scala - 根据 Scala 中类型证据的存在的惯用分支方式

Question

我发现自己不止一次写了以下丑陋的模式：

class Something[A, B](implicit ev: A =:= B = null) {
  ...

  def doStuff {
    if (ev == null) ... // know that A is not the same as B
    else ...            // safely assume A is equal to B
  }
}

更糟糕的是，当 时ev != null，我有时会写出诸如 之类的异端邪说someB.asInstanceOf[A]。

Answer 1

只需使用类型类，

trait DoStuff[A, B] {
  def apply(): Unit
}

trait DoStuff0 {
  implicit def neDoStuff[A, B]: DoStuff[A, B] =
    new DoStuff[A, B] { def apply(): Unit = ... body of your ev == null case ...
}

object DoStuff extends DoStuff0 {
  implicit def eqDoStuff[A]: DoStuff[A, A] =
    new DoStuff[A, A] { def apply(): Unit = ... body of your ev != null case ...
}

class Something[A, B](implicit ds: DoStuff[A, B]) {
  ...
  def doStuff: Unit = ds()
}