scala - How to write a proper null-safe coalescing operator in scala?

Question

Having seen the answers coming out of questions like this one involving horror shows like trying to catch the NPE and dredge the mangled name out of the stack trace, I am asking this question so I can answer it.

Comments or further improvements welcome.

Answer 1

Like so:

case class ?:[T](x: T) {
  def apply(): T = x
  def apply[U >: Null](f: T => U): ?:[U] =
    if (x == null) ?:[U](null)
    else ?:[U](f(x))
}

And in action:

scala> val x = ?:("hel")(_ + "lo ")(_ * 2)(_ + "world")()
x: java.lang.String = hello hello world

scala> val x = ?:("hel")(_ + "lo ")(_ => (null: String))(_ + "world")()
x: java.lang.String = null

Answer 2

添加orElse

case class ?:[T](x: T) {
  def apply(): T = x
  def apply[U >: Null](f: T => U): ?:[U] =
    if (x == null) ?:[U](null)
    else ?:[U](f(x))
  def orElse(y: T): T = 
    if (x == null) y
    else x
}

scala> val x = ?:(obj)(_.subField)(_.subSubField).orElse("not found")
x: java.lang.String = not found

或者，如果您更喜欢命名语法而不是运算符语法

case class CoalesceNull[T](x: T) {
  def apply(): T = x
  def apply[U >: Null](f: T => U): CoalesceNull[U] =
    if (x == null) CoalesceNull[U](null)
    else CoalesceNull[U](f(x))
  def orElse(y: T): T =
    if (x == null) y
    else x
}

scala> val x = CoalesceNull(obj)(_.subField)(_.subSubField).orElse("not found")
x: java.lang.String = not found

更多示例

case class Obj[T](field: T)

  test("last null") {
    val obj: Obj[Obj[Obj[Obj[String]]]] = Obj(Obj(Obj(Obj(null))))
    val res0 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field)()
    res0 should === (null)
    val res1 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field).orElse("not found")
    res1 should === ("not found")
  }

  test("first null") {
    val obj: Obj[Obj[Obj[Obj[String]]]] = null
    val res0 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field)()
    res0 should === (null)
    val res1 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field).orElse("not found")
    res1 should === ("not found")
  }

  test("middle null") {
    val obj: Obj[Obj[Obj[Obj[String]]]] = Obj(Obj(null))
    val res0 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field)()
    res0 should === (null)
    val res1 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field).orElse("not found")
    res1 should === ("not found")
  }

  test("not null") {
    val obj: Obj[Obj[Obj[Obj[String]]]] = Obj(Obj(Obj(Obj("something"))))
    val res0 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field)()
    res0 should === ("something")
    val res1 = CoalesceNull(obj)(_.field)(_.field)(_.field)(_.field).orElse("not found")
    res1 should === ("something")
  }
}