 ) )
}
所有东西都能编译并顺利运行。然而,当我尝试自己实现unapply
方法时,编译器会抛出警告。我使用了与上述相同的Main
类这个类结构:
abstract class BlackOrWhite[A, B]
case class Black[A,B]( val left: A ) extends BlackOrWhite[A,B]
object White {
def apply[A,B]( right: B ): White[A,B] = new White[A,B](right)
def unapply[B]( value: White[_,B] ): Option[B] = Some( value.right )
}
class White[A,B]( val right: B ) extends BlackOrWhite[A,B]
使用-unchecked
标志编译会发出以下警告:
[info] Compiling 1 Scala source to target/scala-2.9.1.final/classes...
[warn] src/main/scala/Test.scala:41: non variable type-argument B in type pattern main.scala.White[_, B] is unchecked since it is eliminated by erasure
[warn] case White(right) => println( "White: " + right )
[warn] ^
[warn] one warning found
[info] Running main.scala.Main
现在,我理解了类型擦除并尝试使用Manifests
来解决警告问题(但迄今为止没有成功),但这两个实现之间有什么区别?Case类是否在做某些需要添加的事情?这可以用Manifests
绕过吗?
我甚至尝试使用-Xprint:typer
标志在Scala编译器中运行Case类实现,但unapply
方法看起来几乎与我预期的相同:
case <synthetic> def unapply[A >: Nothing <: Any, B >: Nothing <: Any](x$0: $iw.$iw.White[A,B]): Option[B] = if (x$0.==(null))
scala.this.None
else
scala.Some.apply[B](x$0.right);