我确实遇到了隐式方法的 Scala 编译器问题。场景很简单。隐式方法的任务是将案例类 A 的对象转换为案例类 B 的对象。隐式方法实现访问 A 的一个不存在的案例类成员。如果案例类成员在案例类 A 或 B 中根本不存在(例如 foobar)，编译器会抛出错误。如果案例类成员确实存在于案例类 B 中，编译器不会抛出错误，即使我使用此名称(即成员 x)访问案例类 A。
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。以下代码将导致编译错误。
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 不是案例类的成员，以下代码也不会遇到编译器错误:
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，我调查了已编译的 scala 类，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$1: scala.Int): scala.Any = { /* compiled code */ }
override def productIterator: scala.collection.Iterator[scala.Any] = { /* compiled code */ }
def canEqual(x$1: scala.Any): scala.Boolean = { /* compiled code */ }
override def productElementName(x$1: 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$1: 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$0: 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$1: scala.Int): scala.Any = { /* compiled code */ }
override def productIterator: scala.collection.Iterator[scala.Any] = { /* compiled code */ }
def canEqual(x$1: scala.Any): scala.Boolean = { /* compiled code */ }
override def productElementName(x$1: 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$1: 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$0: 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 */ }
}
最佳答案
编译器正在执行多项操作来解决缺少的方法/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，或者是否存在添加方法 foobar 的 隐式类。
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，它提供方法/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正在做什么
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。
关于方法尝试访问不存在的案例类成员时的 Scala 隐式方法编译，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/62194645/