Question

我在类型安全构建器模式中使用幻像类型，以确保只调用一次方法，如下面的代码示例所示

  sealed trait TBoolean
  sealed trait TTrue extends TBoolean
  sealed trait TFalse extends TBoolean

  class Builder[MethodCalled <: TBoolean] private() {

    def foo()(implicit ev: MethodCalled =:= TFalse): Builder[TTrue] = {
      new Builder[TTrue]
    }
  }

  object Builder {
    def apply() = new Builder[TFalse]()
  }

我非常感谢这种方法，因为可以使用. - 运算符来链接方法调用（与其他方法不同）但是，如果有许多方法可以防止像

这样的结尾，这就变得不方便了

  class Builder[MethodCalled1 <: TBoolean, MethodCalled2 <: TBoolean, ... ,MethodCalledN <: TBoolean]

有没有办法创建“类型结构”？像下面的伪代码：

  type S {
      type MethodCalled1 <: TBoolean
      type MethodCalled2 <: TBoolean
      ...
      type MethodCalledN <: TBoolean
  }

  class Builder[S] private() {

    def foo()(implicit ev: S#MethodCalled1 =:= TFalse): Builder[S#MethodCalled1.TTrue] = {
      new Builder[S#MethodCalled1.TTrue]
    }
  }

Answer 1

你走在正确的轨道上，你只需要添加一个小类型的改进：

trait BuilderMethods {
  type FooCalled <: TBoolean
  type BarCalled <: TBoolean
}

class Builder[M <: BuilderMethods] private() {
  def foo()(implicit ev: M#FooCalled =:= TFalse): Builder[M {type FooCalled = TTrue}] = {
    new Builder[M {type FooCalled = TTrue}]
  }
  def bar()(implicit ev: M#BarCalled =:= TFalse): Builder[M {type BarCalled = TTrue}] = {
    new Builder[M {type BarCalled = TTrue}]
  }
}

object Builder {
  type UnusedBuilder = BuilderMethods {type FooCalled = TFalse; type BarCalled = TFalse;}
  def apply(): Builder[Builder.UnusedBuilder] = new Builder[UnusedBuilder]()
}

object TestPhantomStruct extends App {
  val newBuilder = Builder()
  val builderFooCalled = newBuilder.foo()
  val builderFooCalledTwice = builderFooCalled.foo() // will not compile
  val builderFooCalledBarCalled = builderFooCalled.bar()
  val builderFooCalledTwiceBarCalled = builderFooCalledBarCalled.foo() // will not compile

  val builderBarCalled = newBuilder.bar()
  val builderBarCalledTwice = builderBarCalled.bar() // will not compile
}

类型安全的构建器：如何组合幻像类型

1 个答案: