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.

You should have answered that question instead of creating a new one. Or, perhaps, in addition to. — Daniel C. Sobral, Sep 01 '09 at 23:59
If you say so. When I answer a question more than about 24 hours after it's asked the answer languishes in obscurity, regardless of its relative merit (and given that I only answer scala questions I have some idea of the value of my answer.) — psp, Sep 03 '09 at 02:28
thanks, it was a horror to see a solution that involves exceptions. unless you want to slow down your application several order of magnitude. — smartnut007, Aug 07 '12 at 03:48
[Best Scala imitation of Groovy's safe-dereference operator (?.)?](http://stackoverflow.com/questions/1163393/best-scala-imitation-of-groovys-safe-dereference-operator) — Mechanical snail, Aug 07 '12 at 11:22

psp · Answer 1 · 2014-08-05T23:43:45.683

39

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

edited Aug 05 '14 at 23:43

answered Sep 01 '09 at 20:01

psp

12,138
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10

I've waited seven years for someone to make that observation! Thanks. – psp Aug 30 '16 at 20:34

Gabriel · Answer 2 · 2021-04-21T21:10:46.847

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")
  }
}

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

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