覆盖Scala中的密封特征

Question

I am using a library that has a sealed trait. I really need to extend this trait.

Is there a way (even a dirty one) to bypass that?

For a bit of background, this is what I am trying to work around : https://github.com/ReactiveMongo/ReactiveMongo/issues/247

Answer 1

Don't extend, include, If you need to consider another option.

// Foo.scala
sealed trait Foo
case class Foo1(n: Int) extends Foo
case class Foo2(s: String) extends Foo

// Bar.scala
sealed trait Bar
case class BarFoo(foo: Foo) extends Bar
case class Bar3(b: Boolean) extends Bar

And the boilerplate can be reduced by implicit conversion

implicit def fooToBar(foo: Foo): Bar = BarFoo(foo)

---

Or if you want to add new methods to an existing class , use implicit classes:

// Foo.scala
sealed trait Foo
case class Foo1(n: Int) extends Foo
case class Foo2(s: String) extends Foo

// Quux.scala
// This class be called FooOps or FooExtra, if you like, name is irrelevant
implicit class Quux(foo: Foo) {
  def quux: Boolean = foo match {
    case Foo1(n) => n == 0
    case Foo2(s) => s.isEmpty
  }
}

Then you can use .quux on Foo values:

scala> Foo1(2).quux
res1: Boolean = false

scala> Foo2("").quux
res2: Boolean = true

This is about the same as extending, but with even less boilerplate, compare to:

case class Quux2(foo: Foo) {
  def quux2: Boolean = foo match {
    case Foo1(n) => n == 0
    case Foo2(s) => s.isEmpty
  }
}

implicit def fooToQuux2(foo: Foo): Quux2 = Quux2(foo)

scala> Foo1(2).quux
res0: Boolean = false

scala> Foo2("").quux
res1: Boolean = true