How can a parameter's default value reference another parameter?

Question

How can a parameter's default value reference another parameter? If it cannot, how to work around that?

case class A(val x:Int, val y:Int = x*2)

Error (reasonably enough):

scala> case class B(val x:Int, val y:Int = x*2)
<console>:7: error: not found: value x
   case class B(val x:Int, val y:Int = x*2)
                                       ^

score 25 · Answer 1 · answered Jun 15 '13 at 16:37

25

This requires that you use multiple parameter lists:

case class A(x: Int)(y: Int = x*2)

Default values can only refer to parameters in preceding lists.

Be careful however with case classes, because their equality only takes into the account the first parameter list, therefore:

A(1)() == A(1)(3)  // --> true!!

answered Jun 15 '13 at 16:37

0__

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Nice answer. That's quite a gotcha when using case classes, and it's still not fixed as of 2.11.6. – Brent Faust Jun 02 '15 at 18:12
2

Actually, I consider it a useful feature to be able define parameters that do not participate in equality and hash. – 0__ Jun 03 '15 at 10:12

score 10 · Accepted Answer · answered Jun 15 '13 at 19:07

10

Since you asked for the work-around, if it's not obvious how to preserve caseness:

scala> :pa
// Entering paste mode (ctrl-D to finish)

case class Foo(x: Int, y: Int)
object Foo {
  def apply(x: Int): Foo  = apply(x, 2*x)
}

// Exiting paste mode, now interpreting.

defined class Foo
defined object Foo

scala> Foo(5,6)
res45: Foo = Foo(5,6)

scala> Foo(5)
res46: Foo = Foo(5,10)

answered Jun 15 '13 at 19:07

som-snytt

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I find this method more useful, but I'm giving 0__'s answer the acceptance, because it seems to be more popular. – Dominykas Mostauskis Jul 29 '15 at 10:40
Actually, the green check is supposed to mean what solved your problem, however you define that. But I would guess you solved your problem a couple of years ago. – som-snytt Jul 29 '15 at 15:24
Yeah, in retrospect, I don't know why I switched the green checks. – Dominykas Mostauskis Jul 29 '15 at 18:18
Took a second look after all these years. This is definitely the better solution! Giving back the green check! – Dominykas Mostauskis Feb 07 '23 at 10:12

How can a parameter's default value reference another parameter?

2 Answers2