hmm.md

Case Classes w/ Smart Ctors

I just learned this trick from @OlegYch and it seems pretty good. Wondering if anyone knows of any problems with it.

People often try to constrain construction of a case class by making the constructor private and providing a partial factory method on the companion. For instance, a natural number class wrapping an Int:

case class Nat private (toInt: Int)
object Nat {
  def fromInt(n: Int): Option[Nat] =
    if (n >= 0) Some(Nat(n)) else None
}

We can construct instances with fromInt that behave as expected.

scala> Nat.fromInt(1)
res4: Option[Nat] = Some(Nat(1))

scala> Nat.fromInt(-1)
res5: Option[Nat] = None

scala> new Nat(42)
<console>:14: error: constructor Nat in class Nat cannot be accessed in object $iw
       new Nat(42)
       ^

However there are two holes:

scala> Nat.fromInt(42).get.copy(-67) // copy method is unsafe
res17: Nat = Nat(-67)

scala> Nat(-1) // oops, companion apply
res18: Nat = Nat(-1)

The traditional solution to this problem is to use a sealed trait or non-case class and hack up the rest of the machinery that makes it look like a case class, which is boilerplatey and irritating.

---

So, it turns out we can get around this by using the unlikely-sounding sealed abstract case class.

sealed abstract case class Nat(toInt: Int)
object Nat {
  def fromInt(n: Int): Option[Nat] =
    if (n >= 0) Some(new Nat(n) {}) else None
}

When we do this the behavior is what we want.

There's no companion apply.

scala> val n = Nat(10)
<console>:15: error: Nat.type does not take parameters
       val n = Nat(10)
                  ^

So we use the factory method.

scala> val on = Nat.fromInt(-1)
on: Option[Nat] = None

scala> val on = Nat.fromInt(10)
on: Option[Nat] = Some(Nat(10))

Pattern-matching and unapply work.

scala> val oi = on.collect { case Nat(n) => n }
oi: Option[Int] = Some(10)

scala> val n = on.get
n: Nat = Nat(10)

scala> Nat.unapply(n)
res11: Option[Int] = Some(10)

Our toInt field is public, and equals seems to do the right thing.

scala> n.toInt
res12: Int = 10

scala> Nat.fromInt(3) == Nat.fromInt(3)
res13: Boolean = true

And copy doesn't work.

scala> n.copy(-1)
<console>:18: error: value copy is not a member of Nat
       n.copy(-1)
         ^

Example with private-package constructor:

object pack {
  sealed abstract case class Nat private[pack](toInt: Int)
  object Nat {
    def fromInt(n: Int): Option[Nat] =
      if (n >= 0) Some(new Nat(n){}) else None
  }
}

scala> new Nat(-9){}
<console>:12: error: constructor Nat in class Nat cannot be accessed in <$anon: pack.Nat>
              new Nat(-9){}
                  ^

This is not needed for the situations where you would use extends AnyVal - extends AnyVal makes a class wrapping an Int (say) into just an Int at runtime, so the equals, toString, hashcode etc. will work as you expect, and you don't want copy anyway.

As of Scala 2.11.11 (with -Xsource:2.12) and Scala 2.12.2, this trick is no longer necessary. Thanks to scala/scala#5730, you can make the constructor private with case class C private (...) and then put your smart apply method in the companion object.

@SethTisue

Using Scala 2.12.2, you cannot seem to implement a smart apply method that returns an Option.

scala> case class Person private (name: String, age: Int)
defined class Person

scala> object Person {
     |   def apply(name: String, age: Int): Option[Person] =
     |     if (age < 0) None
     |     else Some(Person(name, age))
     | }
<console>:16: error: type mismatch;
 found   : Option[Person]
 required: Person
           else Some(Person(name, age))
                           ^

@wjlow You have to write Some(new Person(name, age)) because you've made the default apply private.

@SethTisue
this trick is still necessary with scala 2.12.2 because even with a private constructor, the copy method is still public :(
update: or alternatively also override the copy method (eg. to return an Option.)

@jbgi @SethTisue or

private def copy() = ()

@yawaramin, @ccmtaylor, @mpilquist

Compiler only creates one anonymous class there; it's not per creation, it's per unique inner class.

object NatSealed {
  def apply(toInt: Int): NatSealed = new NatSealed(toInt) {}
}

object Main extends App {
  override def main(args: Array[String]): Unit = {
    println(NatSealed(1).getClass == NatSealed(2).getClass)
  }
}

@jbgi

@SethTisue
this trick is still necessary with scala 2.12.2 because even with a private constructor, the copy method is still public :(
update: or alternatively also override the copy method (eg. to return an Option.)

Eliding the copy method entirely is being tracked (I think) by scala/bug#7884.

The "sealed abstract case class" suggestion is not necessary any more:

https://gist.github.com/longliveenduro/62638ec95a6fd0c6576bdb77ca88cd6b

Tested in Scala 2.11.12 (with -Xsource:2.12), 2.12.10 and Scala 2.13.2: