scala - 如何避免使用路径依赖类型的可怕类型转换？

Question

我是 Scala 的新手，不知道为什么我必须在以下代码中进行与路径相关类型相关的(对我来说不直观)类型转换。
(我不喜欢 getter、setter 或 nulls，它们在这里是为了分离操作并消除错误来源的歧义)

// Module A public API
class ModA {
  trait A
}

// Module B public API that depends on types defined in Module A
class ModB(val modA: ModA) {
  trait B {
    def getA: modA.A;
    def setA(anA: modA.A);
  }
}

// One implementation of Module A
class ModAImpl extends ModA {
  class AImpl extends A
}

// One implementation of Module B
class ModBImpl(mod: ModA) extends ModB(mod) {
  class BImpl extends B {
    private[this] var privA: modA.A = _;
    override def getA = privA;
    override def setA(anA: modA.A) = privA = anA;
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    // wiring the modules
    val modAImpl = new ModAImpl;
    val modBImpl = new ModBImpl(modAImpl);

    // wiring objects
    val a = new modAImpl.AImpl;
    val b = new modBImpl.BImpl;
    b.setA(a); //don't compile and complain: type mismatch;  found: modAImpl.A  required: modBImpl.modA.A

    //i have to do this horrible and coutnerintuitive cast to workaround it
    b.setA(a.asInstanceOf[modBImpl.modA.A]);

    var someA: modAImpl.A = null;
    someA = b.getA; // don't compile with same reason
    someA = b.getA.asInstanceOf[modAImpl.A]; // horrible cast to workaround

    println(a == b.getA); // however this prints true
    println(a eq b.getA); // this prints true too
  }
} 

我已阅读有关单例类型以在两种类型相同时通知编译器的信息，但我不知道如何在此处应用。
提前致谢。

Answer 1

您可以在 ModB 上粘贴类型参数类型:

class ModA { trait A }

class ModB[AA](val modA: ModA { type A = AA }) {
  trait B {
    def getA: AA
    def setA(anA: AA)
  }
}

class ModAImpl extends ModA { class AImpl extends A }

class ModBImpl[AA](
  mod: ModA { type A = AA }) extends ModB(mod) {
  class BImpl extends B {
    private[this] var privA: AA = _
    override def getA = privA
    override def setA(anA: AA) = privA = anA
  }
}

并且类型推断都按需要进行:

scala> val modAImpl = new ModAImpl
modAImpl: ModAImpl = ModAImpl@7139edf6

scala> val modBImpl = new ModBImpl(modAImpl)
modBImpl: ModBImpl[modAImpl.A] = ModBImpl@1dd7b098

scala> val a = new modAImpl.AImpl
a: modAImpl.AImpl = ModAImpl$AImpl@4cbde97a

scala> val b = new modBImpl.BImpl
b: modBImpl.BImpl = ModBImpl$BImpl@63dfafd6

scala> b.setA(a)