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

user_89872 0
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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.

coalescing scala
2 Answers

Added 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

Or if you prefer named syntax as opposed to operator syntax

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

More examples

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")
  }
}
user_2633060
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