当方法尝试访问不存在的案例 Class 成员时，Scala 隐式方法编译

Question

我运行使用隐式方法解决了一个 Scala 编译器问题。场景很简单。隐式方法的任务是将 case class A 的对象变成 case class B 的对象。隐式方法实现访问 A 的 case class 成员，它做不存在。如果 case class 成员在 case class A 或 B（例如 foobar）中根本不存在，编译器会抛出错误。如果 case class 成员在 case class B 中确实存在，编译器不会抛出错误，即使我使用此名称访问 case class A（即成员 x）。

我使用的是 2.13.1 版的 Scala。目前，2.13.2 是最新版本。

以下代码更详细地显示了该场景。以下代码会导致编译错误。

package Hokuspokus

object ImplicitMagic extends App {

  case class A(a: String, b: String, c: String)
  case class B(d: String, e: String, f: String, x: String)

  implicit def AtoB: A => B = a => B(a.a, a.b, a.c, a.foobar)

  def print(b: B): Unit = {
    System.out.println("Print" + b.d)
  }

  val a = A("foo", "bar", "asdf")

  print(a)

}

编译器指出以下错误：

[ERROR]   implicit def AtoB: A => B = a => B(a.a, a.b, a.c, a.foobar)
[ERROR]                                                       ^
[ERROR] one error found

但是，即使 x 不是 case class:

的成员，以下代码也不会运行编译器错误

package Hokuspokus

object ImplicitMagic extends App {

  case class A(a: String, b: String, c: String)
  case class B(d: String, e: String, f: String, x: String)

  implicit def AtoB: A => B = a => B(a.a, a.b, a.c, a.x)

  def print(b: B): Unit = {
    System.out.println("Print" + b.d)
  }

  val a = A("foo", "bar", "asdf")

  print(a)

}

我现在想知道，为什么 Scala 编译器在编译时没有检测到这个问题。为了了解scalac编译器的作用，我调查了编译后的scala classes，但到目前为止我还没有得出结论。

package Hokuspokus
object ImplicitMagic extends scala.AnyRef with scala.App {
  def this() = { /* compiled code */ }
  case class A(a: scala.Predef.String, b: scala.Predef.String, c: scala.Predef.String) extends scala.AnyRef with scala.Product with scala.Serializable {
    val a: scala.Predef.String = { /* compiled code */ }
    val b: scala.Predef.String = { /* compiled code */ }
    val c: scala.Predef.String = { /* compiled code */ }
    def copy(a: scala.Predef.String, b: scala.Predef.String, c: scala.Predef.String): Hokuspokus.ImplicitMagic.A = { /* compiled code */ }
    override def productPrefix: java.lang.String = { /* compiled code */ }
    def productArity: scala.Int = { /* compiled code */ }
    def productElement(x: scala.Int): scala.Any = { /* compiled code */ }
    override def productIterator: scala.collection.Iterator[scala.Any] = { /* compiled code */ }
    def canEqual(x: scala.Any): scala.Boolean = { /* compiled code */ }
    override def productElementName(x: scala.Int): java.lang.String = { /* compiled code */ }
    override def hashCode(): scala.Int = { /* compiled code */ }
    override def toString(): java.lang.String = { /* compiled code */ }
    override def equals(x: scala.Any): scala.Boolean = { /* compiled code */ }
  }
  object A extends scala.runtime.AbstractFunction3[scala.Predef.String, scala.Predef.String, scala.Predef.String, Hokuspokus.ImplicitMagic.A] with java.io.Serializable {
    def this() = { /* compiled code */ }
    final override def toString(): java.lang.String = { /* compiled code */ }
    def apply(a: scala.Predef.String, b: scala.Predef.String, c: scala.Predef.String): Hokuspokus.ImplicitMagic.A = { /* compiled code */ }
    def unapply(x[=14=]: Hokuspokus.ImplicitMagic.A): scala.Option[scala.Tuple3[scala.Predef.String, scala.Predef.String, scala.Predef.String]] = { /* compiled code */ }
  }
  case class B(d: scala.Predef.String, e: scala.Predef.String, f: scala.Predef.String, x: scala.Predef.String) extends scala.AnyRef with scala.Product with scala.Serializable {
    val d: scala.Predef.String = { /* compiled code */ }
    val e: scala.Predef.String = { /* compiled code */ }
    val f: scala.Predef.String = { /* compiled code */ }
    val x: scala.Predef.String = { /* compiled code */ }
    def copy(d: scala.Predef.String, e: scala.Predef.String, f: scala.Predef.String, x: scala.Predef.String): Hokuspokus.ImplicitMagic.B = { /* compiled code */ }
    override def productPrefix: java.lang.String = { /* compiled code */ }
    def productArity: scala.Int = { /* compiled code */ }
    def productElement(x: scala.Int): scala.Any = { /* compiled code */ }
    override def productIterator: scala.collection.Iterator[scala.Any] = { /* compiled code */ }
    def canEqual(x: scala.Any): scala.Boolean = { /* compiled code */ }
    override def productElementName(x: scala.Int): java.lang.String = { /* compiled code */ }
    override def hashCode(): scala.Int = { /* compiled code */ }
    override def toString(): java.lang.String = { /* compiled code */ }
    override def equals(x: scala.Any): scala.Boolean = { /* compiled code */ }
  }
  object B extends scala.runtime.AbstractFunction4[scala.Predef.String, scala.Predef.String, scala.Predef.String, scala.Predef.String, Hokuspokus.ImplicitMagic.B] with java.io.Serializable {
    def this() = { /* compiled code */ }
    final override def toString(): java.lang.String = { /* compiled code */ }
    def apply(d: scala.Predef.String, e: scala.Predef.String, f: scala.Predef.String, x: scala.Predef.String): Hokuspokus.ImplicitMagic.B = { /* compiled code */ }
    def unapply(x[=14=]: Hokuspokus.ImplicitMagic.B): scala.Option[scala.Tuple4[scala.Predef.String, scala.Predef.String, scala.Predef.String, scala.Predef.String]] = { /* compiled code */ }
  }
  implicit def AtoB: scala.Function1[Hokuspokus.ImplicitMagic.A, Hokuspokus.ImplicitMagic.B] = { /* compiled code */ }
  def print(b: Hokuspokus.ImplicitMagic.B): scala.Unit = { /* compiled code */ }
  val a: Hokuspokus.ImplicitMagic.A = { /* compiled code */ }
}

Answer 1

编译器正在执行多项操作来解决缺失的 method/val a.foobar。

它将检查此方法是否属于 case class A，它将检查 A 是否可以隐式转换为包含方法 foobar 的不同类型，或者是否存在 implicit class添加方法 foobar。

最终，它决定此方法不可用，因此您会看到编译器错误。

如果您使用 a.x，编译器会找到从 A 到 B 的隐式转换，它提供 method/val x。不幸的是，它没有捕捉到这种情况发生在实际转换中的事实。编译器在这种情况下所做的如下

implicit def AtoB: A => B = a => B(a.a, a.b, a.c, AtoB(a).x)

这会编译，但会在运行时产生 StackOveflowException。

当方法尝试访问不存在的案例 Class 成员时，Scala 隐式方法编译

Scala Implicit Method Compilation when Method Tries to Access Non-Existing Case Class Members

scala

implicit

scala-compiler