scala - 这是一个类型检查器错误吗？

Question

我将其缩小到以下代码段:

trait A[T] {
  def apply(t: T): Int
}

sealed trait P {
  def apply(): Int
}

case class I[T](a: A[T], t: T) extends P {
  def apply: Int = a(t)
}

case class X[T1, T2](a1: A[T1], a2: A[T2]) extends A[(T1, T2)] {
  def apply(t: (T1, T2)): Int =
    t match {
      case (t1, t2) => a1(t1) + a2(t2)
    }
}

object m {
  def apply(p1: P, p2: P): P =
    (p1, p2) match {
      case (I(a1, t1), I(a2, t2)) =>
        I(X(a1, a2), (t2, t1)) // <-- Here
    }
}

如您所见，我在标记为 <-- Here 的行中有一个类型错误。 .然而，代码编译时甚至没有警告，并以 ClassCastException 失败。在运行时。要玩的代码:

case class E() extends A[Int] {
  def apply(t: Int): Int = t
}

case class S() extends A[String] {
  def apply(t: String): Int = t.length
}

object Test {
  def apply() = {
    val pe: P = I(E(), 3)
    val ps: P = I(S(), "abcd")
    val pp: P = m(pe, ps)
    pp()
  }
}

我知道当模式匹配 scala 有时无法检查值是否为正确类型时，但这通常会导致编译器警告。

那么，这是一个错误，还是我错过了什么？

更新:我担心的是我可能会犯类型错误而编译器甚至不会警告我。我明白 (t1, t2)是正确的顺序；但是如果我写错了，直到执行程序我才会发现它，甚至可能更晚，尽管它显然是一个类型错误。

Answer 1

也许缺乏警告与此有关:

https://issues.scala-lang.org/browse/SI-9188

它似乎对 A 上的类型参数没有任何用处，除非它可以静态证明你弄错了。

这里的最后一场比赛应该警告:

scala> val i = I(E(), 42)
i: I[Int] = I(E(),42)

scala> i match { case I(a: A[Int], x) => }

scala> i match { case I(a: A[String], x) => }
<console>:15: warning: non-variable type argument String in type pattern A[String] is unchecked since it is eliminated by erasure
              i match { case I(a: A[String], x) => }
                                  ^
<console>:15: error: pattern type is incompatible with expected type;
 found   : A[String]
 required: A[Int]
              i match { case I(a: A[String], x) => }
                                  ^

scala> (i: P) match { case I(a: A[String], x) => }
<console>:15: warning: non-variable type argument String in type pattern A[String] is unchecked since it is eliminated by erasure
              (i: P) match { case I(a: A[String], x) => }
                                       ^
<console>:15: error: pattern type is incompatible with expected type;
 found   : A[String]
 required: A[Any]
Note: String <: Any, but trait A is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
              (i: P) match { case I(a: A[String], x) => }
                                       ^

scala> (i: P) match { case I(a: A[Int], x) => }
<console>:15: warning: non-variable type argument Int in type pattern A[Int] is unchecked since it is eliminated by erasure
              (i: P) match { case I(a: A[Int], x) => }
                                       ^
<console>:15: error: pattern type is incompatible with expected type;
 found   : A[Int]
 required: A[Any]
Note: Int <: Any, but trait A is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
              (i: P) match { case I(a: A[Int], x) => }
                                       ^

scala> (i: P) match { case I(a: A[_], x) => }

scala> (i: P) match { case I(a: A[Any], x) => }

补充一下:

scala> (i: P) match { case I(a: A[Any], x) => a("foo") }
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
  at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:105)
  at E.apply(<console>:33)
  ... 33 elided