自制提取器和案例类提取器的区别 [英] Difference between home made extractor and case class extractor

问题描述

根据scala规范，case类构建的提取器如下(scala规范§5.3.2):

According to the scala specification, the extractor built by case classes is the following (scala specification §5.3.2):

def unapply[tps](x: c[tps]) =
  if (x eq null) scala.None
  else scala.Some(x.xs11, ..., x.xs1k)

出于实现原因，我希望能够在非大小写类上模拟此提取器的行为.但是，我的实现无法重现相同的行为.

For implementation reasons, I want to be able to mimic the behavior of this extractor on a non-case class. However, my implementation fails to reproduce the same behavior.

这是我所拥有的差异的一个例子:

Here is an example of the difference i have:

trait A

sealed trait B[X <: A]{ val x: X }

case class C[X <: A](x: X) extends B[X]

class D[X <: A](val x: X) extends B[X]

object D {
  def unapply[X <: A](d: D[X]): Option[X] =
    if (d eq None) None
    else Some(d.x)
}

def ext[X <: A](b: B[X]) = b match {
  case C(x) => Some(x)
  case D(x) => Some(x)
  case _ => None
}

我有以下警告:

<console>:37: warning: non variable type-argument X in type pattern D[X] is unchecked since it is eliminated by erasure
     case D(x) => Some(x)

注意警告只发生在 D 的情况下，而不是在 case-class texttractor 的情况下.你知道警告的原因/我应该怎么做才能避免这个警告吗?

Notice the warning occurs only in the D case, not in the case-class textractor case. Do you have any idea about the cause of the warning / about what I should do to avoid this warning ?

注意:如果你想在 REPL 中测试它，最简单的方法是:

Note: If you want to test it in REPL, the easiest way is:

1. 激活未选中的警告

1. To activate unchecked warning

scala> :power

scala> :power

scala> settings.unchecked.value = true

scala> settings.unchecked.value = true

以粘贴模式复制以上代码:

To copy above code in paste mode:

scala> :paste

scala> :paste

[复制/粘贴]

[ctrl + D]

正如 Antoras 提到的，它应该是一个编译器错误，也许 scala 版本可能有用:scala 2.9.0.1(经过快速测试，仍然存在于 Scala 2.9.1RC2 中)

As Antoras mentioned it should be a compiler bug, maybe the scala version could be useful: scala 2.9.0.1 (after a quick test, still there in scala 2.9.1RC2)

推荐答案

这似乎是一个编译器错误.我已经分析了编译器 AST 的输出(使用 fsc -Xprint:typer .scala).它将两者解释为相同:

This seems to be a compiler bug. I have analyzed the output of the compiler AST (with fsc -Xprint:typer <name_of_file>.scala). It interprets both as the same:

...
    final <synthetic> object C extends java.lang.Object with ScalaObject with Serializable {
      def this(): object test.Test.C = {
        C.super.this();
        ()
      };
      final override def toString(): java.lang.String = "C";
      case <synthetic> def unapply[X >: Nothing <: test.Test.A](x$0: test.Test.C[X]): Option[X] = if (x$0.==(null))
        scala.this.None
      else
        scala.Some.apply[X](x$0.x);
      case <synthetic> def apply[X >: Nothing <: test.Test.A](x: X): test.Test.C[X] = new test.Test.C[X](x);
      protected def readResolve(): java.lang.Object = Test.this.C
    };
...
    final object D extends java.lang.Object with ScalaObject {
      def this(): object test.Test.D = {
        D.super.this();
        ()
      };
      def unapply[X >: Nothing <: test.Test.A](d: test.Test.D[X]): Option[X] = if (d.eq(null))
        scala.None
      else
        scala.Some.apply[X](d.x)
    };
...

两个方法 unapply 的方法签名是相同的.

The method signature of both methods unapply are identical.

此外，代码工作正常(由于相同的方法，正如预期的那样):

Furthermore the code works fine (as expected due to identical methods):

trait A {
  def m = "hello"
}

class AA extends A

sealed trait B[X <: A]{ val x: X }

case class C[X <: A](x: X) extends B[X]

class D[X <: A](val x: X) extends B[X]

object D {
  def apply[X <: A](x: X) = new D(x)
  def unapply[X <: A](d: D[X]): Option[X] =
    if (d eq null) None
    else Some(d.x)
}

def ext[X <: A](b: B[X]) = b match {
  case C(x) => Some("c:"+x.m)
  case D(x) => Some("d:"+x.m)
  case _ => None
}
println(ext(C[AA](new AA())))
println(ext(D[AA](new AA())))

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