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)
                                       ^

0__ · Accepted Answer · 2013-06-15 16:37:55Z

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, 2013 at 16:37

0__

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2 Comments

Brent Faust Over a year ago

Nice answer. That's quite a gotcha when using case classes, and it's still not fixed as of 2.11.6.

0__ Over a year ago

Actually, I consider it a useful feature to be able define parameters that do not participate in equality and hash.

som-snytt · Accepted Answer · 2013-06-15 19:07:42Z

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, 2013 at 19:07

som-snytt

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4 Comments

Dominykas Mostauskis Over a year ago

I find this method more useful, but I'm giving 0__'s answer the acceptance, because it seems to be more popular.

som-snytt Over a year ago

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.

Dominykas Mostauskis Over a year ago

Yeah, in retrospect, I don't know why I switched the green checks.

Dominykas Mostauskis Over a year ago

Took a second look after all these years. This is definitely the better solution! Giving back the green check!

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